THE  CHEMISTRY  AND  TECHNOLOGY 

OF 

GELATIN  AND  GLUE 


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VENEERING  AND  THE  USE  OF  GLUE  IN  ANCIENT  EGYPT. 

The  upper  cut  shows,  to  the  left,  a  piece  of  thin  rare  wood  being  applied  to 
a  plank  of  inferior  quality.  An  adze  is  stuck  into  a  block  of  the  latter  wood. 
Above  this  is  a  box  made  with  inlaid  veneer.  The  central  figure  is  grinding 
something.  Above  him  is  shown  a  pot  of  glue  being  heated  over  a  fire,  and  a 
piece  of  glue  with  its  characteristic  conchoidal  fracture.  The  figure  to  the 
right  is  applying  glue  with  a  brush.  The  lower  cut  shows  a  plank  of  rare  wood 
being  cut  into  thin  pieces  for  veneer.  The  figure  to  the  right  is  smoothing 
the  surface.  Specimens  of  the  wood  showing  its  handsome  grain  are  pictured  at 
the  left. 

Wall  carving  in  the  tomb  of  Rekhmara  in  Thebes,  Period  of  Thothmes  III, 
1500-2000  (?)  B.  C.  Taken  from  "The  Life  of  Rekhmara"  by  P.  E.  Newberry, 
Westminster,  1900.  (Kindness  of  C.  C.  Keller  of  the  Western  Theological  Seminary 
of  Chicago,  III.) 


THE  CHEMISTRY  AND  TECHNOLOGY 

OF 
GELATIN  AND  GLUE 


BY 

ROBERT  HERMAN  BOGUE,  M.S.,  PH.D., 

INDUSTRIAL   FELLOW   OP   THE    MELLON   INSTITUTE    OP   INDUSTRIAL   RESEARCH    OF   THE 

UNIVERSITY    OP    PITTSBURGH,    AND    RESEARCH    CHEMIST    FOR    ARMOUR 

AND    COMPANY    OF    CHICAGO 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
NEW  YORK:  370  SEVENTH  AVENUE 

LONDON:   6  &  8  BOUVERIE  ST.,  E.  C.  4 
1922 


COPYRIGHT,  1922,  BY  THE 
McGRAw-HiLL  BOOK  COMPANY,  INC. 


THE     MAFIAS     PHKSS     Y  O  S  K     PA 


DEDICATED 
TO 

CHARLES  FREDERICK  CHANDLER, 

The  Patriarch  of  Chemical  Progress  in  American  Indus- 
try; than  whom,  among  chemists,  there  is  no  one  more 
beloved,  no  one  more  graced  with  kindly  sympathy, 
and  no  one  more  potent  to  inspire,  in  all  America. 


4835 


PREFACE 

The  dearth  of  reliable  information  upon  the  manufacture, 
testing,  analysis,  and  general  applications  of  gelatin  and  glue  is 
felt  keenly  among  students  and  investigators  who  are  desirous  of 
acquainting  themselves  with  the  principles  of  this  industry.  The 
books  that  are  upon  our  shelves  have  not  kept  up  with  the  latest 
developments  and  improvements  in  the  art,  and  much  information 
that  is  found  in  the  literature  is  astonishingly  inaccurate.  But  of 
even  greater  importance  is  the  failure  of  previous  writers  to  meet 
the  issue  of  the  chemistry  of  gelatin  and  glue.  An  enormous 
amount  of  very  important  and  suggestive  work  has  appeared 
during  the  past  decade  in  the  scientific  literature  dealing  either 
directly  or  indirectly  with  the  chemistry  of  gelatin,  and  it  is  in  the 
correlation  and  summarization  of  this  material  that  the  author 
feels  his  work  is  most  completely  justified.  The  attempt  is 
made  throughout  the  book  to  attack  the  subject  from  the  point 
of  view  of  the  chemist  rather  than -from  that  of  the  plant  tech- 
nologist, and  it  is  primarily  for  the  student  and  investigator, — the 
thinkers  ahead, — that  the  book  is  written. 

There  is  no  field  of  research  that  is  more  rich  in  highly  interest- 
ing problems  awaiting  investigation  than  that  of  the  protein 
colloids,  of  which  gelatin  and  glue  are  the  most  conspicuous 
examples.  We  have  but  to  point  to  the  recent  book  by  Wilder 
D.  Bancroft  on  " Applied  Colloid  Chemistry,"  to  cause  a  seem- 
ingly unending  procession  of  such  problems  to  rise  before  us  and 
to  beckon  us  on  to  master  them.  There  is  no  dearth  for  study 
here.  And  it  has  been  the  constant  aim  of  the  present  author  to 
indicate  lines  upon  which  further  investigation  would  be  highly 
desirable. 

The  advance  in  the  principles  of  industrial  research  that  has 
been  made  during  the  past  ten  years  is  a  seven-league  stride 
when  compared  with  the  meager  progress  previously  attained. 
Industrialists  have  been  slow  to  appreciate  the  value  of  chemical 
research,  and  to  those  among  them  who  have  had  the  vision 
to  foster  it,  too  much  credit  cannot  be  given.  The  pioneer  work 
of  Armour  and  Company  in  providing  for  an  intensive  investiga- 

ix 


x  PREFACE 

tion  -upon  gelatin  and  glue  should  be  mentioned,  together  with  the 
studies  that  are  being  carried  on  upon  somewhat  similar  lines  in 
the  laboratories  of  A.  F.  Gallun  and  Sons,  of  Milwaukee,  by  John 
A.  Wilson,  and  in  the  laboratories  of  the  Eastman  Kodak  Com- 
pany, of  Rochester,  by  S.  E.  Sheppard. 

The  subject  of  gum  and  dextrin  adhesives  is  omitted  purposely 
on  account  of  the  unreliability  of  existing  literature  in  this  field. 
Work  has  been  carried  on  upon  this  subject  at  the  Institute,  how- 
ever, and  it  is  the  intention  of  the  author  to  include  the  results  of 
this  study  in  a  later  edition.  The  subject  of  algal  gelatins  also 
has  been  omitted,  but  a  brief  resume*  of  that  field  may  be  found 
in  Chemical  Age  (New  York),  29  (1921),  485,  by  Irving  A.  Field. 

The  author  wishes  especially  to  express  his  thanks  to  Ralph 
C.  Shuey,  chemical  engineer  with  the  Redmanol  Chemical 
Products  Company,  of  Chicago,  formerly  with  the  Armour  Glue 
Works,  for  his  contribution  of  Chap.  VI  on  "The  Manufacture  of 
Glue  and  Gelatin;"  and  to  Dr.  Donald  K.  Tressler,  formerly  indus- 
trial fellow  of  the  Mellon  Institute  of  Industrial  Research,  for  his 
contribution  of  the  section  on  "Liquid  Fish  Glues"  (Chap.  VII). 
The  author  also  has  made  free  use  with  proper  credit  of  all 
available  literature  bearing  upon  the  subjects  treated. 

It  is  a  pleasure  to  acknowledge  the  indebtedness  which  the 
author  feels  towards  Armour  and  Company  for  the  cooperation, 
without  which  the  present  volume  could  not  have  been  written. 
The  constructive  criticisms  of  J.  R.  Powell,  chief  chemist  of  the 
Armour  Glue  Works  and  of  Adolph  Heicke,  the  plant  super- 
intendent, have  been  of  the  greatest  value.  To  William  A. 
Hamor,  assistant  director  of  the  Mellon  Institute  of  Industrial 
Research,  the  author  is  indebted  deeply  for  inspiration  and 
encouragement,  as  also  for  editorial  advice.  It  is  an  especial 
pleasure  to  be  able  to  express  the  highest  appreciation  to  Doctors 
John  C.  Hessler,  Jacques  Loeb,  Wilder  D.  Bancroft,  Martin  H. 
Fischer,  and  Arthur  W.  Thomas  for  reading  sections  of  the 
manuscript,  and  for  their  very  valuable  criticisms  which  have 
made  possible  an  accurate  and  comprehensive  treatment  of  the 
modern  theories  of  the  emulsoid  colloids. 

ROBERT  HERMAN  BOGUE. 
PITTSBURGH,  PA., 
May,  1922. 


CONTENTS 

PAGE 


PREFACE  '  ............................  ix 

INTRODUCTION.  —  Historical  and  Statistical  Considerations  ......        1 

PART  I.—  THEORETICAL  ASPECTS 

CHAP. 

I.  THE  CONSTITUTION  OP  THE  PROTEINS  ............     15 

II.  THE  CHEMISTRY  OF  GELATIN  AND  ITS  CONGENERS  ......     48 

III.  THE  PHYSICO-CHEMICAL  PROPERTIES  AND  STRUCTURE  OF  GELATIN  91 

IV.  GELATIN  AS  A  LYOPHILIC  COLLOID  ..............    160 

V.  GELATIN  AS  AN  AMPHOTERIC  COLLOID  ............   219 

PART  II.—  TECHNOLOGICAL  ASPECTS 

VI.  THE  MANUFACTURE  OF  GLUE  AND  GELATIN,  BY  RALBH'C.  SHUEY    271 
VII.  WATER-  RESISTANT  GLUES  AND  GLUES  OF  MARINE  ORIGIN.    .    .    .   318 

VIII.  THE  TESTING  OF  GLUE  AND  GELATIN  ............   367 

IX.  THE    CHEMICAL    ANALYSIS,    DETECTION,    AND  ESTIMATION  OF 

GELATIN  AND  GLUE  ....................   425 

X.  THE  EVALUATION  OF  GLUE  AND  GELATIN  ...........   487 

XI.  THE  USES  AND  APPLICATIONS  OF  GLUE  ............   506 

XII.  THE  USES  AND  APPLICATIONS  OF  GELATIN  ..........   555 

APPENDIX  ........................    ...   579 

INDEX.  .   625 


XI 


THE 

CHEMISTRY  AND  TECHNOLOGY 

OF 
GELATIN  AND  GLUE 

INTRODUCTION 
HISTORICAL  AND  STATISTICAL  CONSIDERATIONS 

Glutinum  ferunt  Daedalium  invenisse. 
Varro  (about  20  B.C.) 

The  English  word  " gelatin"1  is  derived  through  the  French 
gelatine,  and  Italian  gelatina,  from  the  Latin  gelata,  which  means 
that  which  is  frozen,  congealed,  or  stiff.  It  is  therefore,  in 
origin,  cognate  with  " jelly,"  which  comes  through  the  French 
gelee  from  the  same  Latin  original. 

The  term  "glue"2  comes  through  the  French  glu  from  the 
Latin  glutem  or  glus  meaning  glue.  The  French  also  use  the 
term  glu  to  refer  to  lime,  the  name  given  to  a  viscous  exudation 
of  the  holly  tree,  used  for  ensnaring  birds  and  for  that  reason 
known  as  bird  lime.  The  German  word  for  glue,  him,  comes 
through  this  source. 

The  term  glutin,  from  the  Latin  glutem,  is  employed  as  the 
German  equivalent  of  gelatin.  This  expression  has  also  found 
occasional  use  by  English  writers  in  the  designation  of  the  pure 
protein  "gelatin, "  as  distinguished  from  the  commercial  material. 

The  Greek  word  for  glue  was  Ko\\a,  from  which  the  term 
"colloid"  was  derived  directly,  and  meaning  glue-like  or  gelati- 
nous. The  protein  "collagen"  was  similarly  derived  from  the 
same  Greek  term,  with  the  ending  -gen,  the  whole  meaning  a 
glue-producing  material. 

HISTORICAL  CONSIDERATIONS 

The  utilization  of  the  skins  of  animals  for  protection  against 
cold  and  in  the  forming  of  rough  shelters  was  doubtless  one  of 
the  earliest  of  the  achievements  of  man  in  his  evolution  from  his 

1  Used  in  1800  by  Hatschett  (Trans.  Roy.  Soc.  London,  90,  366)  as  follows, 
"That  animal  jelly — which  is  distinguished  by  the  name  of  gelatin." 

2  Used  in  1400  by  Lanfranc  (*'  Chirurgia  Magna  et  Parva,"  p.  135)   as 
follows,  "As  it  were  two  bordis  weren  ioyned  togidere  with  cole  or  with  glu." 

1 


2  GELATIN  AND  GLUE 

progenital  ape-like  ancestors  to  the  dawn  of  more  enlightened 
civilization.  A  granite  carving  of  ancient  Egyptian  origin  which 
is  probably  4,000  years  old,  and  is  now  deposited  in  the  British 
Museum,  depicts  the  working  up  of  a  tiger  skin  to  render  it 
suitable  for  service.  Homer  (1000  B.C.?)  wrote  the  picturesque 
lines  descriptive  of  an  ancient  hide-preserving  operation,  in 
visualizing  the  struggle  over  the  body  of  Patroclus: 

"As  when  a  man 

A  huge  ox-hide  drunken  with  slippery  lard 
Gives  to  be  stretched,  his  servants  all  around 
Disposed,  just  intervals  between,  the  task 
Ply  strenous,  and  while  many  straining  hard 
Extend  it  equal  on  all  sides,  it  sweats 
The  moisture  out  and  drinks  the  unction  in." 
(Iliad,  XVII,  389-393.) 

That  skins  were  used  in  the  early  Bible  period  is  shown  by 
many  passages.  In  Kings  (900  B.C.?),  it  is  written: 

"Dost  thou  see  that  I  dwell  in  a  house  of  cedar,  and  the  Ark  of 
God  is  lodged  within  skins."  (Kings,  VII.) 

Leather  ornaments,  straps,  coverings,  etc.,  have  been  found  well 
preserved  on  mummies,  and  shoes  of  morocco  leather  of  early 
Egyptian  periods  are  existent. 

Just  how  early  it  was  discovered  that  a  powerful  adhesive 
could  be  made  by  cooking  up  hide  pieces  in  water,  cannot  be 
ascertained,  but  it  may  be  conjectured  that  such  a  discovery 
could  not  have  been  long  delayed  after  the  experiences  obtained 
in  the  tanning  and  preservation  of  hides  and  pelts.  Among 
the  stone  carvings  of  the  ancient  city  of  Thebes,  of  the  period 
of  Thothmes  III,  the  Pharaoh  of  the  Exodus,  and  at  least  3,300 
years  old,  is  one  representing  the  gluing  of  a  thin  piece  of  a  rare 
wood  of  red  color  to  a  yellow  plank  of  sycamore,  see  Frontis- 
piece. A  pot  of  the  adhesive  is  being  heated  over  a  fire,  and 
several  samples  of  veneered  and  inlaid  wood  are  pictured.  One 
of  the  figures  is  spreading  glue  with  a  brush,  and  a  piece  of  dry 
glue  with  its  characteristic  concave  fracture  is  shown. 

Reference  is  made  to  glue  in  the  Bible  in  Ecclesiastes  (200  B.C.) : 

"He  that  teacheth  a  fool  is  like  one  that  glueth  a  potsherd 
together."  (Ecclesiastes,  XXII,  7.) 

It  is  probable  that  hide  pieces  only  were  employed  for  making 
glue  in  the  earlier  periods.  At  least  no  mention  is  made  of  the 


HISTORICAL  CONSIDERATIONS  3 

use  of  bones  for  such  a  purpose  until  comparatively  recent 
times,  but  reference  is  frequently  made  in  the  Roman  period  to 
the  adhesive  value  of  glue  made  from  hides.  Thus  Lucretius 
about  50  B.C.  wrote: 

"  Materials  are  made  one  from  bullish  glue," 
and  Pliny,  about  a  hundred  years  later  repeated, 
"Glue  is  cooked  from  the  hides  of  bulls," 
and  referred  to  a  glue  made  in  Rhodia  as  being  most  satisfactory : 

"Rhodiacum  glutinum  fidelissimum." 
(Pliny  28,    17,   71,    §  236.) 

Varro  (about  20  B.C.)  advises  us  that  glue  was  discovered  in 
Daedalia, 

"Glutinum  ferunt  Daedalium  invenisse," 
(Varro,  from  Charis,  p.  67  and  106.) 

but  it  was  certainly  in  use  before  that  period.  Pliny  refers  to 
glue  as  being  used,  together  with  gums,  milk,  eggs,  and  wax  as 
a  vehicle  for  the  paints  used  by  the  ancient  Egyptians,  and  a 
glue-like  material  has  been  found  in  other  ancient  applications. 
In  the  Elizabethian  period  (about  1600  A.D.)  Shakespeare 
and  Francis  Bacon  made  frequent  reference  to  glue.  The  adhe- 
sive value  of  the  material  is  well  attested  in  the  lines  of  the  great 
dramatist : 

"Go  to;  have  your  lath  glued  within  your  sheath, 
Till  you  know  better  how  to  handle  it." 

(Titus  Andronicus,  Act  2,  scene  1.) 

And  Bacon,  in  imitation  of  the  earlier  philosophers,  discoursed 
as  follows : 

"Water  and  all  liquors  do  hastily  receive  dry  and  terrestrial 
bodies  proportionable;  and  dry  bodies  on  the  other  hand 
drink  in  water  and  liquors,  so  that  it  was  well  said  by  the 
ancients,  of  earthy  and  watery  substances,  'One  is  glued  to 
the  other." 

The  earliest  practical  manufacture  of  glue  that  can  be  directly 
traced  from  the  present  day  dates  back  to  the  time  of  William  III 
of  Holland.  It  appears  to  have  been  manufactured  there  in 
1690,  and  shortly  after  to  have  been  introduced  into  England  and 
established  as  one  of  her  permanent  industries  about  1700. 

The  first  mention  of  glue  in  patent  literature  is  found  in  a 
British  patent  of  1754,  and  refers  to  the  preparation  of  "a  kind 
of  glue  called  fish  glue."  This  historic  patent  makes  interesting 
reading. 


4  GELATIN  AND  GLUE 

A.D.  1754,  May  23,  No.  691.     To  Peter  Zomer. 

Making  from  the  tails  and  fins  of  whales,  and  from  such  sediment,  trash, 
and  undissolved  pieces  of  the  fish  as  are  usually  thrown  away  as  useless, 
after  the  boiling  of  the  blubber,  a  sort  of  train  oil,  and  afterwards  making 
from  the  remains  of  such  tails,  fins,  sediments,  and  undissolved  pieces  a 
kind  of  glue  called  fish  glue. 

For  making  the  glue,  the  undissolved  blubber  and  pieces  of  fish,  after 
they  have  been  boiled  for  the  train  oil,  are  put  upon  boards  in  a  cold  place  or 
into  small  casks  till  they  are  perfectly  cold  and  run  together  in  one  body 
which  is  then  cut  into  small  pieces  and  put  into  a  trough  made  with  a  grate 
at  the  bottom,  which  must  be  then  shut  and  the  trough  filled  with  water,  in 
which  these  small  pieces  must  lie  about  24  hours  in  order  to  soak,  after  which 
the  water  is  let  off  at  the  bottom  through  the  grate,  and  these  pieces  are  put 
into  a  boiler  on  the  bottom  whereof  is  laid  first  a  wooden  bottom  with  holes 
in  it,  over  that  a  row  of  laths,  and  over  them  a  covering  of  straw,  and  upon 
that  a  second  row  of  laths,  which  must  be  fixed  so  as  to  prevent  their  swim- 
ming, and  upon  this  the  pieces  are  laid;  water  is  added  and  the  pieces  and 
water  are  boiled  together  for  2  or  3  hours.  Then  any  sediment  remaining 
from  former  makings  is  put  in  and  it  is  allowed  to  stand  for  1  hour  to  settle. 
The  glue  water  is  then  drawn  off  from  the  bottom  of  the  boiler  and  after- 
wards the  dross  and  sediment  remaining  in  the  boiler  may  be  put  into  a 
press  with  holes  in  order  to  press  out  the  remainder  of  the  glue  water.  This 
glue  water  must  be  strained  through  a  hair  sieve  to  clear  it,  and  then  stand 
for  a  night  in  order  to  grow  cold  and  stiff,  when  it  is  cut  into  pieces  and  placed 
upon  nets  to  dry  and  harden. 

The  preparation  of  an  isinglass  made  from  "the  internal  and 
external  gelatinous  and  membranous  parts  of  fishes  in  general, 
and  of  the  sturgeon  in  particular/'  was  patented  in  1760,  and  the 
system  used  today  in  Russia  and  some  other  countries  in  making 
isinglass  was  described  in  1812. 

In  1814  the  treatment  of  bones  with  "muriatic,  nitric,  phos- 
phoric, or  acetous  acid,  and  the  mixture  stirred  daily  until 
the  bony,  hard,  or  cartilaginous  parts  shall  have  become  soft," 
was  first  mentioned  in  the  patent  literature.  The  use  of  steam 
under  pressure  "conveyed  into  a  mass  of  bones  in  such  manner 
as  to  extract  therefrom  a  gelatin  adapted  for  the  purpose  of  glue" 
was  first  described  in  a  patent  of  1822.  Sulphurous  acid  was 
introduced  into  glue  and  gelatin  manufacture  in  1838,  and 
"euchlorine,  chlorous,  or  chloric  acid  prepared  from  the  chlorates 
or  chlorides  of  lime,  potass,  soda,  barytes,  or  other  compounds 
by  the  action  of  hydrochloric  or  other  acids,  "was  described  in 
1839.  Vacuo  evaporation  of  glue  liquors  was  introduced  in 
1844. 

The  first  mention  of  the  manufacture  of  a  gelatin  for  edible 
purposes  is  found  in  a  patent  by  Arney  in  1846.  He  prepared  a 


HISTORICAL  CONSIDERATIONS  5 

powdered  gelatin  "for  forming  compositions  from  which  may  be 
prepared  jellies  and  blanc-manges;  also,  when  mixed  with  farina, 
or  starch,  or  starchy  vegetable  flour,  for  thickening  soups, 
gravies,  etc." 

The  application  of  "currents  of  air  artificially  dried  either  by 
heat  or  any  of  the  drying  compounds  in  common  use,  such  as 
concentrated  sulphuric  acid  or  fused  chloride  of  calcium,"  was 
introduced  in  1847.  A  few  other  important  mechanical  devices 
for  speeding  up  the  cooling  of  the  product,  for  pulverizing,  and 
for  making  a  nearly  anhydrous  powdered  glue  have  been  added 
to  the  installment  of  the  larger  glue  factories  in  recent  years,  but 
the  operations  today  are  otherwise  not  greatly  different  than 
those  which  were  in  practice  100  years  ago.  The  modern  glue 
manufacturing  establishment  is  described  in  detail  in  Chap.  VI. 

The  earliest  official  record  of  the  United  States  government 
wherein  glue  is  mentioned  seems  to  have  been  in  a  compendium 
of  all  of  the  manufactures  of  the  several  counties  of  each  state 
which  was  compiled  for  the  year  1810.  In  this  there  were  listed 
six  establishments  making  glue  in  Pennsylvania,  with  a  total 
product  value  for  the  year  of  $53,206,  and  one  in  Maryland  with 
a  product  value  of  $500.  In  no  other  state  is  glue  mentioned  as 
being  produced.  The  American  Glue  Co.  of  Boston  affirm, 
however,  that  the  originator  of  their  company,  Elijah  Upton, 
began  the  first  manufacture  of  glue  in  this  country  in  1808  in 
the  town  of  Peabody,  Mass.,  at  that  time  called  South  Danvers. 
Peter  Cooper  established  his  glue  factory  in  1827  at  Brooklyn, 
N.  Y. 

f       STATISTICAL  CONSIDERATIONS 

The  manufacture  of  glue  and  gelatin  in  this  country  since 
1810,  and  the  most  recent  data  available  upon  imports  and 
exports,  are  given  in  the  following  tables,  taken  mainly  from  the 
Census  Reports  of  the  Department  of  Commerce  and  the  Bulletins 
of  the  Bureau  of  Foreign  and  Domestic  Commerce.1 

In  Table  1  and  Fig.  1  is  shown  a  comparison  of  the  domestic 
production,  the  total  imports,  and  the  total  exports  of^glue  and 
gelatin  for  the  years  1914  and  1919.  The  most  striking  feature 
observed  here  is  the  enormous  increase  in  exports  (about  380 
per  cent)  and  decrease  in  imports  (afeout  510  per  cent)  which 
occurred  in  that  5-year  period.  * 

1  Especially  U.  S.  Bureau  of  Foreign  &  Domestic  Comm..  Bull.  82,  1919 


6  GELATIN  AND  GLUE 

Table  2  and  Fig.  2  show  the  number  of  establishments  em- 
ployed in  manufacturing^  glue  or  gelatin  as  their  principal 
product,  the  capital  invested,  and  the  value  of  the  product,  for 
the  census  years  from  1810  to  1914,  and  the  distribution  of  manu- 
facture among  the  several  states  in  the  latter  year.  It  will  be 
observed  that  the  capital  invested  and  the  value  of  the  product 
have  quite  steadily  increased,  whereas,  the  number  of  establish- 
ments has  decreased  since  1879  from  82  to  57.  This  means  that 
the  industry  has  become  more  centralized  and  that  a  large  number 
of  small  factories  has  given  place  to  a  less  number  of  larger  ones. 
Illinois  led  in  1914  in  the  capital  invested  and  the  value  of  the 
product,  although  Massachusetts  had  a  greater  number  of  glue 
and  gelatin  establishments. 

It  is  sometimes  overlooked  that  glue  and  gelatin  are  also  pro- 
duced in  large  amounts  by  establishments  that  are  devoted 
primarily  to  the  manufacture  of  some  other  product.  The 
slaughtering  and  meat  packing  establishments  especially  produce 
large  amounts  of  glue  and  gelatin,  but  these  are  not  included 
in  Table  2. 

Tables  3  and  4  are  compiled  to  give  data  upon  the  relative 
amounts  of  glue  manufactured  in  glue  and  in  other  establish- 
ments in  1900  and  in  1914,  and  the  relative  amount  of  glue  made 
from  the  several  chief  sources,  as  hides,  bones,  etc.  see  also  Fig.  1. 
In  1900  there  was  very  nearly  as  much  glue  made  in  slaughtering 
as  in  glue  establishments,  but  in  1914  only  15.7  per  cent  of  the 
value  of  the  total  output  came  from  slaughter  houses.  From 
fertilizer  establishments  came  5.7  per  cent,  and  9.0  per  cent 
from  all  others,  as  plants  manufacturing  sand  and  emery  paper, 
tallow,  soap  stock,  food  preparations,  oleo  oil,  and  fish  oil. 
More  than  60  per  cent  of  all  glue  (in  1900)  was  made  from  hide 
pieces,  fleshings,  sinews,  leather  cuttings,  etc.,  and  33.5  per  cent 
from  bones.  Fish  skins  and  refuse  contributed  4.3  per  cent. 

The  imports  of  glue  and  gelatin,  and  glue  stock,  for  the  years 
1914,  1918  and  1919  are  shown  in  Tables  5  and  6,  together  with 
the  percentage  by  quantity  imported  from  each  of  the  principal 
shipping  countries.  In  1918  the  imports  reached  a  very  low 
value,  but  in  1919  had  gained  greatly.  Even  the  latter  values, 
however,  are  much  below  the  prewar  level  of  1914. 

The  exports  of  domestic  and  of  foreign  glue  for  the  years  1914, 
1918  and  1919,  together  with  the  percentage  by  quantity  shipped 
to  the  principal  receiving  countries  are  shown  in  Tables  7,  8  and 


HISTORICAL  CONSIDERATIONS  7 

9.  It  is  noticeable  that  in  1914  the  exports  of  foreign  made 
glue  and  gelatin  exceeded  those  of  the  domestic  product  by  about 
$273,000,  while  in  1919,  the  exports  of  domestic  product  exceeded 
those  of  foreign  manufacture  by  $1,472,000. 


STATISTICS 

20 
19 
18 
17 
16 
!5 
14 

IN( 

:OM 

1 

PLETE            P 

ER  CENT 
100 
95 
90 
85 
80 
75 
70 

i 

/  OTHER 

S  CATTLE 
HOGS  ETC.  i.97o 

MFlSH    3MN5 
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BONES 

33.5% 

j 

13 

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N 

j> 

65 

60 

i 

j  M 

^QN 

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^N 

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55 

H 

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Z    6 

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i 

1 

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£     Ld 

ii 

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sOs 

1 

£  IE 

2    £ 

£  i3 

ii 

50 
45 
40 
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30 
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15 
10 
5 

HIDE  TRIMMINGS 
60.ZL% 

ft 

sfft 

x^i 

\$N 

^£\x$i 

o 

8§ 

1914 


1919 


FIG.  1. — The  domestic  production,  total  Distribution  of  glue  manufac- 
imports,  and  total  exports  of  glue  and  gela-  tured  from  various  materials  in  the 
tin  for  the  years  1914  and  1919.  United  States  in  1900. 

A  chart  of  the  average  prices  of  imported  glue  from  1820  to 
1914  as  compiled  by  Ludwig  A.  Thiele1  is  shown  in  Fig.  3.  Con- 
siderable fluctuation  will  be  observed  within  short  periods  of 
time,  but  the  general  tendency  is  shown  to  be  a  slight  decrease  of 
about  1  cent  per  pound  each  15  years. 

1  LTJDWIG  A.  THIELE,  personal  communication. 


GELATIN  AND  GLUE 


TABLE   1. — DOMESTIC  PRODUCTION,   IMPORTS,   AND  EXPORTS  OF  GELATIN 
AND  GLUE,  1914  AND  1919 

1914 


Material 

Domestic 
production 

Total 
imports 

Total  exports 

Gelatin,  unmanufactured  .  . 
Glue 

$     875,588 
1  810  093 

$       13,595 
266  334 

Mucilage  

$19,725,703 

26  182 

Isinglass  and  fish  glue 

70  212 

Total  

19,725,703 

2  755  893 

306  111 

Hide    cuttings    and    other 
glue  stock   . 

no  data 

1  510  608 

1919 


Gelatin,  unmanufactured  .  . 
Glue  and  size  

|      no  data       > 

$     241,835 

208  882 

{  $1,489,  625  } 

Total  

450  717 

1  489  625 

Hide    cuttings    and    other 
glue  stock  

978  514 

TABLE  2. — GLUE  AND  GELATIN  MANUFACTURED  IN  THE  UNITED  STATES 
FROM  1810  TO  19141 


Year 

Number  of 
establishments 

Capital 

Value  of 
product 

1914 

57 

$17,162,000 

$13,733,000 

1909 

65 

14,289,000 

13,718,000 

1904 

58 

10,673,000 

10,035,000 

1899 

61 

6,144,000 

5,389,000 

1889 

62 

4,859,000 

4,270,000 

1879 

82 

3,917,000 

4,324,000 

1869 

70 

1,955,000 

1,710,000 

1859 

62 

1,053,000 

1,186,000 

1849 

47 

520,000 

652,000 

1810 

7 

53,706 

1  From  establishments  engaged  primarily  in  the  manufacture  of  glue  or 
gelatin. 


HISTORICAL  CONSIDERATIONS 


Distribution  by  States  for  1914 


Illinois  
Massachusetts 

9 
11 

5,552,170 
2  956  497 

3,731,375 
2  588  733 

Pennsylvania  

8 

2  820  250 

2  028  767 

New  York  
Indiana 

9 
3 

2,459,102 
356  295 

2,483,254 
280  251 

All  others*  

17 

3,018,048 

2,620  449 

Canada  (1918)   

11 

816  420 

1  465  163 

*  Includes:  Ohio,  5  establishments;  California,  3;  Wisconsin,  2;  and  1  each 
in  Connecticut,  District  of  Columbia,  Iowa,  Kentucky,  Maine,  Maryland 
and  New  Hampshire. 


o  =  NUMBER  OF  ESTABLISHMENTS 
•  =  CAPITAL  INVESTED 
x  =  VALUE  or  PRODUCT 


1810  1649  1859  1869  1879  1889  1899  1909  1919 

FIG.  2. — The  glue  and  gelatin  manufactured  in  the  United  States  from  1810  to 

1919. 


10 


GELATIN  AND  GLUE 


TABLE  3.     DISTRIBUTION  OF  GLUE  MANUFACTURED  IN  GLUE  ESTABLISH- 
MENTS AND  IN  SLAUGHTERING  ESTABLISHMENTS  IN  1900 


Total 
pounds 

Made  from 
hide     trim- 
mings,  etc. 

Made  from 
bones 

Made 
from 
cattle, 
hogs,    etc. 

Made 
from   fish, 
skins  and 
waste 

Made 
from 
other 
materials 

Glue  establishments  .  .  . 
Slaughtering  establish- 
ments   
United  States  

34,984,448 

34,516,761 
69,501,209 

29,036,901 

12,780,832 
41,817,733 

3,109,165 

20,183,562 
23,292,727 

66,666 

1  ,  282  ,  367 
1  ,  349  ,  033 

2,731,156 

270,000 
3,001,156 

40,560 

40  ,  560 

Percentage       distribu- 
tion, United  States 

100.0 

60.2 

33.5 

1.9 

4.3 

0.1 

TABLE  4. — VALUE  OF  GLUE  MANUFACTURED  IN  GLUE  AND  OTHER  ESTAB- 
LISHMENTS IN  1914 


Quantity, 
pounds 

Value 

Percentage  by 
value  from  the 
different 
establishments 

Glue  establishments  

40,844,650 

$13,732,824 

69.6 

Slaughtering  establishments 
Fertilizer  establishments  
All  others    (including   sand 
and  emery  paper,   tallow, 
soap  stock,  food  prepara- 
tions, oleo  oil,  and  fish  oil). 
Total,  United  States  

no  data 

3,088,764 
1,131,243 

1,772,872 
19,725,703 

15.7 
5.7 

9.0 
100.0 

TABLE  5. — IMPORTS  OF  GELATIN  AND  GLUE,  1918  AND  1919 


1918 

1919 

Percentage  of  quan- 

Material 

tity  by  Countries 

Pounds 

Value 

Pounds 

Value 

in  1919 

1 

Gelatin,      un- 

82,766 

32,353 

449,336 

241,835 

Netherlands  .  .   72  .  5 

in  a  n  u  f  a  c- 

Scotland  16.7 

tured. 

Switzerland.  .     5.3 

France.                2  5 

Glue  and  size. 

732,324 

172,642 

866,042 

208,882 

Chili  28.9 

Netherlands..   20.5 

France  18.5 

England  15.7 

Belgium  12.9 

Hide   cuttings 

9,381,629 

454,838 

13,780,637 

978,514 

Canada  40  .  7 

and  other 

England  16.6 

glue  stock. 

Italy  12.1 

British  India.    11.2 

Uruguay              8  7 

HISTORICAL  CONSIDERATIONS 


11 


TABLE  6. — IMPORTS  OF  GLUE  AND  GELATIN,   1914 


Material 

Quantity 
in  pounds 

Value 
in  dollars 

Percentage  of  quan- 
tity by  countries  in 
per  cent 

Gelatin 
Manufactured 

135,374 

738,731 

481 
1,002 

1,805,543 

4,550 
56,454 

809 
13,758 

1,510,608 

Germany 

41.0 
37.6 
17.6 
55.9 
21.7 

10.5 
6.5 
4.2 
100.0 
84.9 
14.4 

31.1 

23.9 
19.1 
12.2 
5.2 
100.0 

90.1 
3.9 
3.1 
100.0 
61.8 
38.2 

Unmanufactured  
Extra  fine  gold  label 

2,441,337 

1,489 
1,669 

22,714,877 

81,466 
147,438 

352 
300,273 

France  
England 

Germany  
France 

Austria- 
Hungary  
Switzerland  — 
Scotland 

Germany  
England  
Canada 

No.  3,  fine          

Glues 
Animal  glue  . 

Glue  powder 

Germany  
Austria- 
Hungary  
England  
France  
Belgium  
Germany  

Japan  
England  
Russia  
Russia  
Scotland  
England 

Isinglass   and   prepared   fish 
sounds 

Isinglass,  lead     

Marine  glue  pitch 

Hide  cuttings  and  other  glue 
stock 

TABLE  7. — EXPORTS  OF  DOMESTIC  GELATIN  AND  GLUE,   1918  AND   1919 


1918 

1919 

Percentage  of 
countries 

quantity  by 
in  1919 

Pounds 

Value 

Pounds 

Value 

5,809,605 

$1,110,837 

8,486,167 

$1,480,777 

Canada  
England  
China 

29.8 
17.9 
.      .       8  6 

Japan 

6  8 

Cuba 

6  1 

Denmark  .... 

6.0 

12  GELATIN  AND  GLUE 

TABLE  8. — EXPORTS  OF  FOREIGN  GELATIN  AND  GLUE  IN  1919 


Material 

Pounds 

Value 

Percentage  of  quantity  by 
countries 

Gelatin,  unmanufactured  .  .  . 
Glue  and  glue  size  

2,635 

24,895 

$1,187 
7,661 

Cuba  
Peru  .  . 

84  5 

12  5 

Canada.  .  . 
Canada 

3.0 
58  6 

Cuba  
Mexico  . 

.  .  38.0 
3  4 

TABLE  9. — EXPORTS  OF  DOMESTIC  AND  FOREIGN  GELATIN  AND  GLUE  IN  1914 


Material 

Domestic  product 

Foreign  product 

Gelatin,  unmanufactured  .  .  . 
Glue 

$  8  238 

$  13,595 

258  096 

Mucilage.  

26  182 

Total  

34  420 

271  691 

15- 


1820 


1835 


1850 


186 


18SO 


18C5 


1910 


FIG.   3. — -Average  prices  of  imported   glue  from   1820  to   1914.      (Kindness   of 

Ludwig  A.  Thiele.) 


PART  I 
THEORETICAL  ASPECTS 

THE  FIRST   GREAT   MISSION   OP   SCIENCE   IS  TO  EXTEND  THE 

BOUNDARIES   OF   KNOWLEDGE,   THAT   MAN   MAY  LIVE  IN  AN 

EVER-WIDENING   HORIZON 


PART  I— THEORETICAL  ASPECTS 

CHAPTER  I 
THE  CONSTITUTION  OF  THE  PROTEINS 

It  is  the  intuition  of  unity  amid  diversity 

which  impels  the  mind  to  form  a  science. 

F.  S.  Hoffman  (about  1700  A.D.) 

PAGE 

1.  The  Proteins 15 

2.  The  Albuminoids  or  Sclero-proteins 19 

3.  The  Cleavage  Products  of  the  Proteins 20 

4.  Determination  of  the  Nitrogenous  Constituents : 

Protein,  Proteose,  Peptone,  and  Amino-Acid 25 

5.  The  Estimation  of  Amino-Acid 28 

6.  The  Separation  of  the  Amino- Acids 34 

7.  Determination  of  the  "Hausmann"  Numbers 37 

8.  Distribution  of  Nitrogen  by  the  Method  of  Van  Slyke 38 

9.  The  Color  Reactions  of  the  Proteins 42 

1.  THE  PROTEINS 

General  Description. — Living  matter  has  very  succinctly  been 
grouped  into  three  great  classes  of  compounds  with  respect  to 
composition.  The  carbohydrates  comprise  the  great  bulk  of 
plant  life:  the  cellulose  framework  of  the  woody  and  fibrous 
portions;  the  starchy  cells  that  fill  the  seeds  with  a  reserve  of 
energy;  and  the  saps  and  fruit  which  are  rich  in  sugars.  The 
fats  are  less  common  in  the  greater  portion  of  plant  structures, 
but  in  certain  forms,  as  the  seed  of  corn,  cotton,  flax,  soy-bean, 
etc.,  and  in  nuts  of  all  varieties  it  is  abundant.  In  animal 
economy  the  fats  play  a  most  important  role.  The  proteins  are 
by  far  the  most  complex,  and  probably  more  closely  connected 
with  the  living  processes  than  either  of  the  other  groups.  The 
relative  proportion  of  proteins  in  plants  is  usually  very  small. 
The  legumes  are  an  exception,  as  they  are  enabled  by  means  of  a 
saprophytic  azotobacter  in  their  roots  to  utilize  nitrogen  of  the  air 
in  protein  synthesis.  Animals,  however,  are  composed  very 
largely  of  proteins.  The  flesh  consists  mainly  of  myosinogen, 
para-myosinogen,  and  myosin.  The  blood  contains  several  pro- 

15 


16  GELATIN  AND  GLUE 

teins,  as  fibrinogen,  fibrin,  thrombin,  serum  globulin,  pseudoglobu- 
lin,  and  serum  albumin.  The  skin,  connective  tissue,  tendons, 
and  bone  contain  collagen,  chondrigen,  mucin,  and  elastin.  The 
hair,  nails,  horns,  feathers  etc.,  contain  keratin.  Milk  contains 
caseinogen,  lactoglobulin,  and  lactalbumin.  Many  other  proteins 
are  found  in  special  glands  or  secretions  of  the  animal  body,  but 
need  not  be  enumerated. 

When  these  several  proteins  are  studied  systematically,  it  soon 
becomes  apparent  that  they  are  very  unlike  in  many  respects, 
although  there  may  be  found  points  of  marked  similarity.  The 
word  protein  signifies  "of  first  importance,"  and  in  respect  to 
their  role  in  the  animal  and  plant  economy  they  are  well  named 
and  properly  grouped.  The  processes  most  intimately  bound  up 
in  what  we  call  "life  phenomena"  are  primarily  concerned  with 
activities  of  the  protoplasm  and  body  fluids,  and  these  in  turn  are 
for  the  most  part  proteins.  If  the  attempt  is  made  to  separate 
them  from  their  source  it  is  found  that,  while  qualitative  separa- 
tions may  easily  be  made,  a  quantitative  separation  is  a  most 
difficult  if  not  impossible  task.  Many  methods  are  commonly 
employed  to  obtain  any  specific  protein  in  a  state  of  high  purity, 
but  in  the  last  few  years  these  methods,  such  as  coagulation  by 
heat,  precipitation  by  salts,  solubility  in  weak  acids  or  bases,  etc., 
have  been  shown  to  be  qualitative  only. 

Most  of  these  methods  of  separation  depend  upon  differences 
in  solubility:  not,  as  had  formerly  been  propounded,  upon  pro- 
found differences  in  constitution  or  structure.  This  point  is 
further  substantiated  by  the  fact  that,  unlike  the  saccharides, 
practically  all  proteins  may  be  hydrolyzed  by  a  single  enzyme, — 
trypsin,  and  indeed  most  of  them  also  by  pepsin.  In  the  case  of 
the  saccharides  the  active  enzymes  which  will  bring  about 
hydrolysis  are,  in  nearly  every  case,  specific:  that  is,  a  given 
sugar  will  be  acted  upon  by  one  enzyme  only,  and  that  enzyme 
will  hydrolyze  no  other  sugar.  The  various  color-reactions  of 
the  proteins  are  also  applicable  to  nearly  all  proteins.  If  the 
differences  between  them  were  profound  it  would  be  expected 
that  they  would  require  specific  enzymes  to  accelerate  hydrolysis, 
and  that  they  would  not  all  react  to  the  same  color  tests. 

Another  peculiar  property  possessed  by  the  proteins  as  a  group 
is  their  ability  to  neutralize  to  a  considerable  extent  the  acidity 
of  acids,  or  the  alkalinity  of  bases,  which  may  be  added  to  them. 


CONSTITUTION  OF  THE  PROTEINS  17 

They  therefore  function  as  bases  in  neutralizing  acids,  and  as 
acids  in  neutralizing  bases.  That  is,  they  are  amphoteric 
substances.  The  English  physicist,  Graham,  found  that  the 
proteins  would  not  diffuse  through  animal  membranes  or  parch- 
ment paper.  He,  therefore,  called  them  colloids,  meaning  glue- 
like,  because  glue,  being  also  a  protein  substance,  would  not 
diffuse.  This  non-diffusibility  of  the  proteins  has  in  general 
been  accepted  as  being  due  to  the  extraordinary  size  of  the 
molecules,  the  pores  of  the  membranes  being  so  small  that  these 
large  molecules  are  retained  while  the  smaller  molecules  of  the 
crystalloids  may  pass  through  easily. 

In  composition  the  proteins  possess  a  very  marked  similarity. 
They  all  contain  carbon,  oxygen,  hydrogen,  and  nitrogen,  and 
sometimes  sulphur  and  phosphorus.  The  average  composition 
is  approximately:1 

Carbon , 50 . 0  per  cent 

Oxygen 25 . 0  per  cent 

Hydrogen 7.0  per  cent 

Nitrogen 16.0  per  cent 

Sulphur ss^. : 0.3  per  cent 

Phosphorus /> 0.3  per  cent 

From  the  above  consideration  it  is  apparent  that  the  proteins 
have  many  properties  in  common.  They  are  always  found  in 
living  matter,  or  produced  by  living  matter,  and  are  intimately 
associated  with  vital  phenomena;  they  are  separated  quantita- 
tively with  great  difficulty  by  applying  the  principles  of  frac- 
tional precipitation;  nearly  all  are  hydrolyzed  by  trypsin,  and  most 
of  them  by  pepsin,  into  amino-acids  which  are  simple  and  well 
defined  substances;  they  all  react  to  group  color-tests;  they  are 
amphoteric  substances,  are  colloidal,  and  are  very  similar  in  their 
ultimate  chemical  composition.  To  account  for  these  several 
properties  Kossel2  in  a  study  of  the  protamines  established  the 
hypothesis  that  the  proteins  consisted  of  a  large  number  of 
amino-acid  residues  which  were  bound  together  through  their 
carboxyl  and  amino  groups.  This  theory  was  thoroughly  sub- 
stantiated by  Emil  Fischer3  who  succeeded  in  synthesizing 

IMATHEWS,  "Physiological  Chemistry,"  2nd.  ed.,  New  York  (1916),  111. 

2  KOSSEL,  Z.  physiol.  Chem.,  69  (1910),  138. 

3  EMIL  FISCHER,  "  Untersuchungen  iiber  Aminosauren,  Polypeptide,  und. 
Proteine,"  Berlin  (1906). 

2 


18  GELATIN  AND  GLUE 

polypeptides  having  as  many  as  eighteen  amino-acids  linked 
together  as  suggested  by  Kossel.  This  octadecapeptid,  contain- 
ing fifteen  glycyl  radicals  and  three  leucyl  radicals,  possessed  most 
of  the  properties  of  a  natural  protein. 

Classification. — Two  systems  of  classification  of  the  proteins 
are  in  common  use,  one  advanced  by  the  American  Society  of 
Biochemists,  the  other  by  the  English  Society  of  Physiologists. 
As  far  as  possible  chemical  differentiation  has  been  made  the 
basis  of  the  grouping,  but  solubility  differences  are  made  use  of 
in  many  cases. 

The  American  classification  recognizes  three  major  divisions: 

1.  Simple  proteins. 

2.  Conjugated  proteins. 

3.  Derived  proteins. 

I.  Simple  Proteins. — These  are  naturally  occurring  proteins  which  on 
hydrolysis  decompose  only  into  a  amino-acids  or  their  derivatives. 
They  are  grouped  as  follows: 

A.  Albumins. — Simple  proteins,  coagulable  by  heat,  soluble  in  water 
and  dilute  salt  solutions.     Serum  albumin. 

B.  Globulins. — Simple  proteins,  heat  coagulable,  insoluble  in  water, 
but  soluble  in  dilute  solution  of  salts  of  strong  bases  and  acids. 
Serum  globulin. 

C.  Glutelins. — Simple  proteins,  heat  coagulable,  insoluble  in  water  or 
dilute  salt,  but  soluble  in  very  dilute  acids  or  alkalies.     Glutenin. 

D.  Prolamines. — Simple  proteins,  insoluble  in  water,  soluble  in  80  per 
cent  alcohol.     Found  in  grains.     Gliadin,  zein. 

E.  Albuminoids. — Simple  proteins,   insoluble  in  dilute   acids,   alkali, 
water,  or  salt  solution.     Collagen,  gelatin,  keratin,  elastin. 

F.  Histones. — Simple  proteins,   not   coagulable  by   heat,    soluble   in 
water  and  in  dilute  acid;  strongly  basic,  and  insoluble  in  ammonia. 
Histone. 

G.  Protamines. — Simple  proteins,   strongly  basic,   non-coagulable  by 
heat,  soluble  in  ammonia,  and  yielding  large  amounts  of  diamino- 
acids  on  decomposition.     Salmin. 

II.  Conjugated  Proteins. — These  are  compounds  of  proteins  with  some 
other  non-protein  group.  The  other  group  is  generally  acid  in  nature 
They  are  grouped  as  follows: 

A.  Hemoglobins  or  Chromoproteins. — The  prosthetic  group  is  colored. 
Hemoglobin. 

B.  Glyco-  or  Gluco-proteins. — The  prosthetic  group  contains  a  carbo- 
hydrate radicle.     In  mucin  and  cartilage  it  may  be  chondroitic  acid. 
Mudn,  mucoids. 

C.  Phosphoproteins. — Proteins  of  the  cytoplasm.     The  prosthetic  group 


CONSTITUTION  OF  THE  PROTEINS  19 

is  not  known,  but  it  contains  phosphoric  acid,  but  not  nucleic  acid 
or  a  phospholipin.     Casein. 

D.  Neucleoproteins. — Proteins  of  the  nucleus.     The  chromatin.     The 
prosthetic  group  is  nucleic  acid.     Nuclein. 

E.  Lecithoproteins. — Found   in    the    cytoplasm.     Prosthetic   group   is 
lecithin  or  a  phospholipin. 

III.  Derived  Proteins. — This  group  includes  all  the  decomposition  pro- 
ducts of  the  naturally  occurring  proteins,  produced  by  any  means 
whatsoever;  and  also  the  artificially  synthesized  poly  pep  tids. 

A.  PRIMARY  PROTEIN  DERIVATIVES. 

a.  Proteins. — The  first  products  of  hydrolysis  insoluble  in  water. 
Edestan. 

b.  Metaproteins. — Produced    by    further    hydrolysis.     Soluble    in 
weak   acids   and   alkalies,   but  insoluble  in   neutral    solutions. 
Acid  albumin. 

c.  Coagulated  Proteins. — Insoluble  protein  products  produced  by 
the  action  of  heat  or  alcohol. 

B.  SECONDARY  PROTEIN  DERIVATIVES. 

a.  Proteoses. — Hydrolytic     decomposition     products    of    proteins. 
Soluble  in  water,  not  coagulable  by  heat,  precipitated  by  saturat- 
ing their  solutions  with  ammonium  sulphate. 

b.  Peptones. — Produced  by  further  hydrolysis.     Soluble  in  water, 
not  coagulable  by  heat,  not  precipitated    by  saturation  with 
ammonium  sulphate^  generally  diffusible,  and  giving  the  biuret 
reaction. 

c.  Peptids. — Compounds  of  amino-acids  of  which  the  composition 
is  known. 

The  English  classification  is  very  similar.  From  the  fact  that 
the  proteins  of  the  Albuminoid  group,  e.g.  collagen  (from  hides 
and  bones),  keratin,  (from  hair,  horn,  hoofs,  feathers  etc.),  and 
elastin  (from  tendons)  make  up  in  large  measure  the  organic  part 
of  the  skeletal  structure  of  animals,  the  English  Society  has  called 
this  group  the  sclero-proteins.  It  is  at  once  apparent  that  it  is 
with  this  group  that  the  investigator  of  gelatin  and  glue  is  most 
concerned.  Following  the  presentation  of  this  brief  introduction 
upon  proteins  in  general,  attention  will,  therefore,  be  focused 
throughout  the  rest  of  the  chapter  upon  the  albuminoid  group, 
and  its  most  interesting  constituents. 

2.  THE  ALBUMINOIDS  OR  SCLERO-PROTEINS 

The  proteins  of  the  albuminoid  group  are,  according  to  the 
customary  classification,  simple  proteins,  insoluble  in  dilute  acid, 
alkali,  salt  solutions,  or  water.  But  while  all  of  the  members  of 


20  GELATIN  AND  GLUE 

this  group  are  insoluble  in  cold  water,  a  few,  e.g.,  collagen,  gelatin, 
and  sericin  are  readily  soluble  in  hot  water  after  a  preliminary 
soaking  in  cold  water,  during  which  period  a  solvation  or  hydra- 
tion  of  the  molecules  takes  place.  Alexander1  has  suggested  the 
following  classification  of  the  albuminoids,  somewhat  modified 
by  the  author: 

A.  COLLAGENS:  or  Jelly-forming  Albuminoids:      Dissolved  more  or  less 
Collagen;  from  bones,  hides,  etc.,  of  animals,     readily     by     boiling 

and  swimming  bladders,  skins,  and  scales  of    water.     The    solutions 
fish.  gelatinize    on    cooling. 

Gelatin;  from  collagen.  Contain    little    or    no 

Sericin;  from  silk.  sulphur. 

B.  FIBROIDS:  Not  acted  on  by  boiling 
Elastin;  from  elastic  ligaments,  tendons.  water    or   very    dilute 
Fibroin;  from  silk  and  sliders  webs.                        boiling     alkali.       Dis- 
solved     by      stronger 
alkali.     Unaffected  by 
dilute  acids.     Contain 

*  KUO  sirtphur. 

C.  CHITINOIDS:  Not  ^  Qn        boil_ 
Cto;  from  external  coatings  of  invertebrate.     .       water  or  alkalie8 

Chonchiohn;  irom  shells  ol  mollusca.  ^     ,    .  ,   , 

Contain  no  sulphur. 
Spongin;  from  sponges. 

D.  KERATINS:  Not  acted  on  by  boil- 
Keratin;  from  hoofs,  horns,  feathers,  hair,  etc.     ing   water.     Dissolved 
Neurokeratin;  from  brains.                                          by  boiling  with  dilute 

alkali  hydroxide.   Con- 
tain sulphur. 

3.  THE  CLEAVAGE  PRODUCTS  OF  THE  PROTEINS 

Before  consideration  is  given  to  the  properties  and  reactions  of 
the  several  individual  proteins  in  the  above  group  which  are  of 
interest  to  us,  it  is  necessary  to  discuss  the  more  general  reactions, 
which  are  found  to  obtain  with  all  proteins,  upon  treatment  with 
certain  enzymes  and  hydrolyzing  agents.  Whether  we  treat  a 
protein  with  a  solution  of  trypsin  or  pepsin  in  nearly  neutral 
solution  at  ordinary  temperature,  or  with  a  dilute  acid  or  alkali 
at  the  boiling  temperature,  or  with  superheated  steam,  the  results 
are  the  same  in  most  practical  respects.  The  complex  colloidal 
protein  molecule  becomes  broken  into  smaller  and  ever  smaller 
segments.^  The  largest  of  these  derived  proteins  is  called  proteose, 
and  in  its  properties  most  closely  resembles  the  original  protein. 

1  "Allen's  Commercial  Organic  Analysis,"  4th  ed.,  vol.  8  (1913),  583. 


CONSTITUTION  OF  THE  PROTEINS  21 

As  the  cleavage  is  continued,  and  the  segments  become  smaller, 
the  properties  likewise  change  regularly,  and  a  second  group  is 
recognized,  called  peptones,  which  differs  from  proteose  in  much 
the  same  manner  that  proteose  differs  from  protein.  The  final 
products  of  this  hydrolysis,  the  amino-acids,  are,  however,  very 
different  from  the  original  protein.  They  are  definite  chemical 
compounds  of  known  constitution  and  structure,  are  crystalliz- 
able  and  non-colloidal,  and  of  comparatively  simple  composition. 

Nothing  has  aided  the  chemist  in  his  search  for  an  understanding 
of  the  true  nature  and  constitution  of  the  proteins  as  have  these 
end  products.  It  is  very  remarkable  that  of  all  of  the  proteins 
obtained  from  either  plant  or  animal,  all  have  been  found  to 
resolve  upon  hydrolytic  decomposition  into  a  very  few  simple 
amino-acids,  twenty  only  having  been  isolated  from  proteins. 
It  is  believed  with  very  good  reason  that  the  amino-acids  are 
present  in  the  protein  molecule,  a  condensation  being  assumed 
between  the  carboxyl  group  of  one  and  the  amino  group  of  a 
second.  It  makes  no  difference  whether  hydrolysis  is  brought 
about  by  an  enzyme,  by  acid,  by  alkali,  or  by  steam,  the  same 
amino-acids  will  ultimately  be  produced,  and  in  the  same  propor- 
tion. It  must  be  concluded,  trie^efore,  that  the  simple  proteins 
are  nothing  more  than  condensation  products  of  a  few  amino- 
acids,  and  that  the  differences  observed  in  the  several  simple 
proteins  is  brought  about  by  differences  in  the  ratio  and  number 
of  the  several  amino-acids  represented. 

The  Amino-acids. — The  composition  and  structure  of  the 
amino-acids  which  have  been  isolated  from  the  proteins  are  as 
follows:1 

A.  Monoamino  monocarboxylic  adds 

1.  Glycine,  C2H5NO2,  or  amino  acetic  acid. 

CH2-NH2-COOH 

2.  Alanine,  CsHrNO^  or  a  amino  propionic  acid. 

CH3-CHNH2-COOH 

3.  Valine,  C5HnNO2,  or  a  amino  isovalerianic  acid. 

CH3\ 

>CH-CHNH2.COOH      . 
CH3/ 

4.  Caprine,  Cr,Hi3NO2,  or  a  amino  normal  caproic  acid. 

CH3.CH2-CH2.CRS-CHNH2-COOH 

5.  Leucine,  C6Hi3NO2,  or  a  amino  isocaproic  acid. 

1  Taken  mainly  from  PLIMMER,  "The  Chemical  Constitution  of  the 
Proteins,"  Pt.  1,  3rd  ed.,  (London),  (1917),  2-4. 


22  GELATIN  AND  GLUE 

CH3x 

>CH-CH2-CHNH2-COOH 
CH3/ 

6.  Isoleucine,  C6Hi3NO2,  or  a  amino  /3  methyl  /?  ethyl  propionic  acid. 

CH2\ 

>CH-CHNH2-COOH 

C2H6/ 

7.  Phenylalanine,  C9HnNO2  or  /3  phenyl  a  amino  propionic  acid. 

C6H5-CH2-CHNH2.COOH 

8.  Tyrosine,   C9HiiNO3,   or  /3  parahydroxyphenyl  a  amino  propionic 
acid. 

C6H4-OH-CH2-CHNH2-COOH 

•9.  Serine,  C3H7NO3,  or  /3  hydroxy  a  amino  propionic  acid. 
CH2-OH-CHNH2-COOH 

10.  Cystine,  C6Hi2N2O4S2,  or  di  (/3  thio  a  amino  propionic  acid). 

S  CH2-CHNH2-COOH 

I 

S  CH2-CHNH2-COOH 

B.  Monoamino  dicarboxylic  acids 

11.  Afepartic  acid,    C4H7NO4,    or  amino  succinic  acid. 

CH2COOH-CHNH2-COOH 

12.  Glutamic  acid,  C5H9NO4,  or  a  amino  glutaric  acid. 

CH2COOH-CH2-CHNH2-COOH 

C.  Diamino  monocarboxylic  acids 

13.  Arginine,  C6Hi4N4O2,  or  a  amino  6  guanidine  valerianic  acid. 

C6Hi4N4O2 


HN 

[2-CH2-CH2-CHNH«-COOH 

14.  Lysine,  C6Hi4N2O2,  or  a  —  e  diamino  caproic  acid. 

NH2-CH2-CH2-CH2-CH2-CHNH2-COOH 

D.  Heterocyclic  compounds 

15.  Histidine,  C6H9N3O2,  or  /3  imidazole  a  amino  propionic  acid. 

CH 

/\ 

N         NH 

I  I 

CH  =  C— CH2-CHNH2-COOH 

16.  Proline,  C6H9NO2,  or  a  pyrrolidine  carboxylic  acid. 

CH2 CH2 

I  I 

CH2       CH-COOH 

NH 


CONSTITUTION  OF  THE  PROTEINS  23 

17.  Oxyproline,  CsHgNOg,  or  7  hydroxy  a  pyrrolidine  carboxylic  acid. 
HO-CH  ---  CH2 

I 
CH2      CH-COOH 


NH 

18.  Tryptophane,  CnHi2N2O2,  or  /3  indole  a  amino  propionic  acid. 
C—  CH2-CHNH2-COOH 


C6H4      CH 

\/ 

NH 

The  Proteoses  and  Peptones.  —  Many  intermediate  products 
have  been  in  one  way  or  another  separated  between  the  proteose 
and  peptone  divisions.  But  it  must  be  pointed  out  that  although 
such  a  separation  may  be  necessary  in  certain  studies  and  pro- 
duces data  that  are  instructive,  yet  the  differences  between  the 
several  fractions  are  continuous  functions  of  a  solubility  curve, 
and  any  permanent  sub-division  on  such  a  basis  seems  unwar- 
ranted and  unnecessary.  Indeed  Abderhalden1  has  even  gone  so 
far  as  to  suggest  that  the  term  proteose  be  also  dropped,  and  that 
all  products  intermediate  between  protein  and  amino-acids  be 
termed  peptones.  His  suggestion  has  not,  however,  been  favor- 
ably received  by  the  majority  of  biochemists. 

The  proteoses  obtained  from  the  different  proteins  differ 
greatly  in  their  properties  as  is  to  be  expected  from  a  considera- 
tion of  their  varying  origin,  and  of  the  varying  complexity  of 
their  amino-acid  content.  For  this  reason  many  writers  have 
used  a  nomenclature  for  the  proteoses  corresponding  to  the  name 
of  the  protein  from  which  it  is  derived.  Thus  the  terms  albumose, 
gelatose,  caseose,  globulose,  elastose,  etc.,  have  found  some  service, 
but  the  general  term  proteose  is  in  greatest  favor.  The  terms 
proto-,  hetero-,  and  deutero-proteoses  have  been  used  to  designate 
varying  degrees  of  solubility.  .  ' 

The  peptones  are  more  or  less  definitely  defined  chemical 
compounds.  A  great  many  have  been  made  synthetically  in 
vitro,  especially  by  the  master  chemist  Emil  Fischer.2  When 
peptones  are  prepared  synthetically  they  are  known  as  poly- 
peptids.  Any  combinations  of  two  or  more  amino-acids  are 
called  peptids.  Fischer  obtained  hexa-,  hepta-,  y&a-,  dodeca-,  and 

1  OPPENHEIMER'S  "Handb.  der  Biochem.,"  Bd.  1,  (1908). 

2  EMIL  FISCHER,  Op,  cit.     See  also  PLIMMER,  lib.  cit.,  part  II. 


24  GELATIN  AND  GLUE 

even  an  octadecapeptid  by  means  of  reactions  which  enabled  him 
to  produce  condensations  between  the  carboxyl  and  amino 
groups  of  his  original  amino-acids.  This  process  may  be  repre- 
sented empirically  as  follows,  with  two  molecules  of  glycine : 

CH2-NHH-COOH      CH2-NH-COOH 

-»        |        +  H20. 

CH2-NH2  -COOH  CH2-NH2-C  =  O 

The  first  dipeptid  known  had  this  formula  but  was  obtained 
from  the  anhydride  of  glycine  by  treatment  with  hydrochloric 
acid: 

CH2-NH-CO 
|  |        +  H20  -» CH2COOH-NH-CO-CH2-NH2. 

CO    -NH-CH2  glycyl-glycine 

Hydrolyzing  Agents. — Proteoclastic,  or  protein-splitting  enzymes 
are  found  very  widely  distributed  in  both  the  animal  and  the 
plant  world.  Pepsin  is  excreted  by  the  mucous  membrane  of  the 
stomach  of  mammals,  and  trypsin  is  produced  by  the  pancreas. 
Erepsin,  of  later  discovery,  is  obtained  from  the  mucous  mem- 
brane of  the  small  intestine.  In  addition  to  these,  which  are  of 
greatest  importance  in  digestive  processes,  proteoclastic  enzymes 
have  been  isolated  from  nearly  every  organ  of  the  body.1  Simi- 
lar enzymes  may  be  found  in  many  plants  (e.g.,  papainhom  the 
papaw  tree).  It  is  believed  that  a  particular  enzyme  produces 
a  scission  only  between  certain  particular  types  of  linkage  in  the 
catenary  molecule.  For  example  if  a  racemic  peptid  is  prepared 
synthetically  and  subjected  to  the  action  of  an  enzyme  which  will 
attack  that  peptid,  it  is  observed  that  only  that  isomer  which 
exists  in  nature  is  acted  upon,  whereas  its  asymmetric  congener 
remains  unaffected.  For  these  reasons  different  proteins  will  be 
acted  upon  somewhat  differently  by  the  several  proteoclastic 
enzymes. 

Whenever  it  is  desired  to  carefully  and  definitely  control  the 
process  of  hydrolysis,  as  in  experiments  upon  digestion,  an 
enzyme  is  used  as  the  hydrolyzing  agent,  but  if  only  the  ultimate 
products,  the  amino-acids,  are  desired,  the  more  rapid  and 
drastic  method  of  acid  hydrolysis  is  resorted  to.  Hydrochloric 
acid  of  20  per  cent  concentration  is  recommended  by  Van  Slyke.2 
Sulphuric  acid  may  also  be  used,  but  is  more  difficult  to  eliminate 

1  Cf.  papers  by  Abderhalden  and  his  pupils  in  recent  volumes  of  Z.  physiol. 
Chem. 

2  D.  D.  VAN  SLYKE.  J.  BioL  Chem.,  10  (1911),  18. 


CONSTITUTION  OF  THE  PROTEINS  25 

after  the  hydrolysis  is  complete.  With  hydrochloric  acid  of 
this  strength  it  requires  from  12  to  60  hours  at  the  boiling  tem- 
perature, depending  on  the  substance  under  investigation,  to 
reduce  the  protein  completely  to  amino-acids. 

The  hydrolysis  of  gelatin  under  different  conditions  of  hydrogen 
or  hydroxyl  ion  concentration,  and  by  the  use  of  different  enzymes 
has  been  studied  by  Northrup.1  He  finds  that  if  the  hydrogen 
ion  concentration  is  kept  constant  the  hydrolysis  follows  the  laws 
of  a  monomolecular  reaction  for  a  third  of  the  reaction,  but  if 
not  kept  constant  the  hydrolysis  is  proportional  to  the  square 
root  of  the  time.  The  velocity  is  directly  proportional  to  the 
hydrogen  ion  concentration  when  pH  is  greater  than  10.0,  but 
between  these  values  is  approximately  constant  and  greater  than 
would  be  calculated  from  the  pH.  Northrup  accounts  for  this  by 
assuming  that  the  uncombined  gelatin  hydrolyzes  much  more 
rapidly  than  the  gelatin  salt.  The  particular  peptid  linkages  that 
are  most  resistant  to  acid  hydrolysis  are  the  most  rapidly  split 
by  pepsin,  trypsin  and  alkali.  All  linkages  that  are  attacked  by 
pepsin  are  also  hydrolyzed  by  trypsin  (although  with  very  differ- 
ent degrees  of  rapidity),  but  'trypsin  hydrolyzes  linkages  that 
are  not  attacked  by  pepsin. 

4.   DETERMINATION   OF   THE   NITROGENOUS    CONSTITUENTS: 
PROTEIN,  PROTEOSE,  PEPTONE  AND  AMINO-ACID 

By  definition  the  proteoses  consist  of  those  nitrogenous 
cleavage  products  of  protein  which  may  be  precipitated  by  satura- 
tion of  the  solution  with  the  sulphate  of  ammonium,  zinc,  or 
magnesium.  The  unchanged  protein  is  similarly  precipitated 
by  half  saturation  while  the  peptones  are  not  thrown  down  at  any 
concentration  of  the  sulphates.  It  is  an  easy  matter,  therefore, 
to  make  such  a  separation  in  any  mixture  of  protein  cleavage 
products.  Further  separations  by  using  concentrations  of  salt, 
varying  by  10  or  15  per  cent,  between  20  and  100  per  cent  have  been 
used  in  certain  investigations,  but  are  not  in  general  necessary. 

All  precipitations  by  salt  are  best  brought  about  in  acid 
solution.  Schryver2  has  recommended  the  addition  of  2  c.c. 
of  1  to  4  sulphuric  acid  to  each  100  c.c.  of  the  sulphate  solution 
and  of  the  protein  solution.  In  work  upon  gelatins  the  author3 

1  J.  H.  NORTHRUP,  J.  Gen.  PhysioL,  3  (1921),  715;  4  (1921),  57. 

2  "Allen's  Commercial  Organic  Analysis,"  4th.  ed.,  vol.  8  (1913),  482. 

3  R.  H.  BOGDE,  Chem.  Met.  Eng.,  23  (1920),  106. 


26 


GELATIN  AND  GLUE 


has  shown  that  the  maximum  precipitation  occurred  upon  the 
addition  of  only  0.5  c.c.  of  1  to  4  sulphuric  acid  to  each  100  c.c.  of 
the  reacting  solutions.  The  influence  of  the  addition  of  varying 
amounts  of  sulphuric  acid  is  shown  in  Fig.  4. 

Since  the  proportions  of  material  thrown  down  at  any  given 

concentration   of  salt  solution  are   continuous  functions  of  a 

solubility  curve  it  is  evident  also  that  variations  in  temperature 

60, 


35PerCenf5afurafedMeS04  Solution 


Per  Cent  Saturated  ne^QfSolution 


56 
52 
4S 
44 
40 
36 
32 
28 
24 
20 
16 
12 


I          234567 
Cubic  Centimeters  of  1-4  H^Stfy to  I00c.c.  Solution 
FIG.  4. — Effect  of  sulphuric  acid  on  protein  precipitation. 

would  be  expected  to  alter  greatly  the  proportions  of  precipitate 
obtained.  In  the  case  of  gelatin  the  author  found  from  3  to  8 
per  cent  more  protein  nitrogen  thrown  down  at  17°  than  at  25°C. 
It  is  accordingly  evident  that  where  comparisons  are  desired 
between  the  nitrogenous  substances  precipitated  from  mixtures 
of  protein  cleavage  products,  it  becomes  necessary  to  work  under 
carefully  controlled  temperature  conditions. 

The  following  procedure  for  the  examination  of  a  gelatin  or  glue 
has  been  found  to  give  excellent  results:1 

1.  The  moisture  is  first  determined.2 

2.  10  g.  (calculated  on  the  water-free  basis)  are  weighed  out,  dissolved 
in  water,  and  made  up  to  500  c.c.  in  a  volumetric  flask. 

3.  A  50  c.c.  aliquot  is  removed  and  total  nitrogen  determined  by  the 
Kjeldahl  method.3 

4.  A  50  c.c.  aliquot  is  removed  into  a  200  c.c.  erlenmeyer  flask,  0.3  c.c. 

1  For  a  further  discussion  of  this  determination  see  page  448. 

2  See  page  429. 

3  See  page  431. 


CONSTITUTION  OF  THE  PROTEINS  27 

of  1-4  sulphuric  acid  is  added,  and  solid  magnesium  sulphate  is  then  added 
in  excess  of  the  saturation  point.  The  flask  is  stoppered  and  placed  in  a 
constant  temperature  room  or  bath  (between  15°  and  25°C.),  and  shaken 
frequently.  After  24  hours  it  is  filtered  and  washed  with  saturated  mag- 
nesium sulphate  solution  containing  0.5  per  cent  1-4  sulphuric  acid.  The 
filtrate  and  washings  are  retained  for  the  amino-acid  determination.  The 
moist  filter  paper  with  its  contents  is  removed  to  an  800  c.c.  Kjeldahl  flask 
and  nitrogen  determined  in  the  usual  way.  The  value  so  obtained  repre- 
sents the  sum  of  the  protein  and  the  proteose  nitrogen. 

5.  A  50  c.c.  aliquot  is  removed  as  in  No.  4,  but  in  place  of  solid  magnesium 
sulphate  being  added,  50  c.c.  of  a  saturated  solution  of  the  same,  contain- 
ing 0.5  per  cent  1-4  sulphuric  acid,  is  added.     This  is  treated  as  No.  4, 
except  that  the  wash  solution  consists  of  half  saturated  magnesium  sul- 
phate with  the  usual  acid  content.     The  nitrogen  value  obtained  repre- 
sents the  protein  nitrogen. 

6.  The  difference  between  Nos.  4  and  5  is  the  proteose  nitrogen. 

7.  The   combined   filtrate   and  washings   from   the   precipitation   with 
saturated  magnesium  sulphate  (No.  4)  are  used  in  the  determination  of 
the  ammo-acids.     This  may  be  done  by  the  formaldehyde-titration  method 
of  S0rensen  or  by  the  nitrous  acid  method  of  Van  Slyke.1     If  the  former 
method  is  used  the  usual  directions2  have  to  be  modified  before  they  may 
be  applied  to  the  case  in  hand.3     It  is  usually  specified  that  both  the  for- 
maldehyde and  the  solution  containing  the  amino-acids  should  be  made 
faintly  pink  to  phenolphthalein  before  mixing.     If  these  instructions  are 
followed  here,  however,  the  solution  resulting  from  the  mixing  becomes 
intensely  red,  making  a  determination  impossible.     The  same  thing  happens 
with  the  control  of  saturated  magnesium  sulphate.     The  process  should 
therefore  be  modified  as  follows:  the  amino-acid  solutions  and  the  control 
are  made  faintly  pink  to  phenolphthalein.     The  formaldehyde  is  treated 
with  sodium  hydroxide  until,  on  adding  25  c.c.  of  it  to  100  c.c.  of  the  con- 
trol, the  intensity  of  the  original  pink  remains  unchanged.     A  drop  more 
of  the  base  would  produce  an  intensifying  of  the  color;  a  drop  less  a  decrease 
or  removal  of  the  color.     The  amino-acid  solutions  are  then  treated  with 
the  formaldehyde  in  the  usual  way,  and  titrated  back  to  a  uniform,  rather 
deep,  red  with  barium  hydroxide  in  N/5  concentration. 

8.  The    peptone  is  calculated  by  subtracting  the  sum  of  the  protein, 
proteose,  and  amino-acid  nitrogen  from  100.     A  study  of  several  different 
methods  by  the  author  showj  an  error  for  peptone  by  the  above  procedure 
of  only  0.28  to  0.77  per  cent. 

A  large  number  of  glues  and  gelatins  of  all  grades  have  been 
examined  in  the  author's4  laboratory  by  the  procedure  above 
described,  and  some  extraordinarily  illuminating  results  have 
been  obtained.  The  grade  (here  referred  to  jelly  consistency) 

1  These  methods  are  described  below. 

2  Cf.  SCHRYVER,  loc.  ciL,  p.  488. 

3  R.  H.  BOGUE,  loc.  cit.,  p.  106. 

4  R.  H.  BOGUE,  loc.  cit.,  105. 


28 


GELATIN  AND  GLUE 


was  found  to  be  in  direct  proportion  to  the  percentage  of  the 
total  nitrogen  that  was  in  the  form  of  unhydrolyzed  protein,  or  to 
the  ratio  of  the  protein  nitrogen  to  the  nitrogen  representative 
of  the  products  of  protein  hydrolysis.  The  proteose  and  peptone 
nitrogen  varied  inversely  with  the  grade,  while  the  actual  amino- 
acid  nitrogen  was  nearly  constant,  and  very  small. 

Some  of  the  data  obtained  are  given  in  Table  10  and  shown 
graphically  in  Figs.  5  and  6. 

TABLE  10. — NITROGENOUS  CONSTITUTION  OF  GLUES 


Pro- 

Pro- 

Pep- 

Amino- 

Grade1 

tein 

teose 

tone 

Acid 

N 

N 

N 

N 

Hi2 

92.2 

6.3 

1.1 

0.4 

H9 

90.4 

7.0 

2.0 

0.6 

H7 

86.2 

12.0 

1.4 

0.4 

Hide  glues  and  fleshings.  .  . 

H6 

84.6 

12.4 

2.6 

0.4 

H4 

78.7 

16.0 

4.5 

0.8 

H3 

77.6 

17.0 

4.7 

0.7 

H2 

52.0 

38.6 

8.4 

0.9 

B7 

79.1 

14.9 

4.8 

1.2 

B6 

73.5 

16.4 

8.1 

2.0 

B5 

64.6 

28.3 

5.6 

1.5 

B4 

59.8 

32.4 

6.4 

1.4 

Bone  glues  

B3 

53.6 

36.6 

8.4 

1.4 

B3 

52.5 

37.9 

7.8 

1.8 

B2 

48.2 

40.1 

10.1 

1.6 

B, 

36.8 

47.1 

12.5 

2.3 

B! 

31.5 

50.6 

14.8 

3.0 

Russian  isinglass  

HIO 

91.0 

4.4 

4.5 

0.1 

Edible  gelatin  

G8 

87.8 

11.3 

0.7 

0.2 

Fish  glue. 

B, 

33.4 

42.3 

21.9 

2.4 

Pressure  tankage  

•*-*! 

B! 

34.3 

46.4 

16.3 

3.0 

Peptone  

Bi 

0.0 

33.2 

48.5 

18.3 

1  See  page  503. 

5.  THE  ESTIMATION  OF  AMINO-ACID 

Two  other  methods  which  have  found  favor  for  following  the 
course  of  an  hydrolysis,  or  for  determining  the  total  amino-acids 
in  a  solution,  are  based  upon  the  fact  that  as  the  large  protein 
molecule  is  divided  and  broken  up  into  smaller  segments  the 


CONSTITUTION  OF  THE  PROTEINS  29 

number  of  carboxyl  and  of  amino  groups  increases.  This  is 
readily  seen  from  the  following  illustration  of  the  hydrolysis  of 
glycyl  glycine: 

NH2-CH2-CO-NHCH2.COOH  +  H2O  -*  2NH2.CH2-COOH. 
The  original  peptid  had  one  carboxyl  and  one  amino  group.     The 
100 


90 


80 


«o 


50 


40 


2)0 


20 


10  - 


•'2  3  "A.  ""6     7  "9     12 

Grade  in  order  of  increasing  jelly  strength 
FIG.  5. — Relation  between  nitrogen- 
ous constituents    and  jelly  strength, 
hide  glues. 


Per  Cent  of  Total  Nitrogen 

osSsS-s^c 

I 

/ 

<5 

# 

/ 

/ 

N 

\ 

/ 

/ 

\ 

/ 

/ 

\ 

k 

- 

1 

^- 

% 

^% 

^ 

^ 

In'H 

^ 
M,'+ 

- 

X 

4mino_/ 

^L^ 

—    — 

B,  B,     e    3    3    4    5    6    7 

Grade  in  order  of  increasing  jelly  strength 
FIG.  6. — Relation  between  nitrogen- 
ous  constituents  and  jelly  strength, 
bone  glues. 


two  molecules  of  glycine  resulting  from  the  cleavage  possess 
together  two  carboxyl  groups  and  two  amino  groups.  If  the 
original  peptid  had,  for  example,  ten  amino-acids  in  combination, 
it  would  still  have,  as  a  peptid,  only  one  carboxyl  and  amino 
group,  but  on  complete  hydrolysis  it  would  possess  ten  each  of 
these  groups.  If  it  were  half  hydrolyzed,  e.g.,  if  five  of  the 
amino-acids  were  broken  off  and  five  remained  combined  there 
would  be  six  each  of  the  carboxyl  and  amino  groups.  In  this  way 
the  course  of  an  hydrolysis  or  digestion  may  be  followed  by 
measuring  the  groups  mentioned. 


30  GELATIN  AND  GLUE 

The  Formaldehyde  Titration. — S0renson1  has  developed  a  very 
workable  method  whereby  the  carboxyl  groups  may  be  deter- 
mined. Owing  to  the  amphoteric  character  of  protein  and  all 
of  its  clearage  products  including  the  amino-acids  the  acidity  of 
these  substances,  which  is  developed  through  their  carboxyl 
groups,  may  not  be  directly  determined.  S0rensen  pointed  out 
that  the  amino  group,  which  is  responsible  for  the  basic  properties 
of  these  substances,  may  be  converted  by  formaldehyde  into  a 
neutral  methylene  imino  group.  After  this  has  taken  place 
the  substance  is  no  longer  amphoteric,  and  may  be  titrated  with 
an  alkali  in  the  usual  manner.  Again  taking  glycine  as  the 
simplest  amino-acid,  the  reaction  is  expressed : 

/NH2  ,N  =  CH2 

CH/  +  HCHO  -*  CH/  +  H20. 

XCOOH  XCOOH 

In  practice  the  original  solution  and  also  the  formaldehyde  must 
be  made  as  nearly  neutral  as  is  possible,  the  formaldehyde  then 
added  in  excess,  and  the  increase  in  acidity  measured  by  N/5 
barium  hydroxide.  If  carbonates  and  phosphates  are  absent 
N/10  sodium  hydroxide  may  be  used.  Each  cubic  centimeter  of 
alkali  added  is  then  equivalent  to  1.4  mg.  of  nitrogen  existing  as 
amino  nitrogen. 

The  Nitrous  Acid  Method. — The  quantitative  estimation  of 
amino-acid  nitrogen  by  the  decomposition  of  the  latter  with 
nitrous  acid  has  been  used  by  biological  chemists  for  a  great 
many  years,  but  it  is  only  since  the  technique  has  been  highly 
perfected  by  D.  D.  Van  Slyke2  that  very  general  usage  has  been 
made  of  the  method.  At  present  it  is  probably  the  best  and 
most  commonly  used  procedure.  A  complete  determination 
may  be  made  in  about  five  minutes.  The  reaction  involved  is 
very  simple: 

R-NH2  +  HNO2  -»  R-OH  +  H2O  +  N2, 
or  using  glycine  as  the  amino-acid  in  question : 

CH2-NH2-COOH  +  HN02  -»  CH2OHCOOH  +  H20  +  N2. 
The  amino-acid  is  converted  into  an  hydroxy  acid,  which  in  this 
case  is  hydroxy  acetic  or  gly  colic  acid.     The  requirement  in  the 
quantitative  determination  by  this  reaction  is  that  the  evolved 

1  S0RENSEN,  Biochem.  Z.,  7  (1908),  45. 

2D.  D.  VAN  SLYKE,  J.  Biol  Chem.,  9  (1911),  185;  12    (1912),  275;  16 
(1913-14),  121;  23  (1915),  407. 


CONSTITUTION  OF  THE  PROTEINS 


31 


FIG.  7. — The  Van  Slyke  amino-acid   (micro)   apparatus.      (Kindness  of  D.D. 

Van  Slyke.) 


32  GELATIN  AND  GLUE 

nitrogen  gas  be  freed  completely  of  any  other  gaseous  impurities 
and  measured  accurately.  Any  traces  of  air,  or  of  any  other  gas 
which  may  not  be  separated  completely  from  the  evolved  nitrogen, 
which  may  be  present  in  the  apparatus  will,  of  course,  produce 
error.  Nitric  oxide  is  a  gas  which  is  absorbed  readily  by  alkaline 
solutions  of  potassium  permanganate.  Use  is  made  of  this  fact 
by  replacing  completely  all  air  in  the  apparatus  by  nitric  oxide, 
produced  by  the  interaction  of  sodium  nitrite  and  glacial  acetic 
acid.  Nitrous  acid  is  produced  which  rapidly  decomposes  into 
nitric  oxide  and  water: 

NaNO2  +  CH3COOH  -» CH3COONa  +  HNO2; 
3HN02  -»  HN03  +  H20  +  2NO. 

The  nitric  oxide  reduces  the  permanganate  leaving  manganese 
dioxide  and  potassium  nitrate.  The  reaction  representing  the 
ultimate  products  may  be  written: 

KMnO4  +  NO  ->  KNO3  +  Mn02. 

The  nitric  oxide  is  thus  entirely  absorbed,  and  only  the  nitrogen 
gas  of  the  reaction  remains  to  be  measured. 

The  accompanying  photograph  and  diagram,  Figs.  7  and  8, 
taken  from  Van  Slyke1  show  the  appearance  and  modus  operandi 
of  the  apparatus.  There  are  three  principal  steps  in  the  opera- 
tion, (1)  the  displacement  of  the  air  by  nitric  oxide,  (2)  the  decom- 
position of  the  amino  substance,  and  (3)  the  absorption  of  the 
nitric  oxide  and  measurement  of  the  nitrogen. 

1.  Displacement  of  Air  by  Nitric  Oxide.2 — Water  from  F  fills  the  capillary 
leading  to  the  Hempel  pipette  and  also  the  other  capillary  as  far  as  c.  Into 
A  one  pours  a  volume  of  glacial  acetic  acid  sufficient  to  fill  one-fifth  of  D. 
For  convenience,  A  is  etched  with  a  mark  to  measure  this  amount.  The 
acid  is  run  into  D,  cock  e  being  turned  so  as  to  let  the  air  escape  from  D. 
Through  A  one  now  pours  sodium  nitrite  solution  (30  g.  NaNO2  to  100 
c.c.  H2O)  until  D  is  full  of  solution  and  enough  excess  is  present  to  rise  a 
little  above  the  cock  into  A.  It  is  convenient  to  mark  A  for  measuring  off 
this  amount  also.  The  gas  exit  from  D  is  now  closed  at  c,  and,  a  being  open, 
D  is  shaken  for  a  few  seconds.  The  nitric  oxide,  which  instantly  collects, 
is  let  out  at  c,  and  the  shaking  repeated.  The  second  crop  of  nitric  oxide, 
which  washes  out  the  last  portions  of  air,  is  let  out  at  c  also.  D  is  now 
connected  with  the  motor  and  shaken  till  all  but  20  c.c.  of  the  solution  have 
been  displaced  by  nitric  oxide  and  driven  back  into  A.  A  mark  on  D 
indicates  the  20  c.c.  point.  One  then  closes  a  and  turns  c  and  /  so  that 
D  and  F  are  connected.  The  above  manipulations  require  between  1 
and  2  minutes. 

i  D.  D.  VAN  SLYKE,  loc.  tit.,  12  (1912),  277-8. 

*  The  following  description  is  taken  directly  from  VAN  SLYKE,  loc.  tit. 


CONSTITUTION  OF  THE  PROTEINS 


33 


2.  Decomposition  of  the  Amino  Substance. — Of  the  amino  solution  to  be 
analyzed  10  c.c.  or  less,  as  the  case  may  be,  are  measured  off  in  B.  Any 
excess  added  above  the  mark  can  be  run  off  through  the  overflow  tube. 
The  desired  amount  is  then  run  into  D,  which  is  already  connected  with 


FIG.  8. — The  deaminizing  bulb  and  connections  of  the  Van  Slyke  amino-acid 
apparatus.      (Kindness  of  D.  D.  Van  Slyke.) 

the  motor,  as  shown  in  the  photograph.  It  is  shaken,  when  a-amino  acids 
are  being  analyzed,  for  a  period  of  3  to  5  min.  With  a-amino  acids,  pro- 
teins, or  partially  or  completely  hydrolyzed  proteins,  we  find  that  at  the 
most  five  minutes'  vigorous  shaking  completes  the  reaction.  Only  in  the 
cases  of  some  native  proteins  which,  when  deaminized,  form  unwieldly 
coagula  and  mechanically  interfere  with  the  thorough  agitation  of  the 
mixture,  a  longer  time  may  be  required.  In  case  a  viscous  solution  is  being 
analyzed  and  the  liquid  threatens  to  foam  over  into  F,  B  is  rinsed  out  and 
a  little  caprylic  alcohol  is  added  through  it.  For  amino  substances,  such 
3 


34  GELATIN  AND  GLUE 

as  amino-purines,  requiring  a  longer  time  than  five  minutes  to  react,  one 
merely  mixes  the  reacting  solutions  and  lets  them  stand  the  required  length 
of  time,  then  shakes  about  two  minutes  to  drive  the  nitrogen  completely 
out  of  solution. 

3.  Absorption  of  Nitric  Oxide  and  Measurement  of  Nitrogen. — The 
reaction  being  completed,  all  the  gas  in  D  is  displaced  into  F  by  liquid  from 
A  and  the  mixture  of  nitrogen  and  nitric  oxide  is  driven  from  F  into  the 
absorption  pipette.  The  driving  rod  is  then  connected  with  the  pipette 
by  lifting  the  hook  from  the  shoulder  of  d  and  placing  the  other  hook,  on 
the  opposite  side  of  the  driving  rod,  over  the  horizontal  lower  tube  of  the 
pipette.  The  latter  is  then  shaken  by  the  motor  for  a  minute,  which, 
with  any  but  almost  completely  exhausted  permanganate  solutions,  com- 
pletes the  absorption  of  nitric  oxide.  The  pure  nitrogen  is  then  measured 
in  F.  During  the  above  operations  A  is  left  open,  to  permit  displacement  of 
liquid  from  D  as  nitric  oxide  forms  in  D. 

The  permanganate  solution  is  made  up  by  dissolving  50  g. 
of  potassium  permanganate  and  25  g.  of  potassium  hydroxide  in 
water,  and  making  up  to  one  liter. 

Inasmuch  as  the  final  measurement  by  the  above  procedure  is 
the  volume  of  a  gas,  it  is  self-evident  that  the  temperature  and 
pressure  must  be  carefully  noted,  and  considered  in  calculating 
the  results.  A  conversion  table  is  prepared  by  Van  Slyke1  show- 
ing the  milligrams  of  amino  nitrogen  corresponding  to  1  c.c.  of 
nitrogen  gas  at  11°  to  30°C.  and  728  to  772  mm.  pressure.  It 
must  also  be  remembered  that  only  the  a-amino  nitrogen  is  acted 
on  by  nitrous  acid.  Thus  in  arginine  Y±  of  the  nitrogen  is 
unacted  upon  by  nitrous  acid ;  in  histidine  % ;  in  triptophane  %  ; 
and  in  proline  and  oxyproline  none  of  the  nitrogen  is  set  free  by 
that  reaction.  This  fact  however  makes  possible  a  division  of  the 
total  nitrogen  into  amino  and  non-amino  nitrogen  which  is  made 
use  of  by  Van  Slyke2  in  differentiating  the  several  nitrogenous 
groups. 

6.  THE  SEPARATION  OF  THE  AMINO-ACIDS 

The  operation  of  separating  the  several  amino-acids  from  a 
protein  and  from  each  other  is  a  long  and  tedious  process,  and  by 
no  means  satisfactory  from  a  quantitative  point  of  view.  The 
most  painstaking  operations  have,  prior  to  the  work  of  Dakin, 
left  ^  to  %  of  the  molecule  unaccounted  for  when  analyzed  in 
this  way.  For  these  reasons  the  attempt  is  not  often  made  in 
routine  procedures,  and  is  resorted  to  only  for  some  particular 

1  See  Appendix,  page  621. 

2  D.  D.  VAN  SLYKE,  cit.  sup. 


CONSTITUTION  OF  THE  PROTEINS  35 

and  exacting  information  which  may  not  be  obtained  by  other 
methods. 

The  principle  underlying  the  process  developed  by  Emil  Fischer1  con- 
sists in  esterifying  the  products  of  a  protein  hydrolysis,  and  separating  the 
esters  by  fractional  distillation  at  very  low  pressures.  Hydrolysis  may  be 
carried  out  in  a  25  per  cent  solution  of  either  sulphuric,  hydrochloric 
or  hydrofluoric  acid.  On  eliminating  the  acid  after  30  to  150  hrs.,  and 
concentrating  in  vacuo,  cystine  and  tyrosine  separate  out  if  present  in  large 
amounts.  On  saturating  with  hydrogen  chloride  gas  at  0°C.  glutamic  acid 
separates.  Esterification  is  then  brought  about  by  dissolving  the  con- 
centrate in  absolute  alcohol  and  saturating  with  hydrogen  chloride  gas, 
and  evaporating.  This  process  is  repeated  several  times.  Osborne  and  his 
co-workers2  modified  the  process  by  using  alcoholic  hydrogen  chloride 
and  zinc  chloride,  and  distilling  alcoholic  hydrogen  chloride  through  the 
mixture  at  110°.  Glycine  separates  from  the  product  at  0°.  The  esters 
are  set  free  from  their  hydrochlorldes  by  treatment  with  sodium  hydroxide 
and  ether  at  0°,  until  the  free  acid  is  neutralized,  and  then  with  solid  potas- 
sium carbonate  until  a  pasty  mass  is  formed.  This  process  is  repeated, 
and  after  drying  with  fused  sodium  sulphate  and  distilling  off  the  ether, 
it  is  fractionally  distilled.  Pressures  of  from  10  mm.  to  0.5  mm.,  and 
temperatures  from  40  to  160°C.  are  commonly  used.  The  fractions  are 
then  hydrolyzed  with  water  or  barium  hydroxide  and  the  individual  amino- 
acids  separated  as  far  as  possible  by  their  characteristic  reactions  or 
solubilities. 

Dakin3  has  shown  in  a  series  of  most  exacting  investigations 
that  butyl  alcohol  may  be  used  to  great  advantage  in  the  separa- 
tions of  certain  amino-acids  or  amino-acid  groups,  and  he  has 
been  able  to  isolate  and  identify  the  amino-acids  of  91.31  per 
cent  of  the  total  nitrogen  content  of  gelatin.  The  esterification 
procedure  of  Fischer  was  employed  with  some  modifications 
following  an  extended  extraction  with  the  butyl,  and  sometimes 
propyl,  alcohol. 

The  butyl  alcohol  extraction  under  ordinary  pressure  was 
found  to  remove  readily  the  alanine,  leucine,  and  phenylalanine, 
while  the  hydroxyproline  and  serine  were  extracted  more  slowly, 
and  glycine  only  partially.  By  employing  reduced  pressures 
proline  may  be  extracted  without  danger  of  secondary  changes. 
The  last  traces  of  hydroxyproline  are  best  removed  with  propyl 
alcohol.  Aspartic  and  glutamic  acids  are  not  extracted  by  these 
alcohols. 

1  EMIL  FISCHER,  Op.  tit. 

2  See    OSBORNE    and  TELLOTSON,   Am.   J.   Sci.,   24   (1907),    194.     Also 
OSBORNE  and  JONES,  ibid.,  26  (1910),  212. 

3  H.  D.  DAKIN,  Biochem.  J.,  12  (1918),  290;  13  (1919),  398;  J.  Biol  Chem., 
44  (1920),  499. 


36 


GELATIN  AND  GLUE 


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CONSTITUTION  OF  THE  PROTEINS  37 

The  absence  of  hydroxyglutamic  acid,  valine  and  isoleucine 
in  gelatin  was  definitely  established,  but  traces  of  tyrosine  and 
small  amounts  of  serine  were  always  found,  together  with  a 
significant  amount  of  unidentified  sulphur  compounds.  The 
figures  for  glycine,  alanine,  and  hydroxyproline  are  very  much 
higher  than  any  previously  recorded.  The  glutamic  acid  value 
given  by  Skraup  and  von  Biehler  is  probably  much  too  high,  due 
presumably  to  contamination  of  the  glutamic  acid  hydrochloride 
with  glycine  hydrochloride,  and  causing  also  the  glycine  value 
to  be  too  low. 

The  results  of  four  investigations  upon  the  amino-acid  analysis 
of  gelatin  are  shown  in  Table  11,  together  with  similar  analyses 
upon  other  proteins  both  of  plant  and  animal  origin. 

7.  DETERMINATION  OF  THE  "HAUSMANN"  NUMBERS 

As  was  previously  stated,  the  separation  and  determination  of 
the  individual  amino-acids  is  attended  with  many  difficulties, 
and  is  not  quantitative.  But  there  are  groups  of  amino-acids, 
the  individual  members  of  which  are  very  similar  among  them- 
selves, that  present  marked  differences  in  properties  and  reac- 
tions from  other  groups.  Thus  the  diamino-acids  are  more  basic 
than  the  monoamino-acids  and  so  may  be  readily  separated  from 
the  latter  by  precipitation  with  alkaloidal  reagents.  Humin 
(or  melanin)  and  ammonia  are  produced  in  the  hydrolysis  of  the 
protein  and  may  easily  be  measured.  The  distribution  of  the 
total  nitrogen  among  these  four  groups,  the  nitrogen  being 
expressed  as  percentages  of  the  total,  are  known  as  the  "Haus- 
mann"  numbers,  from  its  originator.1 

The  procedure  requires  the  hydrolysis  of  about  1  g.  of  protein  with  20 
per  cent  hydrochloric  acid  for  several  hours.  The  completion  of  the 
hydrolysis  is  noted  by  a  failure  of  the  solution  to  give  the  biuret  reaction. 
It  is  evaporated  at  reduced  pressure  at  40°  to  a  few  cubic  centimeters, 
transferred  with  350  c.c.  of  water  to  a  distilling  flask,  a  slight  excess  of  a 
suspension  of  magnesium  hydroxide  added  and  about  100  c.c.  distilled  over 
in  vacua  at  40°C.  into  standard  N/10  sulphuric  acid.  On  titrating  with 
N/10  alkali  the  ammonia  produced  in  the  hydrolysis  is  determined.  The 
residue  is  then  filtered,  washed  with  water,  and  the  precipitate,  together 
with  the  paper,  treated  for  nitrogen  by  the  Kjeldahl  method.2  The  nitrogen 
so  obtained  is  called  humin  or  melanin  nitrogen.  The  filtrate  obtained  from 

1  Cf.  OSBORNE  and  HARRIS,  J.  Am.  Chem.  Soc.,  25  (1903),  323. 

2  See  page  431. 


38 


GELATIN  AND  GLUE 


filtering  off  the  humin  is  evaporated  to  100  c.c.,  acidified  with  5  g.  of  sul- 
phuric acid,  and  treated  with  30  c.c.  of  phosphotungstic  acid.  (Made  by 
dissolving  20  g.  of  phosphotungstic  acid  and  5  g.  of  sulphuric  acid  in  100 
c.c.  of  water.)  After  standing  24  hrs.  the  precipitated  bases  are  filtered  and 
washed  with  a  dilute  solution  of  phosphotungstic  acid  (made  by  dissolving 
2.5  g.  of  phosphotungstic  acid  and  5  g.  of  sulphuric  acid  in  100  c.c.  of  water). 
The  precipitate  and  paper  are  transferred  to  a  Kjeldahl  flask  and  nitrogen 
determined  in  the  usual  way.  The  result  is  expressed  as  basic  nitrogen. 
The  difference  between  the  sum  of  the  percentages  of  the  nitrogen  of  these 
three  groups  and  100  gives  the  percentage  of  mono-amino  nitrogen. 

A  few  "Hausmann"  numbers  are  presented  in  the  following 
table,  reported  by  Schryver.1 

TABLE  12. — HAUSMANN  NUMBERS  OF  TYPICAL  PROTEINS 


N  (per 
cent) 

Amine 

N 

Mono- 
amino 

N 

Basic 

N 

Humin 

N 

Egg  albumin  

15.51 

8.64 

68.13 

21.27 

1.87 

Caseinogen  
Salmine  . 

15.62 

10.36 

66.00 

22.34 

87.8 

1.34 

Edestine  

18.64 

10.08 

57.83 

31.70 

0.64 

Glutenin  
Gliadin 

17.49 
17  66 

18.86 
23.78 

68.31 
70.27 

11.72 
5.54 

1.08 
0.79 

Gelatin  

1.61 

62.56 

35 

83 

8.  DISTRIBUTION  OF  NITROGEN  BY  THE  METHOD  OF  VAN  SLYKE 

The  principle  of  Hausmann  has  been  extended  by  D.  D.  Van 
Slyke  so  that  instead  of  four  groups  being  determined,  he  obtains 
a  nitrogen  distribution  among  eight  groups,  and  results  obtained 
by  his  method  are  quantitative,  readily  duplicable,  and  of  not 
especially  difficult  technique.  The  ammonia  and  melanin  frac- 
tions are  identical  with  those  of  Hausmann.  The  bases  he 
separates  into  the  four  amino-acids  which  compose  it.  The 
nitrate  from  the  bases  he  separates  into  (a)  amino-acids  contain- 
ing only  primary  amino  nitrogen,  and  (b)  those  which  contain 
non-amino  nitrogen  in  pyrolidine  or  indole  ring  combination. 
The  cystine  of  the  bases  is  estimated  by  a  direct  determination  of 
the  sulphur  content,  cystine  being  the  only  naturally  occurring 
amino-acid  containing  sulphur.  A  rginine  is  found  by  decomposing 
the  molecule  by  a  drastic  treatment  with  potassium  hydroxide, 

1  "Allen's  Commercial  Organic  Analysis,"  4th  ed.,  vol.  8  (1913),  82. 


CONSTITUTION  OF  THE  PROTEINS  39 

ornithine  and  urea  being  produced.  The  urea  in  turn  breaks  up 
into  ammonia  and  carbon  dioxide.  The  complete  reaction  may  be 
written  :  — 

7NH2 
HN  =  C<  +  2H20  -* 

XNH-CH2-CH2-CH2-CHNH2-COOH 

NH2CH2-CH2-CH2-CHNH2-COOH  +  2NH3  +CO2. 

None  of  the  other  bases  are  attacked  by  this  treatment.  It  was 
previously  mentioned  that  not  all  of  the  nitrogen  of  the  bases  is 
amino  nitrogen.  The  amino  nitrogen  is  easily  determined  by  the 
nitrous  acid  method  of  Van  Slyke,  and,  by  subtracting  from 
the  total  nitrogen  of  the  bases  obtained  by  a  Kjeldahl  determina- 
tion, the  non-amino  nitrogen  is  obtained.  By  an  inspection  of 
the  formulas  of  the  bases  it  will  be  seen  that  this  non-amino 
nitrogen  is  derived  from  three-fourths  of  the  arginine  nitrogen, 
and  from  two-thirds  of  the  histidine  nitrogen.  As  the  arginine 
and  total  non-amino  nitrogen  are  known,  the  histidine  nitrogen 
may  easily  be  calculated.  Letting  A  =  arginine  nitrogen,  H  = 
histidine  nitrogen,  and  N  =  total  non-amino  nitrogen,  then 

+  %H  =  N,  or 


N  _  a/r  A 
H  =  --     ^  "  =  ?^(N  ~  %A)  =  L5N  ~  1-125A. 


The  remaining  base,  lysinc.  is  calculated  by  subtracting  the  sum 
of  the  nitrogen  of  the  other  three  bases  from  the  total  nitrogen 
of  the  bases. 

The  eight  groups  obtained  by  Van  Slyke  's  distributions  are 
therefore  as  follows  : 

1.  Ammonia,  or  amide  nitrogen,  considered  to  be  derived  from 
—  CONH2    or    —  CONHOC—  groups    linked    to    the    carboxyl 
groups  of  the  dicarboxylic  acids  in  the  protein  molecule  (glutamic 
and  aspartic  acids). 

2.  Melanin,  or  humin  nitrogen,  from  the  dark  colored  pigment 
and  slight  amount  of  insoluble  matter  always  formed  in  the 
hydrolytic  products  of  acid  hydrolysis  of  proteins.     It  has  been 
shown  by  Gortner  and  Blish1  that  "in  all  probability  the  humin 
nitrogen  of  protein  hydrolysis  has  its  origin  in  the  tryptophane 
nucleus."     They  have  found  that  when  tryptophane  was  boiled 
with  mineral  acids  in  pure  solution  no  humin  was  formed,  but 
when  tryptophane  was  added  to  a  protein,  or  when  carbohydrates 

1  R.  A.  GORTNER  and  M.  BLISH,  J.  Am.  Chem.  Soc.,  37  (1915),  1630. 


40  GELATIN  AND  GLUE 

were  present,  an  abundance  of  humin  was  formed.  They 
recovered  up  to  90  per  cent  of  the  tryptophane  nitrogen  in  the 
humin  fraction. 

3.  Cystine  nitrogen. 

4.  Arginine  nitrogen. 

5.  Histidine  nitrogen. 

6.  Lysine  nitrogen. 

7.  Amino  nitrogen  of  the  filtrate,  which  corresponds  to  all  of  the 
monoamino-acids  except  proline  and  oxyproline. 

8.  Non-amino    nitrogen    of  the  filtrate,  which  corresponds  to 
proline  and  oxyproline,  and  some  of  the  tryptophane.     (Of  the 
tryptophane  which  is  not  retained  in  the  humin  fraction  %  will 
appear  as  amino  and  ^  as  non-amino  nitrogen  of  the  nitrate. 
In  exceptional  cases  a  portion  of  the  tryptophane  may  also  be 
precipitated  by  the  phosphotungstic  acid,  and  be  calculated  as 
histidine  and  lysine.1) 

Inasmuch  as  Van  Slyke's  method  of  studying  proteins  and  their 
products  of  hydrolysis  is  so  generally  used  and  highly  regarded 
where  more  exact  studies  of  the  individual  amino-acids  are  not 
necessary,  or  would  consume  too  much  time  in  their  estimation, 
a  somewhat  detailed  description  of  the  procedure  is  given  below, 
which  has  been  condensed  from  Van  Slyke's  original  papers.2 

From  1  to  3  g.  of  the  proteins  are  boiled  with  10  or  20  parts  of  20  per 
cent  hydrochloric  acid  under  a  reflux  condenser  until  the  hydrolysis  is 
complete.  This  may  take  from  10  to  60  hrs.  depending  upon  the  protein 
used.  The  completion  of  the  reaction  is  ascertained  as  follows:  Portions 
of  1  or  2  c.c.  of  the  mixture  are  withdrawn  by  a  pipette  at  intervals  of  6  to 
8  hrs.  and  their  amino-acid  content  determined  by  Van  Slyke's  method 
previously  described.  As  soon  as  the  amino-acid  nitrogen  becomes  constant 
the  hydrolysis  is  complete.  The  solution  is  then  concentrated  in  vacua 
until  all  of  the  hydrochloric  acid  possible  has  been  driven  off;  the  residue 
dissolved  in  a  little  water,  and  made  up  to  250  c.c.  in  a  volumetric  flask. 
Five  cubic  centimeter  portions  of  this  solution  are  used  for  the  determination 
of  total  nitrogen  by  the  Kjeldahl  process.3  Seventy-five  cubic  centimeter 
portions  are  removed  into  a  Claissen  flask,  diluted  to  200  c.c.,  100  c.c. 
alcohol  added,  and  about  50  c.c.  (enough  to  produce  a  slight  excess)  of  a 
10  per  cent  suspension  of  calcium  hydroxide  introduced.  About  100  c.c. 
are  then  distilled  over  at  50°C.  and  30  mm.  pressure  into  standard  N/10 
sulphuric  acid,  and  the  latter  titrated  back  with  N/10  alkali.  This  gives 
the  ammonia  or  amide  nitrogen. 

1  D.  D.  VAN  SLYKE,  J.  Biol  Chem.,  10  (1911),  40. 

2  Ibid.,  10  (1911),  15-55;  22  (1915),  281-285. 

3  See  page  431. 


CONSTITUTION  OF  THE  PROTEINS  41 

The  residue  is  then  filtered  through  a  folded  filter  and  washed  with  water 
till  free  of  chlorides.  The  precipitate  and  paper  are  then  subjected  to  a 
Kjeldahl  analysis,  using  35  c.c.  of  sulphuric  acid  to  digest  the  large  amount 
of  organic  matter  of  the  filter.  The  nitrogen  obtained  represents  the 
humin  (or  melanin)  nitrogen. 

After  neutralizing  the  filtrate  from  the  humin  filtration  with  hydrochloric 
acid,  it  is  concentrated  in  vacuo  to  100  c.c.  and  removed  to  a  200  c.c.  Erlen- 
meyer  flask.  Eighteen  cubic  centimeters  of  concentrated  hydrochloric 
acid  and  30  c.c.  of  a  50  per  cent  solution  of  phosphotungstic  acid  are  added, 
the  solution  made  up  to  200  c.c.  with  water,  and  warmed  on  a  water  bath 
until  the  precipitate  has  practically  all  dissolved.  It  is  then  set  aside  for 
48  hrs.  The  precipitate  is  best  filtered  through  a  hardened  filter  paper  in  a 
Buchner  funnel,  using  suction.  It  is  washed  with  a  cold  (0°C.)  2.5  per 
cent  solution  of  phosphotungstic  acid  containing  3.5  per  cent  of  hydro- 
chloric acid,  and  stirred  constantly  with  a  glass  rod  to  break  up  lumps. 
The  precipitate  of  "bases"  is  transferred  with  200  to  300  c.c.  of  water  to  a 
500.  c.c.  separatory  funnel,  5  or  10  c.c.  of  concentrated  hydrochloric  acid 
are  added,  and  this  followed  by  100  c.c.  of  a  mixture  of  equal  parts  of  amyl 
alcohol  and  ether.  After  a  few  minutes  shaking  the  precipitate  is  all  dis- 
solved and  the  amino-acids  are  found  in  the  upper  aqueous  layer,  while  the 
phosphotungstic  acid  remains  in  the  heavier  layer  below.  Extraction  of 
the  aqueous  layer  is  repeated  to  remove  all  of  the  phosphotungstic  acid 
and  finally  evaporated  in  vacuo  to  dryness  to  remove  all  free  hydrochloric 
acid.  It  is  then  dissolved  in  water  and  made  up  to  50  c.c.  in  a  volumetric 
flask. 

A  25  c.c.  aliquot  is  removed  for  the  arginine  determination  into  a  200  c.c. 
Kjeldahl  flask,  and  12.5  g.  of  solid  potassium  hydroxide  added.  An  upright 
condenser  is  connected  to  the  flask,  and  the  upper  end  of  this  sealed  with  a 
Folin  bulb  containing  15  c.c.  of  N/10  acid,  colored  with  alizarin  sulphonate. 
The  solution  is  boiled  gently  for  6  hrs.,  then  100  c.c.  of  water  are  added  and 
a  like  amount  distilled  over  into  the  acid  previously  contained  in  the  Folin 
bulb.  On  titrating  the  acid  with  standard  N/10  alkali  the  arginine  nitrogen 
is  determined.  If  much  cystine  is  present  a  correction  must  be  applied, 
as  18  per  cent  of  the  cystine  nitrogen  is  evolved  as  ammonia  in  the  above 
process. 

The  residue  from  the  arginine  determination  is  treated  by  the  Kjeldahl 
method  for  nitrogen.  The  value  obtained  when  added  to  the  arginine 
nitrogen  gives  the  total  nitrogen  of  the  bases. 

Cystine  is  found  by  decomposing  10  c.c.  of  the  solution,  by  the  Benedicts 
and  Dennis1  method  of  oxidation  by  ignition  with  copper  nitrate.  Five 
cubic  centimeters  of  Dennis'  solution  (25  g.  copper  nitrate  crystals,  25  g. 
sodium  chloride,  and  10  g.  ammonium  nitrate  made  up  to  100  c.c.)  are 
added  in  an  evaporating  dish  to  the  10  c.c.  aliquot  of  the  solution  of  the 
bases,  evaporated  to  dryness  on  a  water  bath,  and  ignited  at  red  heat  for 
10  min.  The  residue  is  dissolved  in  10  c.c.  of  10  per  cent  hydrochloric 
acid,  diluted  to  150  c.c.,  heated  to  boiling,  and  10  c.c.  of  a  5  per  cent  solu- 
tion of  barium  chloride  added.  The  barium  sulphate  formed  is  treated  and 

1  BENEDICT  and  DENNIS,  J.  Biol.  Chem.,  6  (1909),  363;  8  (1910),  401. 


42 


GELATIN  AND  GLUE 


weighed  in  the  usual  way.  Each  milligram  of  barium  sulphate  obtained 
indicates  0.06  mg.  of  cystine  nitrogen  in  the  portion  of  solution  analyzed. 

The  amino  nitrogen  in  the  bases  is  determined  by  the  nitrous  acid  method 
of  Van  Slyke,  using  10  c.c.  for  the  large  apparatus,  or  1  or  2  c.c.  if  the  micro- 
apparatus  is  used. 

Histidine  and  Lysine  are  calculated  by  the  method  that  has  already  been 
described. 

The  filtrate  and  washings  from  the  phosphotungstic  precipitation  of 
the  bases  are  carefully  neutralized  and  made  slightly  acid  with  acetic  acid, 
concentrated  under  reduced  pressure  until  crystallization  begins,  and  made 
up  to  150  to  250  c.c.  in  a  volumetric  flask.  Twenty-five  cubic  centimeter 
portions  are  used  for  determining  total  nitrogen  of  the  filtrate  by  the  Kjeldahl 
method,  and  10  c.c.  portions  are  examined  for  amino  nitrogen  in  the  usual 
manner.  The  difference  between  the  nitrogen  obtained  by  these  two 
processes  gives  the  non  amino-nitrogen. 

Blank  analyses  should  of  course  be  made  upon  all  chemicals  used,  and 
corrections  should  likewise  be  made  for  the  solubilities  of  the  bases  in  the 
solutions  in  which  they  are  precipitated.  These  may  be  calculated  from 
the  following  table  prepared  by  Van  Slyke : 

TABLE  13. — SOLUBILITIES  OF  THE  BASES  IN  200  c.c.  OF  THE  SOLUTION  IN 
WHICH  THEY  ARE  PRECIPITATED 


I 

Total  N 
(Add  to  the 
undividual 
bases) 

Amino 

N 

Non-Amino 

N 

Arginine  N  

0.0032 

0.0008 

0.0024 

Histidine  N 

0  .  0038 

0.0013 

0.0025 

Lysine  N 

0  0005 

0  0005 

0  0000 

Cystine  N  
Sum    (subtracted    from    figures    for 
filtrate 

0.0026 

0.0026 
0  .  0052 

0.0000 
0  0049 

The  results  obtained  from  the  examination  of  a  number  of 
proteins  by  the  above  described  method  of  Van  Slyke  are  given 
in  Table  14,  and  curves  showing  the  differences  in  constitution 
observed  between  hide  glues,  bone  glues,  fish  glues,  isinglass, 
and  purified  gelatin  are  shown  in  Figs.  9,  10,  11,  and  12. 


9.  THE  COLOR  REACTIONS  OF  THE  PROTEINS 

The  proteins  yield  many  colored  products  upon  the  action  of 
specific  reagents,  and  in  most  cases  these  colored  substances  may 
be  traced  to  the  presence  within  the  molecule  of  certain  amino- 


CONSTITUTION  OF  THE  PROTEINS 


43 


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OW'O'-HOOOCDOOt^OOCOOOON 


N     _,     ,_     rH  ,_,     .H     .-I     i-H     i-H     l-H 


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Mela 

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fa 

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ft  £ 


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44 


GELATIN  AND  GLUE 


J"  •  *  • ;  •'*  ;°..*y  * 

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FIG.  9. — Variation  in  amino-acid  constituents  between  the  averages  of  the 
hide  and  bone  glues. 


[a)-Hide  Glues 


I 


H/4  -High 


Cb)-Bone  Glues 


I" 


FIG.  10. — Variation  in  the  amino-acid  constituents  on  passing  from  the  highest  to 
the  lowest  grade  glues. 


CONSTITUTION  OF  THE  PROTEINS 


45 


acid  groups.  These  tests  are  used,  therefore,  with  a  two-fold 
purpose,  first,  to  detect  the  presence  of  protein  or  its  decomposi- 
tion products  in  any  material,  and,  second,  to  establish  a  basis  of 
opinion  as  to  the  presence  or  absence  of  certain  amino-acid 


FIG.  11. — Variation  in  the  amino-acid  constituents  between  the  highest  hide 
glue  protein  (His)  and  the  protein  of  the  lowest  bone  glue  (Bi).  Whole  glues 
shown  also. 

groups.  Due  to  the  great  difficulties  entailed  in  obtaining 
absolutely  pure  proteins,  such  tests  cannot  usually  be  taken  as 
altogether  conclusive  evidence  of  the  presence  or  absence  of  a 
particular  group  in  a  particular  protein. 

The  Biuret  Reaction. — The  most  generally  used  and  universal 
test  for  proteins  is  the  biuret  reaction,  as  it  reacts  positive  with 
all  native  proteins  and  with  most  of  the  proteoses  and  peptones, 


46 


GELATIN  AND  GLUE 


The  solution  containing  the  protein  is  made  alkaline  with  sodium 
or  potassium  hydroxide,  and  a  few  drops  of  a  dilute  solution  of 
copper  sulphate  added,  and  well  mixed.  It  may  be  allowed  to 
stand  at  room  temperature,  or  gently  heated.  The  clear  liquid 

-1-4 


FIG.  12. — Variation,  from  the  highest  animal  glue  protein,  of  average  animal 
glues,  isinglass  and  fish  glue. 

is  colored  usually  a  violet,  but  may  vary  from  a  reddish  to  a 
bluish  cast.  The  reaction  was  given  its  name  from  the  fact  that 
the  test  is  also  shown  by  biuret,  NH2-CONH-CONH2. 

The  biuret  test  is  unlike  the  other  color  tests  for  proteins  in 
that  it  is  not  dependent  upon  the  presence  of  a  specific  amino- 
acid,  but  rather  upon  a  particular  configuration  in  the  molecule. 
The  proteins  react  positive  because  they  contain  at  least  one  acid 
amid  group,  and  other  substituted  amid  groups  on  adjacent  car- 
bon atoms. 

Millon's  Reaction. — When  a  protein  is  allowed  to  stand  in  con- 
tact with  a  mixture  of  mercuric  nitrite  and  nitrate,  or  is  heated 
with  these  substances,  a  red  color  is  developed.  The  mixture  of 
the  mercuric  salts  is  known  as  Milton's  reagent.  It  is  made  by 


CONSTITUTION  OF  THE  PROTEINS  47 

dissolving  20  g.  of  mercury  in  40  g.  of  concentrated  nitric  acid 
and,  after  solution  is  complete,  diluting  to  180  c.c.  with  water. 

The  test  with  Millon's  reagent  is  specific  for  the  monohydroxy 
benzene  nucleus.  It,  therefore,  reacts  positive  with  many 
organic  compounds  other  than  proteins.  The  only  amino-acid 
that  has  been  observed  in  the  decomposition  products  of  the 
proteins  that  contains  the  monohydroxy  benzene  group  is 
tyrosine.  The  reaction  as  applied  to  proteins  is,  therefore, 
specific  for  tyrosine.  In  the  presence  of  alcohol  or  chlorides  an 
excess  of  the  reagent  is  necessary. 

Xanthoproteic  Reaction. — When  proteins  are  heated  with 
dilute  nitric  acid  for  a  few  minutes  a  yellow  color  is  developed. 
On  cooling  and  adding  an  alkali  the  yellow  changes  to  a  deep 
orange.  The  reaction  is  specific  for  the  presence  of  the  benzene 
nucleus  in  the  molecule,  which,  with  proteins,  is  confined  to  the 
three  amino-acids  tyrosine,  phenylalanine,  and  tryptophane. 
The  reaction  depends  upon  the  formation  of  a  mono-  or  dinitro- 
benzene. 

Adamkiewicz  Reaction. — If  a  little  glacial  acetic  acid  or  gly- 
oxylic  acid  is  added  to  a  solution  of  a  protein,  and  this  followed 
by  a  small  amount  of  concentrated  sulphuric  acid,  a  violet  color 
develops  in  the  mixture.  The  reaction  appears  to  be  specific 
for  tryptophane.  A  modification  of  this  test  is  used  to  detect  the 
presence  of  formaldehyde  in  milk,  the  formaldehyde  taking  the 
place  of  the  acetic  or  glyoxylic  acid.  The  test  is  delicate  in 
milk  as  casein  is  rich  in  tryptophane. 

Molisch's  Reaction. — A  few  drops  of  a  10  per  cent  alcoholic 
solution  of  a-naphthol  are  added  to  a  solution  of  a  protein,  and 
then  a  little  concentrated  sulphuric  acid  poured  carefully  down 
the  side  of  the  test  tube.  The  development  of  a  violet  ring  at 
the  zone  of  contact  indicates  the  presence  of  a  carbohydrate 
group  in  the  protein  molecule. 

Sulphur  Reaction.— The  protein  is  boiled  with  sodium  hydrox- 
ide, during  which  process  any  sulphur  that  may  be  in  the  protein 
molecule  will  in  part  be  split  off  as  sodium  sulphide.  The  presence 
of  the  sulphide  is  then  observed  by  adding  a  little  lead  acetate 
to  the  tube,  when  a  black  or  dark  brown  coloration  or  precipitate, 
depending  on  the  amount  of  sulphur  which  was  present,  will  be 
produced.  As  sulphur  exists  in  the  protein  molecule  only  as 
cystine,  the  test  is  specific  for  that  amino-acid. 


CHAPTER   II 
THE  CHEMISTRY  OF   GELATIN  AND  ITS  CONGENERS 

Glue  is     an    organic    combination    pre- 
senting itself  in  different  modifications. 
Dawidowski   (1905). 

PAGE 

I.  The  Proteins  Associated  with  Gelatin 49 

1.  Collagen ....  49 

2.  Gelatin ' 52 

3.  Keratin 63 

4.  Elastin .67 

5.  Mucins  and  Mucoids 69 

6.  Chondrigin  and  Chondrin 73 

7.  Melanins  and  Humins 74 

8.  Amyloid 76 

9.  Ichthylepidin .  77 

10.  Comparison  of  the  Properties  of  Gelatin  and  Its  Congeners 78 

II.  The  Tissues  Containing  Gelatin  and  Its  Congeners 79 

1.  The  Skin .    79 

2.  The  Connective  Tissue 84 

3.  The  Cartilage 86 

4.  The  Bones .87 

5.  Fish  Skins,  Scales,  Sounds,  etc 88 

If  it  is  desired  to  appreciate  not  only  the  chemistry  and  physics 
of  pure  gelatin  but  also  the  behavior  of  the  various  commercial 
products  known  as  gelatins  and  glues  it  becomes  necessary  that 
the  other  nitrogenous  substances  with  which  gelatin  is  commonly 
associated  be  understood.  There  seems  to  be,  in  fact,  practically 
no  tissue,  with  the  possible  exception  of  the  inner  membrane  of 
fish  sounds,  that  consists  exclusively  of  collagen,  the  parent 
substance  of  gelatin,  and  gelatin  per  se  is  not  found  in  the  animal 
organism  except  under  pathological  conditions.  Whether  the 
material  be  obtained  from  the  skin,  sinews,  bones,  or  fish  parts, 
the  collagen  is  found  invariably  associated  with  other  protein 
material,  as  keratin,  elastin,  mucin,  chondrin,  etc.,  in  addition 
to  other  non-protein  organic  material  and  inorganic  salts. 

In  the  process  of  extracting  the  gelatin  it  is  inevitable  that 
greater  or  lesser  quantities  of  these  undesirable  proteins  should 
become  hydrolyzed  and  mix  with  the  gelatin  solution,  the  amount 
depending  upon  the  conditions  of  temperature,  pressure,  hydro- 
gen ion  concentration,  etc.  It  seems  desirable  therefore  that 

48 


CHEMISTRY  OF  GELATIN  49 

each  of  these  proteins  be  described,  and  the  combinations  of 
proteins  that  occur  in  the  tissues  made  use  of  in  the  manufacture 
of  gelatin  and  glue  be  set  forth. 

I.  THE  PROTEINS  ASSOCIATED  WITH  GELATIN 

1.  Collagen. — In  plant  physiology  the  principal  structural 
material  producing  turgor  and  rigidity  is  cellulose,  a  highly 
polymerized  carbohydrate.  In  the  animal  a  protein,  often 
fortified  by  inorganic  salts,  serves  in  this  capacity.  Among  the 
invertebrates  the  hard  shell-like  coverings  are  composed  largely 
of  a  protein  called  chitin.  In  the  vertebrates  the  protein  material 
of  the  bones  and  of  the  several  connective  tissues  is  a  mixture  of 
several  proteins,  the  most  important  of  which  is  collagen.  The 
organic  material  of  the  bones,  the  tendons,  the  cartilage  and  the 
skin,  is  to  a  great  extent  comprised  of  collagen.  When  this 
collagen  is  obtained  from  different  tissues  it  is  found  to  vary 
slightly  in  its  composition  and  this  has  led  some  writers  to  regard 
it  not  as  a  definite  chemical  compound,  but  rather  as  a  mixture 
the  composition  of  which  is  variable  within  certain  limits. 

When  collagen  is  heated  in  water  to  80  or  90°C.  it  is 
converted  slowly  into  the  protein  gelatin.  This  conversion  would 
be  greatly  accelerated  if  the  temperature  of  the  water  were  raised 
to  or  above  the  boiling  point  (under  pressure)  or  by  the  use  of 
dilute  acids,  but  such  a  procedure  would,  in  turn,  result  in  a 
further  hydrolysis  of  the  gelatin,  as  soon  as  it  was  produced, 
and  so  greatly  lessen  the  yield  and  quality  of  the  product. 
This  reaction  was  considered  by  Hofmeister1  to  be  an  hydrolysis, 
the  elements  of  one  molecule  of  water  being  added  to  the  collagen 
in  its  conversion  to  gelatin.  He  writes  the  equation, 

Cio2H149O38N3i  +  HgO^CVHisiOggNgi 

Collagen  Gelatin 

and  considers  collagen  as  the  anhydride  of  gelatin.  He  further- 
more regards  the  reaction  as  reversible  for  upon  heating  gelatin 
to  130°C.  he  reports  a  regeneration  of  collagen.  That 
this  is  a  true  conversion  of  gelatin  to  collagen  has  been  ques- 
tioned by  Alexander2  who  considers  it  more  probable  that,  "upon 

1  F.  HOFMEISTER,  Z.  physiol  Chem.,  2  (1878),  299. 

2  J.  ALEXANDER,  "Allen's  Commercial  Organic  Analysis,"  vol.  8  (1913), 
586. 


50 


GELATIN  AND  GLUE 


driving  off  the  water,  the  constituent  particles  of  gelatin  approach 
so  close  as  to  form  an  irreversible  gel,  thus  rendering  it  insoluble. " 

Emmett  and  Gies1  also  contend  that  Hofmeister  was  incorrect 
in  believing  that  gelatin  reverted  to  collagen  upon  heating  to 
130  degrees.  They  find  that  the  dried  product  is  indeed  less 
soluble  than  the  original  gelatin,  but  that  it  is  readily  digested  by 
trypsin  while  collagen  is  not.  They  also  find  that  ammonia 
is  evolved  during  the  hydrolysis  of  collagen  to  gelatin  in  hot 
water,  a  fact  which  goes  further  to  prove  the  irreversibility  of 
the  reaction.  On  boiling  gelatin  no  ammonia  was  evolved. 
They  conclude  that  gelatin  arises  from  an  intramolecular 
rearrangement  of  collagen  on  treating  the  latter  with  boiling 
water,  and  that  the  resultant  gelatin  is  not  a  simple  hydrate  of 
collagen,  as  shown  by  the  liberation  of  ammonia  upon  boiling 
the  latter  with  water. 

Just  as  the  collagens  obtained  from  different  sources  vary 
somewhat  in  their  chemical  composition,  so  also  will  the  gelatins 
obtained  from  different  collagens  vary.  This  is  shown  in  the 
following  table: 

TABLE  15. — ELEMENTARY  COMPOSITION  OF  COLLAGEN  AND  GELATIN 


Material  examined 

Carbon 

Hydrogen 

Nitrogen 

Sulphur 

Oxygen 

Authority 

Collagen                 .  .    •  . 

50.75 

6.47 

17.86 

24.92 

Hofmeister1 

Gelatin  from  bone  

50.00 

6.50 

17.50 

26.00 

Fremy2 

Gelatin  from  ligaments 

50.49 

6.71 

17.90 

0.57 

24.33 

Richards  &  Gies3 

Gelatin  from  tendons  .  . 

50.11 

6.56 

17.81 

0.26 

25.26 

Van  Name4 

Gelatin  from  isinglass 

48.69 

6.76 

17.68 

Faust5 

Gelatin  (commercial)  .  . 

49.38 

6.80 

17.97 

0.70 

25.  13 

Chittendeii* 

Gelatin,  ash  free  

50.52 

6.81 

17.53 

25.15 

C.  R.  Smith? 

HOFMEISTER,  loc.  cit. 

FREMY,  Chem.  Zentr.,  42  (1871),  516. 

RICHARDS  and  GIES,  Am.  J.  Physiol.,  8  (1903). 

VAN  NAME,  J.  Exptl.  Med.,  2  (1897). 

FAUST,  Arch,  exptl.  Path.  Pharm.,  41  (1898). 

CHITTENDEN,  J.  Physiol.,  12  (1891),  33. 

C.  R.  SMITH,  J.  Am.  Chem.  Soc.,  43  (1921),  1352. 

Although  the  sulphur  content  is  very  low,  it  seems  from  very 
careful  work  by  Sakidoff2  and  by  Morner3  that  it  is  nevertheless 
a  necessary  constituent  of  the  gelatin  molecule.  On  preparing 
gelatin  from  tendons  which  had  been  previously  digested  with 

1  A.  EMMETT  and  N.  GIES,  J.  Biol  Chem.,  3  (1907),  xxxiii. 

2  SAKIDOFF,  Z.physiol  Chem.,  39  (1903),  396;  41  (1904),  15. 

3  MORNER,  ibid.,  28  (1899),  471. 


CHEMISTRY  OF  GELATIN  51 

either  trypsin,  or  0.25  per  cent  potassium  hydroxide,  or  with 
sodium  hydroxide  followed  by  sodium  carbonate,  Sakidoff  found 
the  resulting  gelatin  to  have  in  each  case  a  sulfur  content  of  0.30 
to  0.526  per  cent.  Morner  obtained  one  gelatin  with  as  little 
as  0.2  per  cent  of  sulphur. 

Of  the  greatest  importance  in  our  every-day  life  is  the  effect 
which  tannic  acid  and  certain  other  substances  have  upon  colla- 
gen in  converting  it  into  leather.  The  use  of  extractions  from 
the  bark  of  trees,  such  as  the  oak  or  hemlock,  or  the  wood  of  the 
chestnut,  the  quebracho,  etc.,  has  been  applied  from  the  earliest 
ages  in  the  preservation  and  tanning  of  hides  and  pelts.  Of 
more  recent  origin  is  the  introduction  of  chrome  tanning,  in  which 
process  the  hides  are  treated  first  with  a  solution  of  sulphuric  acid 
and  sodium  chloride,  and  later  with  a  solution  of  basic  chromic 
sulphate.  Neither  of  these  processes  is  by  any  means  completely 
understood  even  today,  but  it  is  known  that  these  liquors  produce 
an  insolubilization  in  the  collagen  of  the  hides,  for  when  fully 
tanned  boiling  water  will  have  practically  no  effect  upon  them. 

Collagen  may  be  prepared  by  extracting  bones  first  with  dilute 
hydrochloric  acid  to  remove  the  inorganic  salts,  and  second  with 
dilute  alkali  to  remove  extraneous  organic  matter.  Or  it  may 
be  obtained  from  tendons  or  the  corium  layer  of  skin  by  extract- 
ing with  lime  water  or  dilute  alkali,  and  thoroughly  washing 
with  water.  It  is  a  colorless  substance  which  swells  in  cold 
water,  in  dilute  acids,  and  in  dilute  alkalies,  but  is  insoluble  in  all 
of  the  above,  and  in  organic  solvents.  When  the  temperature  is 
raised,  in  water,  dilute  acid  or  alkali,  it  is  changed  into  gelatin. 
It  is  soluble  in  strong  alkalies,  but  not  in  carbonates.  It  is 
readily  dissolved  by  pepsin  hydrochloride,  but  trypsin  has  no 
effect  upon  collagen,  unless  the  latter  has  first  been  heated  with 
water  to  70°C.,  or  swollen  with  acids  and  again  contracted 
by  heating.  It  seems  probable  that  the  collagen  molecule  is 
completely  resistant  to  the  action  of  trypsin,  but  that  as  soon  as 
a  little  hydration  has  taken  place,  i.e.,  as  soon  as  a  small  amount 
of  gelatin  has  been  produced  from  the  collagen  molecule,  then 
the  trypsin  will  become  active;  its  attack  is  probably  confined  to 
the  gelatin  molecule. 

The  different  behavior  of  pepsin  and  trypsin  on  the  hydrolysis 
of  proteins  is  accounted  for  by  Plimmer1  by  assuming  that  the 

1  R.  PLIMMER,  "Chemical  Constitution  of  the  Proteids,"  2nd  ed.  (1912), 
part  II,  p.  11. 


52  GELATIN  AND  GLUE 

latter  is  unable  to  open  up  a  closed  ring  compound.  He  says, 
"Trypsin  will  hydrolyze  a  chain  of  amino  acids  with  a  terminal 
amino  or  carboxyl  group.  Pepsin  will  open  the  anhydride  ring 
at  one  or  more  junctions  and  give  several  proteoses  and  peptones 
with  free  terminal  —  NH2  and  —  COOH  groups  capable  of  being 
attacked  by  trypsin.  Those  proteins  which  are  resistant  to  the 
action  of  trypsin  until  they  have  been  acted  upon  by  pepsin  will 
have  all  their  units  contained  in  the  anhydride  ring."  This 
statement  is  generally  accepted  although  Procter1  urges 
that  it  seems  to  require  confirmation.  Since  gelatin  is  readily 
hydrolyzed  by  trypsin,  while  collagen  is  not  attacked,  it  seems 
probable  that  the  latter  possesses  a  closed  ring  structure,  and 
as  the  conversion  to  gelatin  seems  to  involve  the  addition  of  the 
elements  of  water,  an  anhydride  formation  may  be  assigned  to 
collagen  : 


NH 


R 
X 


CO  COOH 

Collagen  Gelatin 

The  swimming  bladders  of  fish  are  composed  of  nearly  pure 
collagen,  and  this  variety  is  more  readily  soluble  than  any  other. 
The  scales  and  skin  of  fish  also  contain  a  large  amount  of  collagen 
which  is  likewise  very  easily  dissolved. 

The  cleavage  products  of  collagen  are  identical  with  those 
obtained  from  gelatin,  and  will  be  discussed  in  the  next  section. 

2.  Gelatin.     General  Description  and  Properties.  —  Gelatin  is  a 

.  nearly  colorless,  transparent,  amorphous  substance,  flexible  and 

horny  when  in  the  normal  dry  condition,  in  which  state,  however, 

it  retains  about  16-18  per  cent  of  water.     The  natural  color  is  of 

a  slightly  yellowish  cast.     Precipitated  from  alcohol  or  salts, 

:'  however,    it    is    pure    white,    and    nearly    water-free.     Gelatin 

J  swells  to  many  times  its  normal  volume  when  immersed  in  cold 

;     water  or  in  dilute  acids  or  alkalies.     The  degree  of  acidity,  or  of 

alkalinity,  or  of  salt  content  of  the  water  greatly  modifies  the 

extent  to  which  the  gelatin  will  swell.     A  slightly  acid  solution 

seems  most  favorable  for  maximum  swelling.2     On  raising  the 

temperature    to    about    35°C.    the   swollen    jelly  goes  readily 

1  H.  R.  PROCTER,  "First  Report  on  Colloid  Chemistry,"  British  Assoc. 
for  the  Adv.  of  Science  (1917),  7. 

2  Cf.  Chap.  IV. 


CHEMISTRY  OF  GELATIN  53 

into  solution.  The  absorption  of  water  has  been  shown  by 
Quincke1  to  result  in  a  volume  contraction,  the  volume  of  the 
swollen  jelly  being  less  than  the  sum  of  the  volumes  of  the  original 
dry  gelatin  and  the  absorbed  water.  Wiedemann  and  Ltideking2 
later  showed  that  the  process  was  also  accompanied  by  a  liberation 
of  heat,  as  would  be  expected  from  Quincke's  findings.  Apply- 
ing the  principle  of  LeChattelier  this  means  that  low  temperatures 
will  favor  the  absorption  of  water  by  gelatin,  while  if  the  opposite 
effect  is  required,  e.g.,  if  it  is  desired  to  hasten  the  drying  out 
process,  a  relatively  high  temperature  will  be  found  most 
favorable. 

Gelatin  is  a  typical  colloid  of  the  emulsoid  type,  and  many  of  ' 
its  most  important  and  striking  properties  are  dependent  upon 
this  condition.  As  an  emulsoid  colloid,  the  viscosity  of  its  solu- 
tions is  high  and  very  variable  with  slight  alterations  in  the  tem- 
perature, the  concentration,  the  hydrogen-ion  concentration,  etc. 
As  an  emulsoid  colloid  gelatin  exerts  a  marked  protective  action 
upon  salts  which  are  precipitated  in  its  presence,  such  precipita- 
tions usually  coming  down  in  a  very  finely  divided  suspensoid 
condition.  In  analytical  determinations  such  effects  are  often 
very  troublesome  and,  unless  well  understood,  may  lead  to 
incorrect  postulations.  As  an  emulsoid  colloid  gelatin  finds 
favor  among  manufacturers  of  ice-cream,  as  it  will  prevent  the 
crystallization  of  water  upon  the  long-standing  of  the  cream. 

When  a  solution  containing  one  or  more  per  cent  of  gelatin  is 
allowed  to  stand  at  about  10°C.  a  firm  jelly  will  be  formed.  This 
is  probably  the  most  characteristic  and  important  property  of 
gelatin,  and  a  large  part  of  the  investigational  work  that  has  been 
done  upon  gelatin  has  been  centered  about  this  property.  As  an 
adhesive,  gelatin,  in  the  form  of  glue,  finds  one  of  its  most 
important  uses.  All  of  these  important  properties  will  be  taken 
up  at  length  in  subsequent  chapters. 

Wheri  heated  in  the  air  gelatin  swells  to  many  times  its 
original  volume,  becomes  soft,  and  finally -disintegrates,  evolving 
ammonia  and  a  large  amount  of  pyridine  bases,  and  leaving  a 
residue  of  hard,  difficultly  combustible  charcoal. 

Gelatin  has  the  peculiar  property  of  lowering  the  solubility  of 
easily  soluble  salts  and  of  increasing  the  solubility  of  difficultly 


1  QUINCKE,  Arch.  ges.  PhysioL,  3  (1870),  332. 

2  WIEDEMANN  and  LUDEKING,  Ann.  Physik.  Chem.,  26  (1885),  145. 


54 


GELATIN  AND  GLUE 


soluble  salts.     The  following  figures  of  Pauli  and  Samec1  illus- 
trate this  property: 

TABLE   16. — CHANGES  IN  SOLUBILITY  THROUGH  GELATIN  ADDITIONS 


Solute 

100  g.  water 

+4  Per  cent 
gelatin 

+  10  Per  cent 
gelatin 

Ammonium  chloride  
Magnesium  chloride  
Ammonium       sulphocya- 
nate 

28.49 
35.94 

62  46 

27.55 
35.22 

61  46 

26.48 
35.13 

58  92 

100  g.  water 

1.5  Per  cent 
gelatin 

Calcium  sulphate 

0  223 

0.295 

Tricalcium  phosphate.  .  .  . 

0.011 

0.018 

Calcium  carbonate  
Silicic  acid  

0.004 
0.023 

0.015 
0.027 

According  to  Bechhold  and  Ziegler2  the  melting  point  of  gelatin 
is  altered  by  the  presence  of  either  inorganic  salts  or  organic 
substances.  They  give  the  following  figures: 


Composition  of  solution 


Melting  point 


10  per  cent  gelatin 

10  per  cent  gelatin  +  1  mol.  NaCl 

10  per  cent  gelatin  +  2  mols.  Na2SO4. .  . . 

10  per  cent  gelatin  +  1  mol.  Nal 

10  per  cent  gelatin  +  1  mol.  grape  sugar. 
10  per  cent  gelatin  +  2  mols.  glycerin.  .  . 
10  per  cent  gelatin  +  2  mols.  alcohol. .  .  . 
10  per  cent  gelatin  -f  1  mol.  urea 


31.66 

28.5 

34.2 

10.0 

32.25 

32.17 

30.0 

26.3 


When  solutions  of  gelatin  are  examined  with  the  ultramicro- 
scope,  some  investigators  have  failed  and  others  have  succeeded 
in  obtaining  visible  amicrons.  Zsigmondy3  reported  that  a  hot 
solution  of  pure  gelatin  showed  a  clear  field  except  for  a  slight 
Tyndall  effect  which  he  ascribed  to  traces  of  impurities.  After 

1  Wo.  PAULI  and  M.  SAMEC,  Biochem.  Z.,  17  (1909),  235. 
'   2  H.  BECHHOLD  and  J.  ZIEGLER,  Z.  physiol.  Chem.,  62  (1905),  185. 
3  ZSIGMONDY,  "Colloids  and  the  Ultramicroscope." 


CHEMISTRY  OF  GELATIN  55 

standing  for  2  days,  however,  a  0.2  per  cent  solution  of  gelatin 
appeared  filled  with  particles  of  about  5/x/i  diameter.  Elliott,1 
however,  experienced  no  difficulty  in  observing  freely  moving 
amicrons  in  fresh  dilute  solutions  of  gelatin. 

Solubility. — Most  of  the  organic  solvents  fail  to  dissolve 
gelatin,  it  being  completely  insoluble  in  ether,  chloroform,  carbon 
disulphide,  benzene,  fixed  oils,  volatile  oils,  and  absolute  alcohol. 
It  is  also  insoluble  in  water  at  the  freezing  point  containing  as 
little  as  10  per  cent  of  alcohol.  A  procedure  for  the  determina- 
tion of  gelatin  is  based  upon  this  fact.  Alcohol  in  85  to  95  per 
cent  concentration  is  used  as  a  precipitant  for  gelatin,  the  latter 
being  thrown  down  as  a  white  precipitate,  stringy  if  the  alcohol 
is  at  20°C.  or  above,  but  finely  divided  and  almost  granular  if 
the  temperature  is  kept  at  about  10°.  Tannic  acid  and  phospho- 
tungstic  acid  precipitate  gelatin  quantitatively  if  kept  below 
17°C.  These  reagents  serve,  therefore,  in  both  the  qualitative 
and  quantitative  estimation  of  gelatin.  It  was  pointed  out  by 
Ricevuto,2  however,  that  tannin  did  not  precipitate  gelatin  unless 
the  latter  were  in  a  negative  condition  or  unless  salts  were  pres- 
ent. He  found  that  carefully  dialyzed  gelatin  was  not  precipi- 
tated by  tannin.  If  this  may  be  interpreted  in  terms  of  Loeb's3 
findings  it  would  mean  that  only  when  the  gelatin  is  in  the  form 
of  a  gelatinate  may  it  react  with  tannin.  The  tannin  molecule 
is,  therefore,  positive  with  respect  to  the  gelatin.  If  the  reac- 
tion is  chemical  in  its  nature  tannin  gelatinate  must  be  the  sub- 
stance precipitated.  There  is,  however,  some  objection  to 
considering  this  as  a  chemical  reaction,  because  the  product 
formed  is  variable  in  composition.  It  is,  however,  not  only 
possible,  but  could  hardly  be  averted,  that  some  of  either  the 
gelatin  or  the  tannin,  depending  on  which  was  present  in  excess, 
should  be  carried  down  with  the  precipitate,  contaminating  it, 
as  is  well  known  to  happen  in  thoroughly  understood  inorganic 
precipitations.  This  would  account  for  any  variations  noted 
in  the  composition  of  the  tannin  gelatinate.  On  the  other  hand 
it  is  as  well  recognized  that  oppositely  charged  colloids  mutually 
precipitate  each  other,  and  von  Schroeder4  has  shown  that  the 

1  F.  ELLIOTT,  Communication  to  60th  General  Meeting,  Am.  Chem.  Soc., 
Chicago  (1920). 

2  RICEVUTO,  Kolloid-Z.,  3  (1908),  114. 

3  Cf.  Chap.  V. 

<  J,  VON  SCHROEDER,  Kolloidchem.  Biehefte,  1  (1909),  1. 


56  GELATIN  AND  GLUE 

adsorption  isotherm  is  closely  followed  in  the  system  tannin- 
gelatin,  during  precipitation.  An  objection  to  the  colloid  explana- 
tion, however,  is  found  in  the  fact  that  gelatin  is  not  readily 
precipitated  by  electrolytes,  nor  by  other  colloids  of  opposite 
electric  charge. 

Certain  salts,  notably  the  sulphates  of  ammonium,  zinc,  and 
magnesium  precipitate  gelatin  quantitatively  if  added  to  the 
point  of  saturation,  and  if  the  temperature  is  kept  low.  These 
salts  have  found  extensive  use  in  the  separation  of  gelatin  (and 
other  proteins)  from  their  products  of  hydrolysis,  for  by  arbi- 
trarily varying  the  percentage  of  saturation  of  the  salt  solution, 
the  protein  and  its  cleavage  products  may  be  fractionally 
separated.  It  is  customary  to  consider  the  unchanged  protein 
as  insoluble  in  a  half-saturated  solution  of  these  sulphates,  the 
proteoses  as  insoluble  in  a  saturated  solution,  and  the  peptones 
and  amino-acids  as  soluble  at  all  concentrations  of  the  salts. 

Reactions. — Potassium  dichromate  reacts  with  gelatin  in  the 
presence  of  light  to  produce  a  jelly,  which,  on  drying  out,  is 
insoluble.  This  property  is  made  use  of  in  photolithography. 
Many  methods  of  applying  the  principle  have  been  developed, 
but  in  general  the  process  consists  in  exposing  a  plate  of  dichro- 
mated  gelatin  to  light  through  a  photographic  negative.  The 
light  acts  on  the  gelatin  producing  the  insoluble  phase  under  the 
thinner  parts  of  the  negative.  On  soaking  in  water  the  protected 
portions  swell  more  than  the  exposed  parts  and  a  reproduction  of 
the  picture  is  secured. 

A  large  number  of  reactions  have  been  reported  for  gelatin  with 
divers  reagents,  but  many  of  these  are  of  transient  interest  only. 
When  chlorine  gas  is  passed  through  a  dilute  solution  (about  1 
per  cent)  of  gelatin,1  the  liquid  first  remains  clear,  then  it  froths 
strongly,  and  when  the  chlorine  is  present  in  excess  the  frothing 
subsides  and  the  gelatin  is  precipitated  as  a  white  granular  mass. 
On  washing  and  drying  in  vacuo  over  sulphuric  acid  a  yellowish- 
white  powder  is  obtained  which  is  insoluble  in  water  or  alcohol, 
but  soluble  in  alkalies.  This  reaction  has  been  made  the  basis 
for  the  estimation  of  gelatin  in  tub-sized  papers  by  Cross,  Bevan 
and  Briggs.2  They  found,  by  allowing  the  chlorine  gas  to  act 
upon  the  gelatin,  spread  out  into  very  thin  layers  on  cotton  yarn 
by  immersing  the  latter  in  a  gelatin  solution,  that  15.4  per  cent 

1  S.  RIDEAL  and  C.  G.  STEWART,  Analyst,  22  (1897);  228. 
CROSS,  BEVAN,  and  BRIGGS,  /.  Soc.  Chem.  Ind.,  27  (1908),  260. 


CHEMISTRY  OF  GELATIN  57 

of  chlorine  gas  was  retained,  forming  a  gelatin  chloramine.  On 
washing  with  sulphurous  acid  the  chlorine  was  liberated. 

Bromine1  and  iodine2  have  been  shown,  when  applied  in  the 
proper  manner,  to  give  results  which  are  analogous  to  those 
obtained  with  chlorine.  There  seems  to  be  much  difficulty 
however,  in  so  adjusting  the  technique  of  the  process  that  the 
results  may  be  certain  and  readily  duplicated. 

Potassium  ferrocyanide  or  ferricyanide  do  not  ordinarily 
produce  precipitates  with  gelatin,  but  Morner3  has  shown  that 
if  potassium  ferrocyanide  and  acetic  acid  are  added  to  gelatin, 
and  the  temperature  kept  below  30°C.,  and  only  dilute  solutions 
are  used,  a  precipitate  may  be  obtained,  but  is  dissolved  by  the 
least  excess  of  either  the  ferrocyanide  or  the  gelatin.  The 
presence  of  salts,  organic  acids,  or  bases  prevent  the  formation 
of  the  precipitate. 

Formaldehyde  produces  a  condensation  product  with  gelatin 
rendering  the  latter  insoluble.  Lumiere  and  Seyewetz4  report  that 
from  4.0  to  4.8  per  cent  of  formaldehyde  combines  chemically 
with  the  gelatin  when  a  10  per  cent  solution  of  the  former  is 
allowed  to  act  upon  dry  gelatin.  This  product  is  known  as 
formo-gelatin,  and  was  shown  to  be  decomposed  by  cold  15 
per  cent  hydrochloric  acid,  by  repeated  treatment  with  boiling 
water,  and  by  heating  to  110°C.  The  author5  has  found  in  a 
study  of  this  substance  that  when  a  10  per  cent  solution  of  for- 
maldehyde is  added  to  a  solution  of  gelatin  of  from  about  5.5  to 
22  per  cent  concentration: 

1.  The  viscosity  varies  directly  as  the  amount  of  formalde- 
hyde added.     The  greater  the  purity  of  the  gelatin,  and  the 
higher  the  concentration,  the  less  the  amount  of  formaldehyde 
required  to  produce  insolubility. 

2.  The    jelly-strength    varies    inversely    as    the    amount    of 
formaldehyde  added.     This  effect  is  the  more  marked  in  the 
weaker  grades  of  gelatin,  and  in  the  lower  concentrations. 

3.  The  viscosity  increases  with  the  time. 

4.  The  viscosity  decreases  with  rise  in  temperature  up  to 
about    40°C.     Above   this   temperature   the   viscosity    rapidly 

1  ALLEN  and  SEARLE,  Analyst,  12  (1887),  258. 

2  HOPKINS  and  BROOKE,  J.  Physiol,  22  (1897),  184. 

3  MORNER,  Z.  physiol.  Chem.,  28  (1899),  471. 

4  LUMIERE  and  SEYEWETZ,  Bull.  soc.  chim.,  35  (1906),  872. 

5  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  7-9. 


58  GELATIN  AND  GLUE 

increases  to  the  setting  point.  Attempts  have  been  made  to 
utilize  the  properties  of  formo-gelatin  in  the  preparation  of  an 
insoluble  glue,  but  the  effort  has  not  been  successful  for  several 
reasons.  These  will  be  discussed  in  a  subsequent  chapter. 

Picric  acid  precipitates  gelatin,  when  added  in  excess,  throwing 
down  a  stringy,  sticky,  yellow  precipitate.  On  raising  the  tem- 
perature this  becomes  soluble,  but  again  appears  on  recooling 
the  solution. 

Platinic  sulphate  precipitates  gelatin  in  the  form  of  small  brown 
flakes  which  quickly  turn  black.  Davy  regards  this  as  a  very 
delicate  test  for  gelatin,  more  sensitive  even  than  the  tannin 
test,  and  unaffected  by  the  presence  of  albumin,  but  Alexander 
has  been  unable  to  confirm  this  point.  The  chloride  of  platinum 
throws  down  gelatin  as  a  yellow  precipitate. 

Gelatin  is  unlike  many  others  of  the  proteins  of  this  group  in 
that  it  is  not  precipitated  by  mineral  acids  nor  by  acetic  acid. 
Neither  is  it  thrown  out  of  solution  by  alum,  lead  acetate,  ferric 
chloride,  or  silver  nitrate.  Gelatin  may  be  precipitated,  however, 
by  the  chlorides  of  gold  and  tin  (stannous),  which,  together  with 
the  chloride  of  platinum  above  described,  are  dissolved  by  bring- 
ing the  solution  to  the  boiling  point,  but  reappear  upon  cooling. 
If  neutral  salts  or  a  little  hydrochloric  acid  are  present,  gelatin 
may  be  thrown  out  of  solution  by  mercuric  chloride  or  nitrate 
and  by  basic  lead  acetate.  Chromic  acid  and  mercury-potassium 
iodide  also  precipitate  gelatin,  but  the  precipitate  becomes  soluble 
on  heating. 

Constitution. — Since  gelatin  is  obtained  from  collagens  which 
vary  widely  in  their  source,  and  since  it  is  exceedingly  difficult 
to  be  certain  that  one  is  dealing  with  absolutely  pure  gelatin, 
the  composition  of  the  material  as  reported  by  different  investiga- 
tors shows  conspicuous  variations.  The  elementary  composi- 
tion of  several  gelatins  was  shown  on  page  50.  Sulphur  has 
usually  been  reported  in  small  amounts,  but  it  is  still  an  open 
question  whether  this  small  amount  of  sulphur  is  a  necessary  and 
constant  constituent  of  the  gelatin  molecule,  or  is  an  impurity. 
The  carbon  content  is  low,  which  accounts  for  the  low  calorific 
value  of  gelatin.  According  to  Berthelot  and  Stohmann1  this 
value  is  some  500  to  700  calories  below  the  average  of  the 
albuminous  substances.  The  nitrogen  content  is  relatively  high. 

The  first  record  which  we  have  of  the  amino-acid  constitution 

1  BERTHELOT  and  STOHMANN,  /.  prakt.  Chem.,  44  (1891),  336. 


CHEMISTRY  OF  GELATIN  59 

of  gelatin  was  reported  by  Braconnot1  in  1820.  He  discovered 
that  glycocoll  (glycine)  is  an  abundant  constituent  of  gelatin. 
The  analyses  of  H.  D.  Dakin,  given  on  page  36,  show  that 
glycine  is  present  to  the  extent  of  25.5  per  cent  in  gelatin.  Argi- 
nine,  proline,  oxyproline,  lysine,  and  leucine  are  also  present  in 
conspicuous  amounts.  Tryptophane  and  tyrosine  are  apparently 
absent. 

The  distribution  of  nitrogen  among  the  several  groups  accord- 
ing to  the  method  of  Van  Slyke  is  shown  in  the  table  on  page  43. 
In  this  arrangement  the  value  of  the  figures  obtained  for  non- 
amino  nitrogen  of  the  filtrate  stand  out  as  noticeably  large.  This 
,group,  it  will  be  recalled,  comprises  the  proline  and  oxyproline. 
The  amino  nitrogen  of  the  filtrate  is  also  large,  due  principally  to 
the  glycine.  Cystine  is  noticeable  by  its  absence.  Lysine  is 
rather  higher  in  gelatin  than  in  most  proteins,  and  histidine  and 
ammonia  are  low. 

Trypsin  acts  very  slowly  upon  gelatin,  and  produces  only 
albumoses  and  peptones.  Even  after  10  months'  action  Levene2 
obtained  only  small  amounts  of  amino-acids'by  the  tryptic  diges- 
tion of  gelatin.  He  observed,  however,  that  the  glycine  content 
bf  the  albumoses  was  greater,  and  of  the  peptones  less,  than  the 
glycine  content  of  the  original  gelatin.  He  accounted  for  this 
by  showing  that  in  the  conversion  of  the  albumose  to  peptone 
only  glycine,  and  traces  of  leucine,  were  split  off  from  the  mole- 
cule. The  action  of  pepsin  is  very  similar  to  that  of  trypsin,  being 
slow  and  producing,  for  the  most  part,  only  the  relatively  large 
intermediate  molecules.  The  action  of  the  proteolytic  ferments 
of  the  liver  on  gelatin  is  somewhat  more  rapid  and  complete  than 
that  of  trypsin  and  pepsin.3  Peptone,  diamino  acids,  and  mono- 
ammo  acids  are  produced. 

The  action  of  oxydising  agents,  as  permanganates,  upon  gelatin 
results  in  the  production  of  oxalan,  NH2.CO.NH.C2O2.NH2; 
ammonium  oxaminate,  C2O3.NH2.NH4;  ammonium  oxalate; 
and  the  acids:  oxalic,  succinic,  benzoic,  formic,  acetic,  and  buty- 
ric. Benzaldehyde,  propionic,  and  valerianic  acids  have  also 
been  observed.1 


1  BRACONNOT,  Ann.  chim.  phys.,  13  (1820),  113. 

2  LEVENE,  Z.  physiol.  Chem.,  41  (1904),  8;  99. 

3  ARNHEIM,  Z.  physiol.  Chem.,  40  (1903),  234. 

4  SEEMANN,  Zentr.  Physiol.,  18  (1904),  285. 


60  ,  GELATIN  AND  GLUE 

i>  i 

*r 

Gelatin  gives  a  distinct  violet  biuret  reaction.1  The  Millon's 
reagent  gives  a  faint  pink  coloration,  and  the  xanthoproteic 
reaction  results  in  a  faint  yellow  coloration.  Since  Millon's 
reaction  is  specific  for  tyrosine  and  the  xanthoproteic  reaction  is 
usually  accredited  to  tryptophane,  and  since  both  of  these 
amino-acids  have  been  reported  absent  in  gelatin,  it  follows 
that  either  the  positive  tests  obtained  are  the  result  of  impurities 
in  the  gelatin,  or  else  that  those  amino-acids  upon  which  the 
reactions  depend  are  present,  even  though  they  have  not  yet 
been  isolated,  in  the  gelatin  molecule.  The  lead  sulphide  reaction, 
the  Molisch  reaction,  and  the  Adamkiewicz  reaction  are  usually 
reported  negative  in  pure  gelatin,  but  the  former  is  observed  in 
the  commercial  product,  and  the  Molisch  reaction  has  been 
reported  positive  by  Hofmeister2  and  by  Klug.3 

Gelatin  has  been  reported  to  produce  symptoms  of  poisoning 
when  injected  subcutaneously  or  intravenously. 

The  Decomposition  of  Gelatin  by  Bacteria. — A  very  large  number 
of  bacteria  have  been  described  which  produce  liquefaction  of 
gelatin  with  more  or  less  rapidity.  In  most  cases  the  action  is 
relatively  slow,  and  it  is  in  fact  difficult  to  draw  a  sharp  line 
between  those  varieties  which  do  and  those  which  do  not  produce 
liquefaction,  for  some  types  have  been  found  to  produce  this 
effect  only  after  several  months.  The  Society  of  American 
Bacteriologists  has  recommended  that-  the  tests  be  continued  for 
6  weeks  before  the  term  non-liquefying  be  applied  in  any  specific 
case.  A  few  types  have  had  the  term  liquefaciens  appended  to 
their  name  in  signification  of  their  exceptional  ability  in  this 
direction,  as,  for  example,  the  B.  liquefaciens,  B.  liquefaciens 
boris,  B.  liquefaciens  parvis,  and  B.  fluorescens  liquefaciens. 

The  products  of  decomposition  by  bacteria  appear  to  be,  firstly, 
peptones  and,  secondly,  amino-acids.  This  fact  has  led  to  the 
term  peptonization,  as  descriptive  of  the  process.  The  following 
table  by  Rideal  and  Stewart4  shows  the  products  of  the  decompo- 
sition of  gelatin  by  B.  fluorescens  liquefaciens,  and  B.  prodigiosus, 
after  varying  intervals  of  incubation  at  20  to  21°C. 

It  will  be  observed  that  the  gelatin  and  albumoses  are  first 
hydrolyzed  to  peptones,  thereby  producing  liquefaction,  and 

1  See  page  45. 

2  HOFMEISTER,  Z.  physiol.  Chem.,  2  (1878),  299. 

3  KLUG,  Pfliiger's  Arch.  Physiol.,  48  (1891),  100. 

4  RIDEAL  and  STEWART,  Analyst,  22  (1897),  255. 


CHEMISTRY  OF  GELATIN  61 

TABLE   17. — THE  DECOMPOSITION  OF  GELATIN  BY  BACTERIA1 


Gelatin 
and 
albu- 
moses 

Gelatin 

Albu- 

moses 

Ammonia 
and    vola- 
tile   bases 

Bases 
and 
extrac- 
tives 

Peptone 
and  nitro- 
gen unac- 
counted 
for 

Albumin 
and  celu- 
lose 

Original  gelatin 

89  3 

47  9 

41  4 

0  24 

4  91 

5  55 

B.  flor.  liquefaciens, 

20-2  1°C      1  day  .  . 

85.9 

55.5 

30.4 

2  12 

2  days    . 

54.3 

30.7 

23.6 

3.58 

8.37 

33:8 

3K  days    . 

19.15 

8.45 

10.7 

4.98 

38.4 

33.6 

16  days    . 

17.12 

10.7 

6.42 

13.33 

58.9 

9.85 

0.8 

B.  prodigiosus, 

20-21°C.     14  days 

83.1 

49.9 

33.2 

3.97 

1  The  figures  are  expressed  as  percentage  of  total  nitrogen.  These  were 
calculated  by  the  author  from  the  figures  of  RIDEAL  and  STEWART. 

rendering  the  consistency  of  the  solution  limpid  and  non-viscous, 
but  by  continued  action  the  bacteria  attack  the  peptones  pro- 
duced and  convert  them  to  amino-acids.  Indole,  skatole,  and 
similar  putrefactive  products  are  either  not  produced  at  all  from 
pure  gelatin,  or  are  formed  only  in  very  small  amounts.  The 
presence  of  such  substances  in  conspicuous  amounts  in  decom- 
posed glue  is  evidently  due  to  impurities  which  are  inevitably 
present.  Ammonia  is  slowly  and  regularly  produced  as  decompo- 
sition continues,  but  is  not  present  in  large  amounts  until  the 
action  has  proceeded  for  a  long  time. 

Preparation  and  Purification. — Whenever  the  housewife  boils 
a  bone  for  a  soup  she  is  manufacturing  gelatin,  and  this  gelatin 
is  very  much  the  same  substance,  except  for  fat  and  portions  of 
flesh  which  were  not  previously  removed,  as  is  obtainable  at 
every  grocery  store.  The  ossein  of  the  bone,  which  is  mainly 
collagen,  is  converted  by  the  boiling  process  into  gelatin.  If  it 
is  desired  to  obtain  the  pure  protein,  a  pure  collagen  must  first 
be  prepared  and  this  heated  to  70  to  80°C.  for  several  hours. 
Heating  with  fresh  portions  of  water  may  be  repeated  several 
times,  and  the  combined  solutions  concentrated  at  a  low  tem- 
perature under  diminished  pressure.  This  process  eliminates 
the  possibility  of  the  gelatin  molecules  undergoing  further 
hydrolysis  into  proteoses,  peptones,  or  amino-acids.  The 
gelatin  may  then  be  allowed  to  gel,  cut  into  sheets,  and  dried  in 
the  air.  If  it  is  desired  to  free  it  as  completely  as  possible  of  all 
extraneous  salts  the  thin  sheets  of  gelatin  may  be  suspended  in 
cold  distilled  water.  This  should  be  changed  frequently  or 


62  GELATIN  AND  GLUE 

arranged  so  that  the  supply  and  discharge  are  continuous. 
Further  purification  may  be  obtained  by  precipitating  in  alcohol. 
For  this  purpose  a  dilute  solution  of  the  gelatin  should  be  poured 
into  cold  95  per  cent  alcohol,  redissolved  in  warm  water,  and 
this  process  repeated  several  times.  Gelatin  prepared  in  the 
above  manner  may  be  considered  as  chemically  pure.  Some 
investigators  have  carried  the  purification  process  a  step  farther, 
however,  by  deaminizing  the  product.  This  was  done  by  Blasel 
and  Matula1  as  follows:  200  g.  of  the  best  obtainable  commercial 
gelatin  were  dissolved  in  1  liter  of  distilled  water.  A  liter  of  a 
20  per  cent  solution  of  sodium  nitrite  was  added.  The  mixture 
was  cooled  and  140  g.  of  glacial  acetic  acid  was  carefully  added. 
After  standing  for  12  hours  the  mixture  was  heated  on  a  water 
bath  for  2  hours.  Solid  ammonium  sulphate  was  then  added  to 
precipitate  the  gelatin,  and  it  was  subsequently  purified  by 
dialysis  against  running  distilled  water  for  2  weeks.  None  of  the 
properties  of  the  deaminized  gelatin  differ  greatly,  however, 
from  the  product  as  usually  obtained  and  this  process  is  not 
essential  to  the  obtaining  of  the  pure  protein  gelatin.  The 
product  of  any  of  the  above  methods  may,  according  to  Loeb,2  be 
either  a  metal  gelatinate,  a  neutral  gelatin,  or  a  gelatin  salt.  It 
is  most  probably  a  calcium  or  sodium  gelatinate.  To  obtain  the 
simple  gelatin  it  is  necessary  to  bring  this  salt  to  the  isoelectric 
point,  which  for  gelatin  is  pH  =  4.7.  This  may  be  done  by 
allowing  hydrochloric  acid  of  the  proper  concentration  — N/128 
to  N/512 — to  react  with  the  granulated  gelatin  for  %  hour  at 
10°C.,  and  afterward  thoroughly  washing  out  the  excess  acid 
with  cold  distilled  water.  The  hydrochloric  acid  reacts  with  the 
calcium  gelatinate  forming  calcium  chloride,  which  is  washed  out, 
and  pure  gelatin  remains.3 

The  Distinction  between  Gelatin  and  Glue. — The  chemical 
distinction  between  gelatin  and  glue  is  merely  a  distinction  of 
purity.  Gelatin  is  a  protein  of  a  supposedly  definite  molecular 
constitution,  derived  by  a  (chiefly)  hydrolytic  decomposition  of 
collagen,  and  possessing  certain  well-defined  physical  and  chem- 
ical characteristics  which  have  already  been  set  forth.  Among 
these  the  precipitability  in  a  half  saturated  solution  of  magnesium 

1  BLASEL  and  MATULA,  Biochem.  Z.,  68  (1914),  417. 

2  J.  LOEB,  J.  Gen.  PhysioL,  1  (1918),  45. 

3  J.  LOEB,  loc.  cit.;  Cf.  also  ADA  M.  FIELD,  /.  Am.  Chem.  Soc.,  43  (1921), 
667. 


CHEMISTRY  OF  GELATIN  63 

sulphate  may  be  emphasized  as  a  point  of  differentiation  of  the 
pure  protein  from  its  products  of  hydrolytic  decomposition.  If 
the  material  in  hand  contains  such  cleavage  products  in  quantity, 
or  if  the  protein  gelatin  is  intermixed  with  other  proteins  as 
mucin,  keratin,  elastin,  etc.,  then  it  cannot  of  course  be  regarded 
as  pure  gelatin.  The  cleavage  products,  especially  the  proteose 
obtained  upon  heating  gelatin  in  water,  have-  been  spoken  of  as 
]8  gelatin,  but  the  mixture  must  not  be  considered  as  true  gelatin. 

In  commercial  parlance  a  gelatin  differs  from  a  glue  only  in  j^ 
that  the  former  is  a  very  high  grade  product,  is  of  high  jelly 
strength,  is  light  in  color,  gives  solutions  that  are  reasonably 
clear,  is  sweet,  and  does  not  contain  excessive  impurities.  An 
edible  gelatin  differs  from  a  technical  gelatin  in  containing  only 
such  traces  of  harmful  ingredients  as  are  permitted  by  the  Pure 
Food  Laws,  and  which  is  produced  from  such  stock,  and  by  such 
sanitary  methods,  that  objection  may  not  be  had  +&  it  from  an 
ethical  point  of  view. 

The  highest  grades  of  glue  are  usually  designated  as  technical 
gelatins.  In  glue  manufacture,  however,  provision  is  not  usually 
maintained  to  carefully  separate  the  pure  collagen  from  other 
impure  stock,  with  the  result  that  many  impurities,  both  organic 
and  inorganic,  may  be  introduced.  And  in  order  to  extract 
the  maximum  amount  of  glue  the  cooking  is  prolonged  at  high 
temperatures  with  the  result  that  a  considerable  part-  of  the 
protein  is  hydrolyzed  to  smaller  molecules. 

The  best  grades  of  gelatin  are  converted  into  chemical  gelatin 
by  dialyzing  out  the  mineral  impurities,  using  a  dilute  acid 
solution  and  by  precipitation  with  alcohol. 

3.  Keratin. — The  keratins  are  found  in  the  hard  structure  of 
the  nails,  hair,  horns,  hoofs,  feathers,  wool,  tortoise  shell,  whale- 
bone, etc.  Keratin  is  also  found  in  brain  and  nerve  tissue,  and 
known  as  neurokeratin.  The  membrane  of  some  varieties  of 
eggs  is  likewise  a  keratin. 

The  most  characteristic  property  of  the  keratins  is  the  unusu- 
ally high  content  of  cystine,  and  consequently  of  sulphur,  which 
they  reveal.  The  elementary  composition  of  keratins  obtained 
from  various  sources  is  shown  in  the  following  table. 

It  is  observed  that  while  the  sulphur  is  relatively  high  in  most 
cases,  yet  it  varies  quite  considerably.  It  seems  to  be  present, 
for  the  most  part,  in  a  somewhat  loose  combination,  and  may  be 
largely  removed  by  the  action  of  alkalies  or  water  at  high  tern- 


64  GELATIN  AND  GLUE 

TABLE  18. — ELEMENTARY  COMPOSITION  OF  KERATINS 


Source  of  keratin 

Car- 
bon 

Hydro- 
gen 

Nitro- 
gen 

Sulphur 

Oxy- 
gen 

Authority 

Human  hair  
Wool  

50.65 
50  65 

6.36 

7  03 

17.14 
17  71 

5.00 
4  61 

20.95 
20  00 

v.  Laar 
Schorer 

Rabbit  fur  

49.45 

6.52 

16.81 

4.02 

23.20 

Klihne     and 

Nails 

51  00 

6  94 

17  51 

2  80 

21  75 

Chittenden 
Mulder 

Horn  (cows)  
Hoof  (horses)  

51.03 
51.41 

6.80 
6.96 

16.24 
17.46 

3.42 
4.23 

22.51 
19.94 

Tilanus 
Mulder 

Feathers  

52.46 

6.94 

17.74 

? 

22.86 

Tortoise  shell  

54.89 

6.56 

16.77 

2.22 

19.56 

Mulder 

Whalebone  
Egg  membrane 

51.86 
53  92 

6.87 
7  33 

15.70 
15  08 

3.60 
1  44 

21.97 

v.  Kerkhoff 
Pregl 

Neurokeratin  

56.99 

7.53 

13.15 

1.87 

20.46 

Kiihne      and 
Chittenden 

peratures,  especially  under  pressure.  With  alkalies  it  forms 
sulphides,  and  with  water  it  decomposes  into  hydrogen  sulphide 
and  mercaptans,  leaving  what  Bauer1  calls  atmidkeratin  and  atmid- 
keratose.  Despite  the  looseness  of  the  combination,  Morner 
concludes  that  the  sulphur  is  practically  all  present  in  the  form  of 
cystine.  The  following  table  shows  the  cystine  content  of  a 
number  of  keratins : 


TABLE  19. — CYSTINE  CONTENT  OF  KERATINS 


Source  of  keratin 

Cystine 

Authority 

Source  of  keratin 

Cystine 

Authority 

13  9 

Morner 

Ox  hoof           .... 

5  4 

Buchtala 

14  0 

Buchtala 

Horse  hair        

8.0 

Human  hair 

13.0 

Buchtala 

Horse  hoof  

3  2 

Buchtala 

Human  hair       ... 

14.5 

Buchtala 

Pig   bristles  

7.2 

Buchtala 

Human  hair  (white)  . 
Human  nails   

11.6 

5.2 

Buchtala 
Buchtala 

Pig    hoof  
Sheep  wool  

2.2 
7.3 

Buchtala  i 
Abderhalden 

Ox  hair  

7.3 

Buchtala 

Sheep  horn  

7.3 

Abderhalden 

Ox  horn  

6.8 

Morner 

Egg  membrane  (hen) 

7.62 

Kdrner 

The  keratins  from  different  sources  also  vary  greatly  in  their 
content  of  other  amino-acids  besides  cystine.  Thus  glutamic 
acid  is  present  to  the  extent  of  17.2  per  cent  in  the  keratin  from 

1  BAUER,  Z.  physiol  Chem.,  35  (1902),  343. 


CHEMISTRY  OF  GELATIN 


65 


sheep  horn,1  but  to  only  2.3  per  cent  in  that  from  goose  feathers.2 
Tyrosine  was  found  absent  in  the  keratin  from  the  shell  membrane 
of  the  hen's  egg,3  but  present  to  the  extent  of  10.6  per  cent  in  the 
egg  membrane  of  scyllium  stellar e.*  Other  variations  will  be 
noticed  from  the  following  table: 


TABLE    20. — DISTRIBUTION    OF    NITROGEN    IN    KERATINS 


Keratin1 

Keratin2 

Keratin3 

Keratin4 

Shell 

Egg 

from 
horse 
hair 

from 
sheep 
wool 

from 
goose 
feathers 

from 
sheep 
horn 

mem- 
brane5 of 
hen's 

mem- 
brane2 of 
scyllium 
stellare 

egg 

Glycine        

4.7 

0.58 

2.6 

0.45 

3.9 

2.6 

Alanine 

1.5 

4.40 

1.8 

1.6 

3.5 

3.2 

Valine       

0.9 

2.80 

0.5 

4.5 

1.1 

Leucine 

7.1 

11.5 

8.0 

15.3 

7.4 

5.8 

Serine  

0.6 

0.1 

0.4 

1.1 

Aspartic  acid.  .  .  . 

0.3 

2.3 

1.1 

2.5 

1.1 

2.3 

Glutamic   acid.  .  . 

3.7 

12.9 

2.3 

17.2 

8.1 

•7.2 

Cystine  

7.98 

7.3 

7.5 

7.62 

? 

Phenylalanine  .  .  . 

0.0 

0.0 

1.9 

3.3 

Tyrosine  

3.2 

2.9 

3.6 

3.6 

0.0 

10.6 

Proline  

3.4 

4.4 

3.5 

3.7 

4.0 

4.4 

Histidine  

0.61 

.... 

1.7 

Arginine  

4.45 

.... 

2.7 

.... 

3.2 

Lysine 

1.12 

0.2 

3.7 

1  ABDERHALDEN  and  WELLS,  Z.  physiol.  Chem.,  46  (1905),  31. 

2PREGL,  IOC.  Cli. 

3  ABDERHALDEN  and  LE  CONNT,  loc.  cit. 

4  ABDERHALDEN  and  VOITINOVICI,  loc.  cit. 

5  ABDERHALDEN  and  EBSTEIN,  loc.  cit. 

The  horny  material  of  birds'  gizzards,  known  as  koilin,  which  is 
very  low  in  its  cystine  content,  and  egg  membranes  are  not 
considered  to  belong  to  the  keratin  group  by  Hofmann  and 
Pregl5  because  of  their  low  sulphur  content. 

The  keratins  are  resistant  to  the  ordinary  solvents,  being 
insoluble  in  alcohol  and  ether,  and  unaffected  by  boiling  at 

1  ABDERHALDEN  and  VOITINOVICI,  Z.  physiol.  Chem.,  62  (1907),  348. 

2  ABDERHALDEN  and  LECONNT,  ibid.,  48  (1905),  40. 

3  ABDERHALDEN  and  EBSTEIN,  ibid.,  48  (1906),  530. 

4  PREGL,  ibid.,  66  (1908),  1. 

5  HOFMANN  and  PREGL,  Z.  physiol.  Chem.,  62  (1907),  448. 

5 


66  GELATIN  AND  GLUE 

ordinary  pressures.  Under  pressure  at  150°C.  and  higher  they 
decompose,  evolving  hydrogen  sulphide  or  mercaptans,  and  pro- 
duce a  solution  which,  unlike  gelatin,  does  not  produce  a  jelly  on 
cooling,  and  if  it  is  evaporated  to  dryness  the  residue  will  again 
become  insoluble. 

The  keratins  are  hygroscopic,  but  do  not  swell  appreciably  in 
moist  air  or  water.  In  alkalies  no  effect  is  noted  until  the  con- 
centration becomes  rather  high,1  but  in  20  per  cent  solutions  of 
alkali  the  keratins  swell  and  dissolve  on  boiling.  If  such  a 
solution  is  neutralized  with  an  acid  a  white  flocculent  precipitate 
is  formed,  and  hydrogen  sulphide  is  evolved.2 

Acids  in  general  dissolve  the  keratins  more  or  less  completely, 
swelling  taking  place  in  the  cold  acid,  and  solution  being  effected 
on  boiling.  Sulphuric  acid  seems  to  be  best  suited  for  the  decom- 
position of  the  keratins,  for  although  hydrochloric  acid  attacks 
most  of  these  substances,  hair  remains  unaffected  even  by  fuming 
hydrochloric  acid. 

Smith3  states  that  keratin  is  unaffected  by  either  pepsin  or 
trypsin,  but  Dreaper4  questions  the  correctness  of  the  statement 
so  far  as  it  pertains  to  trypsin. 

Keratin  reacts  with  Millon's  reagent,  and  gives  the  xanthopro- 
teic  reaction. 

Keratin  may  be  prepared5  by  subjecting  a  horny  structure, 
quills,  etc.,  to  digestion  with  a  mixture  of  ether  and  alcohol,  and 
subsequently  with  an  acid  solution  of  pepsin  at  40°C.  The 
residue  is  dissolved  by  prolonged  boiling  in  acetic  acid,  and  the 
solution  strained,  concentrated,  and  dried  to  a  powder  which  will 
be  brownish-yellow,  and  insoluble  in  water,  alcohol,  ether,  dilute 
acids,  and  pepsin  hydrochloride. 

A  very  interesting  proposition  has  resulted  from  the  relation 
of  keratin  to  hair  and  wool.  Professor  Zuntz6  of  Berlin  conceived 
the  idea  that  inasmuch  as  the  keratinous  tissues  were  exception- 
ally rich  in  cystine,  why  should  it  not  be  a  fruitful  proceeding  to 
feed  hydrolyzed  keratin,  or  cystine,  to  the  bald  and  thereby 
abolish  man's  concern  over  hirsute,  or  absence  of  hirsute,  develop- 

1  SMITH,  J.  Chem.  Soc.,  46  (1884),  1398. 

2  DREAPER,  "Allen's  Commercial  Organic  Analysis,"  4th  ed.  vol.  8  (1913), 
675. 

3  SMITH,  loc.  tit. 

4  DREAPER,  loc.  cit. 

5  Official  German  Pharmacopoea  3rd  ed. 

6  ZUNTZ,  Deut.  med.  Wochschr.,  46  (1920),  145. 


CHEMISTRY  OF  GELATIN  67 

ment.  Professor  Zuntz  first  experimented  upon  himself,  and  in 
his  report  stated  that  whereas  before  he  had  begun  his  experi- 
ments his  average  growth  of  hair  on  his  head  and  face  had  been 
5  mg.  per  day,  after  treatment  for  2  months,  during  which  time 
he  consumed  from  1  to  1.5  g.  of  hydrolyzed  keratin  daily,  his 
average  growth  of  hair  had  increased  to  9.22  mg.  He  extended 
his  investigation  to  others  and  reported  that  two  youths  who  had 
become  nearly  bald  due  to  the  war  were  greatly  benefited,  but 
another  became  disgusted  with  the  treatment  since  it  made  it 
necessary  for  him  to  shave  twice  a  day.  On  applying  the  experi- 
ment to  sheep  Professor  Zuntz  found  that  the  yield  of  wool  was 
increased  to  174  per  cent  of  the  original  production,  and  the 
diameter  of  the  individual  wool  fibers  increased  one-third. 

The  idea  was  at  once  exploited  by  a  German  firm  who  put  two 
preparations  of  hydrolyzed  keratin,  or  Humagsolan,  on  the 
market:  one  to  be  used  by  man  and  the  other  by  sheep. 

A  test  of  the  efficacy  of  such  treatment  was  made  by  Fuchs1 
in  Vienna  upon  fifteen  men  and  two  women,  but  his  experiments 
failed  to  confirm  those  of  Zuntz.  The  Journal  of  the  American 
Medical  Association2  also  is  inclined  to  regard  the  proposition 
with  amusement,  albeit  the  record  of  Professor  Zuntz  entitles 
the  subject  to  a  thorough  test. 

4.  Elastin. — The  connective  tissues  of  the  higher  animals 
contain  a  large  amount  of  the  protein  known  as  elastin.  The 
cervical  ligament,  ligamentum  nuchce,  of  the  ox  contains  as  much 
as  74.6  per  cent  of  its  dry  weight  of  elastin.  Other  forms  of 
connective  tissue  contain  much  less  elastin,  but  it  is  present  to 
the  extent  of  4.4  per  cent  in  the  dry  matter  of  the  Achillis 
tendon  of  the  ox. 

Elastin  is  a  simple  protein  belonging  to  the  albuminoid  class. 
Its  elementary  composition  is  given  below. 

It  has  been  a  matter  of  dispute  whether  or  not  sulphur  was  a 
necessary  constituent  of  elastin.  Schwarz  obtained  a  small 
amount  of  sulphur  from  the  elastin  of  the  aorta,  but  he  found  it 
could  be  removed  by  the  action  of  alkalies  without  altering  the 
properties  of  the  substance.  It  is  generally  conceded,  however, 
that  sulphur  is  a  constituent  part  of  the  elastin  molecule.3 

The  amino-acid  constitution  of  elastin  is  given  in  the  table  on 

1  FUCHS,  Weiner.  klin.  Wochschr.,  33  (1920),  707. 

2  /.  Am.  Med.  Assn.,  75  (1920),  676;  748. 

3  See  RICHARDS  and  GIES,  Am.  J.  Physiol.,  7  (1902). 


68 


GELATIN  AND  GLUE 


page  36.  An  examination  of  the  data  will  show  that  the  three 
simple  amino-acids,  glycine,  leucine,  and  alanine,  comprise  more 
than  50  per  cent  of  the  elastin  molecule.  It  will  be  recalled  that 
gelatin  is  also  very  high  in  glycine,  but  the  latter  protein  differs 
from  elastin  in  containing  only  7.1  per  cent  of  leucine  as  com- 
pared with  more  than  21  per  cent  of  that  amino-acid  in  elastin. 
This  protein  is  also  rich  in  phenylalanine,  but  very  poor  in  the 
diamino-acids  and  the  hexone  bases.  This  fact  has  been  used  as 
an  argument  against  the  existence  of  the  elastin  molecule  as  a 
unit  substance. 

TABLE  21. — ELEMENTARY  COMPOSITION  OF  ELASTIN 


Source 

Carbon 

Hydrogen 

Nitrogen 

Sulphur 

Oxygen 

Observer 

Elastin     from     ligamentum 

54.32 

6.99 

16.75 

21.94 

Horba- 

nuchce. 

czewski1 

Elastin     from     ligamentum 

54.24 

7.27 

16.70 

.... 

21.79 

Chitten- 

nuchce. 

den    and 

Hart" 

Elastin  from  the  aorta  

53.95 

7.03 

16.67 

0.38 

Schwarz3 

1  HORBACZEWSKI,  Z   physiol.  Chem.,  6  (1882),  330. 

2  CHITTENDEN  and  HAKT,  Z.  Biol.  25  (1889). 

s  SCHWARZ,  Z.  physiol.  Chem.,  18  (1894),  487. 

When  elastin  is  partially  hydrolyzed  by  the  action  of  pro- 
teolytic  enzymes,  superheated  water,  or  dilute  acids,  two  pro- 
teoses  are  obtained  which  have  been  called  by  Chittenden  and 
Hart1  proteoelastose  and  deuteroelastose.  The  two  are  distin- 
guished by  differences  in  solubility,  the  former  being  the  less 
soluble  and  separating  out  upon  boiling  in  water.  It  is  precipi- 
tated by  acids  and  potassium  ferrocyanide.  The  latter  is  not 
acted  upon  by  these  reagents. 

Pure  elastin  is  a  yellowish-white  powder,  when  dry,  and  in  the 
moist  condition  is  stringy  and  tacky.  It  does  not  dissolve  in 
water,  alcohol,  or  ether,  nor  in  dilute  acids  or  alkalies.  When 
heated  with  stronger  acids,  however,  it  goes  readily  into  solution. 
Strong  caustic  alkalies  dissolve  elastin  only  with  difficulty  and 
upon  heating.  Elastin  responds  to  the  test  with  Millon's 
reagent,  and  to  the  xanthoproteic  reaction. 

Elastin  is  best  prepared  from  the  ligamentum  nuchce  by  freeing 
the  ligament  of  other  proteins.  This  is  best  accomplished  by 

1  CHITTENDEN  and  HART,  loc.  cit. 


CHEMISTRY  OF  GELATIN  69 

boiling  with  successive  portions  of  water,  1  per  cent  caustic 
potash,  water,  and  acetic  acid.  The  residue  is  then  treated  for 
24  hours  with  cold  5  per  cent  hydrochloric  acid,  again  washed 
and  boiled  with  water,  and  lastly  treated  with  alcohol  and  ether. 
The  residue,  dried  and  powdered,  is  practically  pure  elastin. 

5.  Mucins  and  Mucoids. — The  mucins  are  found  to  occur 
generally  in  the  skins  of  the  amphibia  and  of  the  fishes,  and  are 
an  important  constituent  of  the  saliva,  the  tendons,  connective 
tissue,  cartilage,  skin  and  the  umbilical  cord  of  vertebrate 
animals.  These  proteins  are  found  also  in  the  skin  secretions  of 
snails.  Similar  substances  occur  frequently  in  the  shell-like 
coverings  of  the  invertebrates.  The  mucins  belong  to  the  class 
of  conjugated  proteins,  and  to  the  sub-group  glycoproteins.  That 
is,  they  contain  both  a  protein  and  a  carbohydrate  group  in 
their  molecule.  The  carbohydrate  moiety  consists  probably  of 
chondroitic  acid. 

The  mucoids  are  very  similar  to  the  mucins,  but  present  a 
slightly  different  set  of  properties,  and  therefore  have  been  given 
a  distinguishing  name.  The  true  mucins  are  described  as 
giving  a  mucilaginous  and  ropy  solution  in  water,  or  water  con- 
taining a  trace  of  alkali,  and  giving  a  precipitate  with  acetic  acid 
which  is  not  soluble  in  an  excess  of  the  acid.  The  mucoids,  on 
the  other  hand,  do  not  produce  mucilaginous  and  ropy  solutions, 
and  the  precipitate  obtained  with  acetic  acid  is  soluble  in  an 
excess  of  the  acid.  The  chemical  basis  of  differentiation  has  not 
been  satisfactorily  accounted  for.  The  mucoids  show  a  higher 
sulphur  content  than  the  mucins,  and  they  occur  probably  as  the 
calcium  salt,  while  the  mucins  exist  chiefly  in  the  form  of  the 
potassium  salt. 

The  elementary  composition  of  mucins  and  mucoids  obtained 
from  different  sources  is  given  below. 

In  a  study  of  the  hydrolysis  of  mucoid  obtained  from  tendons 
Levene1  found  that  with  dilute  acids  the  mucoid  was  decomposed 
into  galactose,  galactosamine,  and  sulphuric  acid,  and  with  stronger 
acids  he  o'btained  leucine,  tyrosine,  levulinic  acid,  and  acetic  acid. 
Chondroitic  acid  was  also  decomposed  into  glucosamine,  or 
levulinic  acid  derived  from  it,  glycuronic  acid,  sulphuric  acid,  and 
acetic  acid.  Since  the  same  cleavage  products  are  obtained  by 
the  hydrolysis  of  mucoid  as  are  derived  from  chondroitic  acid  it 
was  a  natural  inference  that  chondroitic  acid,  or  a  very  similar 
,  Z.  physiol.  Chem.,  31  (1900),  395. 


70  GELATIN  AND  GLUE 

TABLE  22. — ELEMENTARY  COMPOSITION  OF  MUCINS  AND  MUCOIDS 


Source 

Carbon 

Hydrogen 

Nitrogen 

Sulphur 

Oxygen 

Observer 

Mucin  from  saliva  

48.26 

6.91 

10.70 

1.40 

Mulleri 

Mucin  from  submaxillary 

48.84 

6.80 

12.32 

0.84 

41.30 

Hammer- 

stein2 

Mucin  from  snail  

50.32 

6.84 

13.65 

1.75 

27.44 

Hammer- 

stein2 

Mucoid  from  tendon    .  .  . 

47.47 

6.68 

12.58 

2.20 

31.07 

Cutter    and 

Gies« 

Mucoid  from  cartilage  .  .  . 

47.30 

6.42 

12.58 

2.42 

31.28 

Morner4 

Mucoid  from  ossein   .... 

47.07 

6.69 

11.98 

2.41 

31.85 

Hawk  and 

Gies5 

1  MULLER,  Z.  Biol.,  42  (1901). 

2  HAMMERSTEIN,  Z.  physiol.  Chem.,  12  (1888),  163 
s  CUTTER  and  GIES,  Am.  J.  Physiol.,  6  (1901),  155. 
«  MORNER,  Skand.  Arch.  Physiol.,  1. 

»  HAWK  and  GIES,  Am.  J.  Physiol.,  5  (1901),  388. 

substance,  constitutes  the  prosthetic  group  of  mucoid.  Chon- 
droitic  acid  is  also  one  of  the  major  constituents  of  cartilage, 
which  fact  brings  the  mucins  and  cartilages  into  a  close  chemical 
relationship.  It  is  also  of  interest  to  the  biochemist  that  chitin, 
the  principal  constituent  of  the  hard  parts  of  the  anthropoda, 
also  yields  on  hydrolysis  glucosamine,  acetic  acid,  and  sulphuric 
acid.  From  these  analogies  it  would  seem  that  chitin,  cartilage, 
and  mucin  are  closely  related  from  an  evolutionary  point  of  view, 
and  this  relationship  has  been  the  basis  of  a  theory  by  Gaskell 
and  Putten  to  the  effect  that  the  anthropods  were  the  ancestors 
of  the  vertebrates.  The  chitin  is  considered  as  having  combined 
with  glycuronic  and  sulphuric  acids  to  form  the  matrix  of  the 
mucin,  mucoid,  and  cartilage  of  the  vertebrates. 

The  reactions  of  chondroitic  acid  may  be  written  as  follows: 


Ci8H27NSO17 

Chondroitic  acid 

Ci8H27NOi4 

Chondoitin 

Ci2H21NOn 

Chondrosin 


H2O 
3H2O 


Ci8H27NO14 

Chondroitin 


Chondrosin 


H2SO4 
3CH3COOH 

Acetic  acid 


H20  -»   C6Hi0O7  +  C6HnNH2O5. 

Glycuronic  acid  Glucosamine 


The  terms  chondromucoid,  tendomucoid,  osseomucoid,  etc.,  are 
used  to  designate  the  source  of  the  mucoid  in  question,  the  above 
referring  respectively  to  the  mucoid  obtained  from  cartilage 
(chondro-  signifying  cartilaginous),  from  tendons,  and  from  the 
organic  matrix  of  bones  (os-  meaning  bone).  The  term  chondro- 


CHEMISTRY  OF  GELATIN  71 

protein,  however,  refers  to  a  conjugated  protein  in  which  the 
prosthetic  group  is  chondroitic  acid. 

The  mucins  are  not  all  equally  resistant  to  chemical  decomposi- 
tion. Some,  as  the  submaxillary  mucin,  are  readily  affected  by 
very  dilute  alkalies,  as  lime  water,  while  others,  as  tendomucoid, 
are  not  altered  by  such  treatment.  If  the  strength  of  the  alkali 
be  increased  to  the  equivalent  of  5  per  cent  of  potassium  hydroxide 
the  submaxillary  mucin  will  be  decomposed  into  alkali  albu- 
minate,  proteose-  and  peptone-like  substances,  and  acid  organic 
substances.  Submaxillary  mucin  is  soluble  in  very  dilute  hydro- 
chloric acid,  while  the  mucin  obtained  from  snails  and  from  the 
saliva  is  not  soluble.  The  precipitates  obtained  with  acetic 
acid  differ  also,  that  from  saliva  being  flaky  while  the  sub- 
maxillary mucin  is  thrown  down  by  acetic  acid  in  tough  fibrous 
masses. 

In  many  respects,  however,  the  properties  are  similar  for  all 
mucins.  In  the  moist  condition  they  form  tough  rubbery  lumps. 
On  drying  they  form  a  white  or  slightly  amber  or  grayish  colored 
powder.  They  are  insoluble  in  water  or  dilute  acids,  but  if 
traces  of  alkali  are  present  they  will  go  readily  into  solution  and, 
due  to  the  natural  acidity  of  their  molecule,  if  the  alkalinity  of  the 
water  solution  is  not  greater  than  a  certain  small  value,  the 
solution  resulting  may  be  perfectly  neutral.  Most  acids  will 
throw  the  mucin  out  of  solution  but,  as  the  precipitate  is  soluble 
ifa  an  excess  of  strong  acid,  acetic  acid  is  ordinarily  used  for  this 
purpose.  The  presence  of  5  to  10  per  cent  of  sodium  chloride 
will,  however,  prevent  precipitation  by  acetic  acid.  Tannic  acid 
does  not  ordinarily  precipitate  mucin,  but  if  added  to  the  salt- 
acetic  acid  solution,  a  heavy  precipitation  results.  Potassium 
ferrocyanide  added  to  a  similar  solution  makes  it  highly  viscous. 
Alcohol  has  no  effect  upon  mucin  in  a  neutral  water  solution,  but 
if  salts  are  present  the  mucin  will  be  thrown  out.  Typical 
protein  coagulation  does  not,  however,  take  place  by  the  action 
of  any  of  the  above  reagents,  nor  by  boiling  with  water.  Lead 
acetate  and  alum  may  be  used  as  precipitating  agents.  The 
mucins  react  in  the  usual  way  to  the  Millon's  and  Adamkiewicz' 
reagents,  but  yield  a  rose-red  coloration  with  the  biuret  test. 

Landwehr1  found  that  by  the  action  of  superheated  steam  or 
alkali  a  complex  carbohydrate,  which  he  called  animal  gum,  was 

LANDWEHR,  Z.  physiol.  Chem.,  8  (1884),  122;  9  (1885),  361. 


72  GELATIN  AND  GLUE 

split  off.  Other  investigators1  have  failed  to  confirm  this  finding, 
but  have  obtained  instead  a  nitrogenous  carbohydrate. 

Mucin  is  best  obtained  from  the  submaxillary  glands  by 
extracting  the  macerated  glands  with  water.  The  filtrate  is 
treated  with  25  per  cent  hydrochloric  acid  until  the  solution 
contains  0.15  per  cent  of  the  acid.  A  precipitate  which  appears 
on  the  first  additions  of  acid  redissolves  as  more  acid  is  added. 
The  mucin  is  then  thrown  out  of  solution  by  pouring  into  two  or 
three  volumes  of  water.  Mucin  from  other  sources,  as  the  ten- 
don, is  best  prepared  by  extracting  with  lime  water  and  precipi- 
tating with  acetic  acid.  The  material  is  purified  by  repeated 
solution  in  dilute  alkali  and  precipitation  with  acetic  acid. 

Mucin  is  of  an  unusual  personal  interest  to  the  human  race 
through  its  functions  as  a  salivary  secretion  and  from  the  fact  that 
two  of  the  most  prevalent  diseases  of  the  teeth,  e.g.,  dental  caries 
and  salivary  calculi,  are  the  result  of  its  chemical  behavior.  The 
physiological  function  of  mucin  in  the  mouth  appears  to  be  asso- 
ciated with  the  "buffer  action"2  which  it  possesses.  That  is, 
mucin  is  capable  of  reducing  the  acidity  of  fruit  juices  or  other 
acids  introduced  into  the  mouth  to  a  point  where  such  acids  are 
no  longer  injurious  to  the  teeth.  If  mucin  were  not  present  these 
acids  would  in  the  course  of  years  seriously  corrode  the  teeth  and 
render  them  valueless.  The  film  of  mucin  protects  them  from 
this  disaster. 

But  although  acids  introduced  as  such  are  largely  inhibited 
in  their  solvent  action,  the  mucin  cannot  prevent  particles  of 
food  from  becoming  lodged  between  the  teeth  and  in  protected 
spots  at  the  margin  of  the  gums.  These  food  particles,  especially 
the  carbohydrates,  are  attacked  readily  by  the  bacterial  flora  of 
the  mouth  and  upon  fermentation  produce  organic  acids.  Dental 
caries  is  primarily  a  decalcification  of  the  enamel  and  dentin  of 
the  tooth  by  such  organic  acids.3  In  order,  however,  for  the 
bacterial  action  to  be  effective  there  must  not  only  be  particles  of 
food  lodged  in  the  teeth,  but  also  a  protective  covering  of  "mu- 
coid  plaque"  by  which  acids  formed  are  held  against  the  tooth 
and  protected  from  dilution  and  neutralization  by  the  salivary 
secretions.  Under  this  mucoid  plaque  the  process  of  acid  pro- 
duction and  enamel  dissolution  may  go  on  unmolested  with  the 
result  that  a  lesion  in  the  tooth  is  accomplished. 

1  Cf.  HAMMERSTEIN,  ibid.,  12  (1888),  163;  and  FOHN,  ibid.,  23  (1897),  347. 

2  See  page  587. 

3  BUNTING,  "  Ward's  American  Textbook  of  Operative  Dentistry,"  p.  136. 


CHEMISTRY  OF  GELATIN  73 

Tartar  or  salivary  calculi  that  becomes  deposited  on  the  teeth 
is  composed  of  "masses  of  tricalcium  phosphate  and  calcium 
carbonate  built  upon  a  mucinous  or  colloidal  matrix  and  arranged 
in  concentric  layers  about  a  central  nidus."  When  first  deposited 
the  flaky  white  precipitate  is  soft  and  viscous,  insoluble  in  water, 
but  easily  removable  by  a  brush  or  finger.  This  soon  becomes 
hard  and  not  easily  removable.  The  calculi  is  supposed  to  be 
produced  by  precipitation  of  the  mucin  from  the  saliva  by  means 
of  the  acids  resulting  from  fermentation  of  food  particles,  or  by 
the  free  acid  of  fruits,  etc.  The  phosphates  and  carbonates  of 
calcium  that  are  present  in  the  saliva  are  carried  down  and 
deposited  with  the  mucin. 

The  customary  practice  of  tooth  preservation  lies  in  the 
removal  of  these  mucinous  precipitates  and  films  by  means  of 
brushing  with  an  abrasive  as-  precipitated  chalk,  mixed  with  a 
little  soap.  A  far  more  scientific  procedure  would  be  to  dissolve 
the  precipitated  mucin  and  mucinous  films  by  some  solvent. 
Any  alkali  or  alkali  salt  of  a  weak  acid  of  a  pH  value1  of  10.5  to 
11.0  is  found  to  be  a  ready  solvent  for  mucin.  The  extraordinary 
efficiency  of  the  liquid  dentifrice  recently  prepared  by  Vogt2  is 
attributable  to  the  scientific  adaptation  of  this  principle. 

6.  Chondrigin  and  Chondrin. — In  1900  and  as  late  as  1910  the 
terms  chondrigin  and  chondrin  were  in  common  usage.  Chondri- 
gin was  considered  to  be  the  principal  albuminoid  constituent 
of  the  matrix  of  hyaline  cartilage  and,  upon  boiling  with  water, 
to  slowly  go  into  solution  with  the  formation  of  chondrin.  The 
chondrigin  was  described  as  a  substance  quite  analogous  to 
collagen,  but  somewhat  less  soluble  in  water  and  possessing 
many  of  the  reactions  of  mucin.  On  cooling  an  aqueous  solution 
of  the  chondrigen  a  jelly  was  produced,  but  it  possessed  less 
strength  than  an  equivalent  one  of  gelatin. 

In  1893  the  researches  of  C.  Morner3  and  of  Morochowetz4 
indicated  that  what  had  been  regarded  hitherto  as  chondrigin 
was  in  reality  a  mixture  of  collagen  with  other  substances,  chiefly 
chondromucoid,  albuminoid,  and  chondroitic  acid.  These  sub- 
stances have  already  been  described.  Morner  in  an  examination 
of  cartilage  found  only  16.4  per  cent  of  nitrogen.  Gelatin 
averages  about  17.7  per  cent  nitrogen.  By  an  examination  of 

1  See  page  581. 

2  C.  C.  VOGT,  personal  communication. 

3  C.  MORNER,  Skand.  Arch.  PhysioL,  1. 

4  MOROCHOWETZ,  Verh.  d.  naturh.  med.  Vereins  zu  Heidelberg,  1,  Heft  5. 


y 


74  GELATIN  AND  GLUE 

sections  of  the  cartilage  under  a  microscope  he  found  that  chon- 
droitic  acid  and  chondromucoid  surrounded  the  cells  as  spherical 
shells.  These  have  been  known  as  Morner's  chondrin-balls. 
By  staining  with  methyl-violet  he  was  able  to  observe  that  these 
balls  lay  in  the  meshes  of  a  net-like  structure  of  collagen,  and  by 
treating  these  sections  with  dilute  hydrochloric  acid,  followed  by 
dilute  potassium  hydroxide,  he  was  able  to  dissolve  out  the 
chondrin  balls,  leaving  the  super-structure  of  collagen.  This 
latter  yielded,  on  boiling  with  water,  a  gelatin  having  all  of  the 
usual  properties  and  reactions  of  such. 

Morochowetz  examined  a  number  of  specimens  of  cartilage 
obtained  from  a  variety  of  sources,  and  succeeded  in  separating 
that  material  into  mucin  and  gelatin  by  dissolving  out  the  former 
with  a  dilute  solution  of  an  alkali.  The  gelatin  obtained  by 
boiling  the  residue  with  water  possessed  the  usual  properties 
ascribed  to  that  substance. 

These  investigations  have  been  confirmed  by  Krukenberg 
and  Landwehr,  but  exception  is  taken  to  them  by  Schiitzen- 
berger  and  Bourgeois  who  claim  that  the  products  of  hydrolysis 
of  chondrin  by  boiling  with  barium  hydroxide  do  not  contain 
any  glycine  whatsoever,  and  show  three  times  as  much  acetic 
acid  as  ordinary  gelatin  treated  in  a  similar  way.  Dawidowski1 
has  also  considered  chondrin  as  a  distinct  substance,  but  convert- 
ible to  gelatin  upon  boiling  with  a  caustic  alkali. 

The  material  which  has  been  known  as  chondrin  may  be 
prepared  by  boiling  the  cartilage  of  the  ribs  or  larynx  for. 24  to  48 
hours  with  water,  or  under  pressure  at  120°C.  for  3  or  4  hours. 
The  undissolved  residue,  consisting  of  nonsoluble  proteins, 
elastin,  mucin,  etc.,  is  filtered  off,  and  the  chondrin  is  precipi- 
tated by  pouring  into  a  large  volume  of  alcohol.  The  precipi- 
tate may  be  redissolved  in  hot  water,  concentrated,  allowed  to  gel 
by  cooling,  and  dried  similarly  to  gelatin.  The  product  is  a 
hard,  amber-colored,  transparent  substance,  insoluble  in  cold 
water,  alcohol,  or  ether.  It  is  differentiated  from  normal  gelatin 
by  being  precipitated  from  solution  by  mineral  acids,  organic 
acids,  alum,  the  sulphates  of  iron  and  aluminum,  and  the  acetate 
and  sub-acetate  of  lead.  It  resembles  gelatin  in  being  thrown 
out  of  solution  by  alcohol,  tannin,  and  mercuric  chloride. 

7.  Melanins  and  Humins. — The  melanins  are  dark  brown, 
reddish-brown,  or  black  pigments  which  are  found  in  hair,  in  the 
i  DAWIDOWSKI,  "Glue,  Gelatin,  etc.,"  2nd  ed.,  Philadelphia  (1905),  5. 


CHEMISTRY  OF  GELATIN  75 

epidermis  of  dark-colored  animals  and  races,  and  in  a  few  other 
less  conspicuous  places,  as  the  choroid  coat  of  the  eye  and  in 
pathological  growths,  as  tumors. 

Humins  have  been  described  as  the  dark-colored  pigments 
which  are  usually  formed  upon  the  acid-hydrolysis  of  proteins, 
and  have  been  variously  ascribed  as  due  to  the  presence  of 
tryptophane,  tyrosine,  glucosamine,  lysine,  etc.1  A  combination 
especially  favorable  for  the  formation  of  humin  is  the  simultane- 
ous presence  of  tryptophane  and  a  carbohydrate.2  Whether  or 
not  these  two  groups  of  pigments  are  one  and  the  same  is  a 
point  which  has  not  been  settled.  The  composition  and  the 
reactions  are  very  similar,  and  the  tendency  of  physiological 
chemists  is  to  regard  them  as  identical,  although  conclusive 
evidence  to  that  end  is  lacking. 

The  ultimate  source  of  the  color  of  these  pigments  has  been 
variously  attributed  to  iron,  to  sulphur,  and  to  particular  organic 
combinations.  Many  of  the  melanins  contain  iron,  and  con- 
siderable work  has  been  done  to  trace  their  origin  through  this 
element  but,  since  the  neucleo-proteins  and  many  albumins  also 
contain  iron,  and  since  melanins  have  been  found  which  were 
free  from  iron,  no  important  conclusions  could  be  derived  from 
these  investigations.  The  sulphur  content  has  also  been  found  not 
only  to  vary  within  very  wide  limits,  but  a  few  melanins  have  been 
found  in  which  sulphur  was  absent.  In  one  case  the  sulphur  con- 
tent was  as  high  as  10.1  per  cent.  It  is  possible  that  such  variations 
may  have  been  due  to  impurities,  for  there  has  been  no  criteria 
of  purity  of  the  melanins.  Extraneous  pigments  derived  from 
the  blood  or  bile  or  other  sources  might  easily  contaminate  the 
substance  despite  the  utmost  care  of  the  investigator. 

The  intensity  of  the  coloring  power  of  the  melanins  is  very 
great  as  may  be  judged  by  results  obtained  by  Abel  and  Davis3 
who  found  that  the  skin  of  an  average  negro  contained  only  3.3  g. 
of  pigmentary  granules,  of  which  only  1  g.  comprised  the  actual 
pigment,  the  balance  being  a  colorless  substratum. 

The  elementary  composition  of  some  melanins  is  shown  below. 

The  melanins  are  resistant  to  chemical  action,  being  insoluble 
in  water,  alcohol,  ether,  benzene,  neutral  salt  solutions,  and 

1  SAMUELY,  Hofmeister's  Beitr.,  1  (1902),  229. 

2  GORTNER  and  BLISH,  /.  Am.  Chem.  Soc.,  37  (1915),  1630;  GORTNER  and 
HOLM,  ibid.,  39  (1917),  2477;  42  (1920),  632;  821. 

3  ABEL  and  DAVIS,  J.  Exptl.  Med.,  1  (1896),  361. 


76 


GELATIN  AND  GLUE 


dilute  acids.  They  are  readily  soluble,  however,  in  alkali  or 
alkali  carbonate  solutions,  from  which  they  may  be  reprecipitated 
by  neutralization  or  acidification  with  acids,  and  by  the  addition 
of  lead  acetate,  magnesium  sulphate,  or  barium  hydroxide.  Indol, 
skatol,  and  ammonia  have  been  obtained1  from  the  decomposition 
products  of  the  melanins,  but  bases,  phenol,  cystine,  tyrosine, 
and  leucine  have  not  been  found.2  The  hydro-aromatic  sub- 
stance, xyliton,  has  also  been  isolated  by  Wolff.3  The  melanins 
are  most  satisfactorily  extracted  from  pigmented  skin  by  boiling 
with  dilute  alkali  (0.05N  NaOH  for  one  hour),  and  precipitated 
from  the  solution  by  the  addition  of  acid  (HC1  to  0.3N).4 

TABLE  23. — ELEMENTARY  COMPOSITION  OF  MELANINS 


Source  of  melanin 

Carbon 

Hydrogen 

Nitrogen 

Sulphur 

Iron 

Observer 

Human  hair  

56.  14   to 

4.2       to 

8.5       to 

2.1        to 

0 

Sieber* 

57.6 

7.57 

11.6 

4.1 

Horse  hair  

58.44 

5.55 

11.7 

3.64 

.... 

Nencki    and 

Sieber" 

Skin  of  negro  

51.83 

3.86 

14.01 

3.6 

.... 

Abel     and 

Davis' 

Choroid  coat  of  eye   .... 

60.34 

5.02 

10.81 

0 

Sieber1 

Melanotic  sarcoma  

48.95   to 

4.23     to 

12.  58     to 

1.92     to 

0.41 

Zdenek  and  v 

54.93 

5.  15 

13.02 

8.23 

Zeynek4 

Sepia  

56.34 

3.61 

12.34 

0.  52 

Nencki    and 

Sieber* 

1  SIEBER,  Arch,  exp'l.  Path.  Pharm.,  20  (1885),  362. 

2  NENCKI  and  SIEBER,  ibid.,  24  (1888),  17. 

3  ABEL  and  DAVIS,  loc.  cit. 

4  ZDENEK  and  v.  ZEYNEK,  Z.  physiol.  Chem.,  36  (1902),  493. 

8.  Amyloid.  —  Amyloid  is  usually  considered  as  a  protein 
produced  by  pathological  processes,  but  Krawkow5  has  shown 
that  it  is  a  normal  constituent  of  old  cartilage  and  of  aortse. 
Under  pathological  conditions  it  is  present  in  the  liver,  kidneys, 
and  other  organs.  It  has  been  shown  to  be  an  albumin,  and  in 
composition  is  rather  high  in  sulphur,  containing  about  2.75 
per  cent  of  that  element.  Chondroitic  acid  was  obtained  from 
amyloid  by  Krawkow  which  places  it  in  the  class  of  glycoproteins 


1  HOPPE-SEYLER,  Hofmeister's  Beitr.,  5  (1904),  476. 

2  NENCKI  and  SIEBER,  loc.  cit. 

3  WOLFF,  Hofmeister's  Beitr.,  5  (1904),  476. 

4  YOUNG,  Biochem.  J.,  15  (1921),  118. 

6  KRAWKOW,  Arch,  exptl.  Path.  Pharm.,  40  (1897),  195. 


CHEMISTRY  OF  GELATIN  77* 

with  mucin.     This  constitution  has  been  denied,  however,  by 
Cohn.1 

Amyloid  is  insoluble  in  water  and  salt  solutions,  alcohol,  ether, 
and  dilute  acids.  In  boiling  water,  especially  under  pressure, 
and  in  dilute  alkalies  it  dissolves  readily.  Amyloid  differs  from 
mucin  and  keratin  in  that  it  will  also  slowly  dissolve  in  dilute 
acids.  In  concentrated  acids  and  alkalies  it  is  soluble  with 
decomposition.  Neuberg2  reports  that  amyloid  is  digested  by 
both  pepsin  and  trypsin,  but  it  is  usually  conceded  that  pepsin 
is  without  effect  on  the  pure  material.  Amyloid  reacts  positive 
to  all  the  usual  protein  color  tests. 

9.  Ichthylepidin. — Morner3  has  described  a  protein  which  he 
obtained  from  the  scales  of  certain  species  of  fish,  and  to  which  he 
gave  the  name  ichthylepidin.  In  the  scales  of  the  teleostean 
fishes  he  found  it  to  the  extent  of  24  per  cent,  but  in  the  scales  of 
the  elasmobrauchs,  the  mola  mola,  and  the  spheroides  maculatus 
it  was  apparently  absent.  The  balance  of  the  organic  matter  of 
the  scales  was  found  to  consist  almost  entirely  of  collagen. 

Green  and  Tower4  have  followed  up  the  early  work  of  Morner, 
and  have  analyzed  the  scales  of  a  large  variety  of  fishes  for 
ichthylepidin.  They  found  that  if  it  was  present  at  all  it  con- 
stituted about  one-fourth  of  the  total  organic  matter  of  the 
scales.  Some  chondroitic  acid  and  guanin  was  also  obtained. 
The  remaining  75  per  cent  was  collagen.  It  was  especially 
remarked  that  a  great  difference  exists  apparently  in  the  collagen 
of  scales  containing  no  ichthylepidin  and  of  those  containing 
this  protein.  If  ichthylepidin  is  present  the  collagen  is  very 
loosely  combined,  a  large  proportion  of  it  being  removed  by 
boiling  for  two  hours,  and  also  by  digestion  at  40°C.  with  0.1  per 
cent  hydrochloric  acid.  But  if  ichthylepidin  is  absent  the  col- 
lagen is  very  firmly  combined,  and  is  dissolved  only  by  long 
continued  boiling  (30  to  40  hours),  and  is  much  less  affected  by 
dilute  acid  digestion. 

From  a  large  number  of  analyses  Green  and  Tower  report  the 
following  data.  The  organic  matter  of  the  scales  consisted  of 
ichthylepidin  23.74  per  cent,  and  collagen  76.26  per  cent. 

1  COHN,  Z.  physiol  Chem.,  22  (1896),  153. 

2  NEUBERG,  Verh.  physiol.  Ges.,  (1904). 

3  MORNER,  Z.  physiol.  Chem.  24  (1897),  125;  37  (1902),  88. 

4  GREEN  and  TOWER,  U.  S.  Fish  Commission,  Bull.  21    (1901),   97-102. 


78 


GELATIN  AND  GLUE 
COMPOSITION  OF  MENHADEN  SCALES 


Air  dry 

Dried  at  105°C. 

Water 

20.58 
32.61 

0 
41.07 

Ash  

Organic  matter  

46.80 

Ichthy- 
lepidin      11.11 
Collagen    36  .  69 

58.93 

Ichthy- 
lepidin     13  .  99 
Collagen    44.94 

In  its  properties,  ichthylepidin  stands  very  close  to  elastin. 
It  is  insoluble  in  cold  and  hot  water,  and  in  dilute  acids  and 
alkalies  at  the  ordinary  temperature.  On  boiling  with  dilute 
acids  or  alkalies,  or  on  standing  in  the  cold  concentrated  solutions, 
it  dissolves.  It  is  digested  by  pepsin  in  acid  solution  and  by 
trypsin  in  alkaline  solution.  It  reacts  positive  to  the  biuret  test, 
to  Millon's  reagent,  and  to  the  xanthoproteic  reaction. 

In  composition  it  differs  from  elastin  in  containing  a  larger 
proportion  of  proline  and  glutamic  acid,  but  a  smaller  amount  of 
glycine.1  It  contains  a  considerable  amount  of  loosely  bound 
sulphur  as  shown  by  a  blackening  of  the  solution  when  boiled 
with  alkaline  solutions  of  lead  acetate. 

10.  Comparison  of  the  Properties  of  Gelatin  and  Its  Congeners. 
If  the  several  conspicuous  properties  of  the  proteins  which 
have  just  been  described  are  compared  there  will  be  a  few  which 
will  stand  out  in  each. case  as  more  or  less  characteristic  of  the 
protein  in  question.  Gelatin  is  easily  dissolved  in  hot  water 
(after  swelling  in  cold  water)  and  upon  cooling  will  form  a  firm 
jelly  even  in  dilute  solutions.  (Pure  gelatin  will  form  a  firm 
jelly  in  1  per  cent  solution  at  10°C.)  The  only  other  protein  of 
the  group  which  is  soluble  in  hot  water  is  amyloid,  but  the  latter 
is  dissolved  much  less  readily,  and  does  not  set  to  a  jelly  on 
cooling.  Gelatin  is  furthermore  the  only  member  that  is  soluble 
in  dilute  acids,  and  to  a  solution  of  which  the  addition  of  dilute 
acids  will  fail  to  produce  a  precipitate.  Of  the  color  tests  the 
biuret  reaction  is  the  only  one  that  gives  a  pronounced  and 
undeniable  test  with  gelatin. 

Keratin  and  elastin  are  characterized  by  their  great  resistance 
to  solution.  The  fish  protein,  ichthylepidin,  should  also  be 
included  in  this  class.  Pepsin  does  not  attack  keratin,  but  does 


1  ABDERHALDEN  and  VOITINOVICI,  Z.  physiol.  Chem.,  52  (1907),  368. 


CHEMISTRY  OF  GELATIN  79 

slowly  digest  the  other  two.  The  color  tests  of  this  group  are 
identical.  The  physical  properties  of  the  original  material  are, 
however,  vastly  different,  as  hair,  horn,  etc.,  yellow  connective 
fibers,  and  fish  scales  are  not  physically  similar. 

Mucin  and  chondrin  are  easily  differentiated  by  their  content 
of  carbohydrate,  by  virtue  of  which  they  react  to  Molisch's 
reagent,  and  their  easy  solubility  in  dilute  alkalies  and  the 
readiness  with  which  they  may  be  thrown  out  of  such  solution 
by  dilute  acids.  Chondrin,  which  is  by  some  regarded  as  a 
mixture,  probably  of  mucin  and  collagen,  gives  most  of  the 
positive  reactions  of  both  of  these  proteins.  It  may  be  differ- 
entiated from  mucin  by  being  precipitated  with  tannic  acid. 

Melanin  is  in  a  class  by  itself  as  the  only  pigmented  member  of 
the  group.  Its  solubility  closely  resembles  that  of  mucin.  It 
gives  a  negative  test  to  Millon's  reagent,  but  positive  to  all  of 
the  other  color  tests. 

A  few  of  the  characteristic  tests  that  have  been  described  are 
collected  in  Table  24. 

II.  THE  TISSUES  CONTAINING  COLLAGEN  AND  ITS  CONGENERS 

1.  The  Skin.  Structure  of  the  Skin. — The  skin  of  all  animals 
is  very  much  alike  in  most  of  its  essential  features.  It  consists, 
in  all  cases,  of  an  outer  layer,  or  epidermis,  of  a  hard,  horny, 
hairy,  or  scaly  material,  and  an  inner  layer,  or  corium,  which  is 
composed  of  bundles  of  fibers,  and  constitutes  what  is  generally 
known  as  the  true  skin.  The  epidermis  may  be  very  thin  and 
flexible,  as  in  the  skin  of  a  child,  or  very  thick  and  tough,  as  in  the 
hide  of  an  elephant  or  a  walrus.  It  may  be  covered  with  hair  or 
wool,  as  are  most  animals,  or  it  may  become  flattened  and 
hardened  into  scales,  as  in  the  fishes.  Special  growths  may  be 
evolved  from  it,  as  the  nails,  claws,  horns,  hoofs,  etc. 

The  skin  and  its  usually  abundant  crop  of  hair  constitutes  the 
covering  for  the  animal,  enclosing  the  body  and  affording  pro- 
tection, but  it  is  normally  much  more  than  that.  It  is  an  organ 
of  respiration,  of  transpiration  and  of  sense.  Numerous  measure- 
ments have  been  made  by  different  investigators  upon  the 
amount  of  oxygen  absorbed  and  the  amount  of  carbon  dioxide 
evolved  by  means  of  the  skin.  The  results  show  that,  with  the 
exception  of  the  non-scaly  amphibia,  it  is  relatively  low,  when 
compared  with  the  activity  of  the  lungs  in  this  capacity,  but 


80 


GELATIN  AND  GLUE 


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CHEMISTRY  OF  GELATIN 


81 


although  the  amount  is  small,  it  is  none  the  less  important,  and 
the  body  will  not  suffer  serious  interference  with  this  function 
without  disastrous  consequences.  The  work  of  Zuntz1  shows 
that  the  skin  respiration  equals  about  2J/2  per  cent  of  the  simul- 
taneous lung  respiration.  In  transpiration,  or  the  elimination  of 
water  and  watery  solutions  from  the  body,  the  skin  is  highly 


FIG.  13. — Section  of  fresh  untreated  calf  skin.     25  diameters.     (Kindness  of  J ohn 
Arthur  Wilson,  A.  F.  Gallun  &  Sons  Co.,  Milwaukee.) 

important,  it  having  been  shown2 "that  17  per  cent  of  the  total 
water  excreted  by  the  body  is  evolved  through  the  skin. 

The  epidermis  is  very  thin  in  comparison  with  the  corium.  It 
is  composed,  as  reference  to  the  accompanying  figure  will  show, 
of  two  layers,  the  under  one  of  which  rests  upon  the  corium. 
This  under  layer,  the-  rete  malpighi,  is  composed  of  living  cells, 
but  as  they  approach  the  surface  they  dry  out,  become  flattened 
and  form,  eventually,  the  hard  horny  layer  at  the  surface.  This 
is,  in  turn,  being  constantly  worn  away,  and  replaced  by  fresh 
material.  The  hairs  are  developed  in  the  under  layer  of  the 
epidermis  by  a  downward  growth,  but  not  until  they  have  pene- 
trated deeply  into  the  corium  do  they  break  through  the  surface. 

1  ZUNTZ,  Arch.  ges.  Physiol.  (1894). 

2  Cf.  MATHEWS'  "  Physiological  Chemistry"  (1916),  682. 


82  GELATIN  AND  GLUE 

The  sebaceous,  or  fat  glands,  and  the  sudoriferous,  or  sweat  glands, 
are  also  formed  in  the  rete  malpighi  of  the  epidermis.  Each  hair  is 
provided,  furthermore,  with  a  small  involuntary  muscle,  called 
the  erector  pili,  which  is  caused  to  contract  by  cold  or  by  fear, 
causing  the  hairs  to  " stand  on  end,"  and  producing  " goose 
flesh."  The  quills  of  the  porcupine  are  in  fact  only  an  exagger- 
ated kind  of  a  hair,  and  the  muscles  controlling  them  more  highly 
developed  than  those  in  other  animals,  and  are  probably 
voluntary. 

The  epidermis  is  separated  from  the  corium  by  a  very  fine 
membrane,  called  the  pars  papillaris,  and  known  to  tanners  as 
the  glassy  layer  which  is  responsible  for  the  " grain"  of  leathers. 

The  corium  or  true  skin  is  of  an  altogether  different  nature 
from  the  epidermis  just  described.  It  is  composed  for  the  most 
part  of  long  tender  fibers.  Sometimes  these  lie  in  one  plane  and 
are  parallel,  and  in  what  might  be  called  well-ordered  layers, 
but  frequently,  especially  near  the  surface,  the  fibers  seem  to  be 
interwoven  into  a  hopeless  tangle  and  are  very  tightly  packed. 
The  central  portion  is  often  loose,  and  contains  a  greater  propor- 
tion of  fat  globules,  while  on  the  flesh  side,  as  well  as  near  the 
surface,  the  fibers  are  more  closely  interwoven. 

The  white  fibers  which  constitute  by  far  the  greater  part  of  the 
corium  are  of  the  material  known  to  physiologists  as  white 
connective  tissue.  In  addition  to  this  the  corium  contains  also  a 
small  proportion  of  fibers  of  a  different  nature.  These  are  yellow, 
very  elastic,  and  are  known  as  yellow  connective  tissue.  They 
are  easily  differentiated  by  the  fact  that  they  do  not  swell  in  a 
mixture  of  equal  parts  of  water,  glycerine,  and  acetic  acid,  while 
the  white  fibers  become  swollen  and  transparent. 

On  many  animals,  as  the  ox  and  horse,  the  corium  is  covered 
on  its  under  side  with  a  layer  of  voluntary  muscle.  This  is 
removed  with  the  hide,  in  the  slaughter  houses,  but  is  usually 
separated  from  the  hide  in  the  fleshing  process,  and  constitutes 
one  of  the  important  raw  materials  for  the  manufacture  of  glue. 

The  Composition  of  the  Skin. — The  foregoing  description  will 
serve  to  show  that  the  skin  is  by  no  means  a  simple  substance, 
but  rather  a  mixture  of  a  number  of  components  which,  by 
specialization  along  varying  lines,  have  resulted  in  the  formation 
of  the  various  types  of  surface  growths. 

The  epidermis,  hair,  horns,  hoofs,  etc.,  are  composed  almost 
entirely  of  keratin.  This  has  been  described  on  page  63. 


CHEMISTRY  OF  GELATIN  83 

They  are  easily  dissolved  by  caustic  alkalies,  but  in  the  depilatory 
processes  of  the  tannery  it  is  necessary  to  remove  the  hair 
without  impairment  to  the  hide.  This  is  most  readily  accom- 
plished by  treatment  with  alkaline  sulphides,  which  easily 
dissolve  the  hair  but  do  not  attack  the  connective  tissue  of  the 
corium.  Solution  of  the  keratin  may  also  be  attained  by  heating 
in  water  for  prolonged  periods,  especially  under  pressure. 

The  solution  resulting  has,  however,  no  jellying  power,  and  must  be  con- 
sidered as  an  adulterant  of  no  value  if  present  in  gelatin  or  glue.  The 
statement  has  so  often  been  found,  that  horns  and  hoofs  are  an  important 
source  of  glue  stock,  that  it  should  be  emphatically  pointed  out  that  this  is 
not  the  case.1  This  misstatement  may  have  found  its  origin  in  that  the 
old  time  glue  makers  put  into  the  glue  kettle  everything  from  the  animal 
carcass  for  which  they  could  not  find  use  elsewhere;  or  it  may  have  been 
given  credence  from  the  loose  usage  of  the  term  "buckhorn"  which  is  stated, 
even  by  Dawidowski,2  to  be  an  important  source  of  gelatin.  Properly 
speaking,  buckhorn  is  the  antler  material  of  sheep,  but  this  substance,  and 
certain  hard  bones  of  the  ox,  as  the  lower  shin  bone,  have  both  been  used  for 
the  preparation  of  buttons,  knife  handles,  piano  keys,  etc.,  and  the  name 
of  the  former  has  been  unfortunately  appropriated  by  manufacturers  and 
dealers  of  the  latter  less  desirable  material. 

Besides  the  keratin  of  the  epidermis,  there  are  of  course  several 
other  substances  present  in  small  amounts.  The  fat  glands  are 
in  this  layer,  and  the  sweat  glands,  with  their  fatty  and  salty 
excretions,  and  the  linings  of  the  tubes  are  of  an  albuminous 
material. 

The  corium,  as  has  been  previously  stated,  consists  mainly 
of  white  connective  tissue,  with  fibers  of  the  yellow  variety 
scattered  through  it.  These  tissues  are  described  on  page  84. 
Albumin  is  also  present  in  the  corium  in  small  amounts,  but  in  the 
fleshings  which  are  cut  from  the  corium  and  which  contain  the 
muscles  underlying  the  skin,  the  amount  of  albuminous  material 
may  be  large.  The  albumins  are  easily  coagulated  by  heating  in 
solution  to  about  70  or  75°C.  The  presence  of  acids  lowers  the 
temperature  of  coagulation,  while  the  presence  of  alkalies  raises 
it.  Coagulated  albumins  will  not  readily  pass  again  into  solution, 
and  they  resemble  the  keratins  in  their  solubilities. 

Another  substance  intermediate  between  hide-fiber  and  gelatin 

1  We  refer  only  to  the  outer  horny  portion.     The  inner  portion  of  horns, 
called  the  pith,  is  prized  as  a  .high  grade  of  glue  or  gelatin  stock. 

2  DAWIDOWSKI,  "Glues,  Gelatin,  etc."  (1905),  4. 


84 


GELATIN  AND  GLUE 


has  been  described  by  Rollet1  and  by  Reimer2  and  called  coriin 
by  the  latter  investigator.  He  subjected  calf  skins  to  a  prolonged 
treatment  with  water  to  remove  every  trace  of  soluble  substance, 
and  then  digested  the  skins  with  lime  water  for  7  to  8  days.  On 
adding  acetic  acid  to  the  filtered  solution  a  white  flocculent 
precipitate  was  thrown  down.  He  considered  this  as  the 
cementing  substance  which  held  the  fibers  together.  But  he 
also  found  that  the  same  portion  of  hide  could  be  extracted  in 
this  manner  repeatedly  without  becoming  exhausted,  the  fibers 
becoming  finer  and  finer  until  they  could  be  distinguished  only 
with  difficulty.  This  supports  the  theory  that  there  exists  no 
distinct  cementing  material,  but  only,  perhaps,  a  partially 
dissolved  portion  of  the  fibers  themselves,  which  acts  as  a  binding 
agent. 

The  elementary  composition  of  the  corium  of  different  animals 
is  given  in  the  following  table : 

TABLE    25. — ELEMENTARY    COMPOSITION    OF    THE    CORIUM 


Animal 

Carbon 

Hydro- 
gen 

Nitro- 
gen 

Oxygen 

Observer 

Ox  

50.2 

6.4 

17.8 

25  A 

Von  Schroeder  and 

Goat  and  deer. 
Sheep  and  dog. 
Cat  

50.3 
50.2 
51.1 

6.4 
6.5 
6.5 

17.4 
17.0 
17.1 

25.9 
26.3 
25.3 

Paessler1 

1  VON  SCHROEDER  and  PAESSLER,  Dingler's  polytech.  /.,  287  (1893),  258. 

2.  The  Connective  Tissue. — Several  varieties  of  connective 
tissue  have  been  described,  but  only  two  of  these  are  of  especial 
interest  in  their  relationship  to  this  study.  One  of  these  is 
commonly  known  as  yellow  connective  tissue,  which  consists  of 
tough,  elastic,  yellowish  fibers.  It  is  found  in  tendons,  in  the 
walls  of  blood  vessels,  in  the  lungs,  and  less  prominently  in 
other  parts  of  the  body.  The  most  conspicuous  occurrence  of 
this  variety  is  in  the  ligamentum  nuchue  of  the  ox.  The  second 
type,  commonly  known  as  white  connective  tissue,  is  found  chiefly 
in  the  tendons  of  the  muscles,  and  quite  generally  throughout 


1  ROLLET,  Sitz.  Akad.  Wiss.  Wien.,  39,  305. 

2  REIMER,  Dinglers  polytech.  J.,  205  (1872),  143. 


CHEMISTRY  OF  GELATIN 


85 


the  body,  even  the  fibers  in  the  organic  matrix  of  bone  being  of 
that  substance.  The  most  abundant  source  of  this  variety  of 
tissue  is  in  the  Achillis  tendon. 

These  two  types  of  connective  tissue  differ  in  their  chemical 
constitution  mainly  in  that  the  white  variety  consists  essentially 
of  the  protein  collagen,  85  per  cent  of  the  dry  matter  being  of  this 
material,  while  the  yellow  variety  is  mostly  elastin,  74.6  per  cent 
being  represented  by  this  protein,  and  only  17  per  cent  being 
collagen.  The  composition  of  these  tissues  is  shown  in  the 
table  following. 


TABLE  26. — COMPOSITION  OF  WHITE  AND  YELLOW  CONNECTIVE  TISSUE 


Constituents 

Tendo  Achillis  of 
ox1 

Ligamentum 
nuchse  of  ox  2 

Fresh 
tissue 

Dry 

tissue 

Fresh 
ligament 

Dry 

ligament 

Water  

62.870 
37.130 
0.470 
0.031 
0.039 
0.147 
36.660 
1.040 
0.220 
1.283 
2.633 
31.588 
0.896 

57.570 
42.430 
0.470 
0.026 
0.035 
0.136 
41.960 
1.120 
0.616 
0.525 
31.670 
7.230 
0.799 

1.100 
0.062 
0.081 
0.318 
98.900 
2.640 
1.452 
1.237 
74.641 
17.040 
1.883 

Solids 

Inorganic  matter  

1.266 
0.084 
0.106 
0.397 
98.734 
2.801 
0.593 
3.455 
4.398 
85.074 
2.413 

SO3   .  .                                   

P,O5  

Cl    

Organic  matter 

Fat  (ether-sol,  matter)        

Albumin  globin 

Mucoid  

Elastin 

Collagen  (gelatin)  

Extractives  and  undetermined       .  . 

1  BUERGER  and  GIES,  Am.  J.  Physiol.,  6  (1901),  219. 

2  VANDERGRIFT  and  GIES,  ibid.,  5  (1901),  288. 

A  small  amount  of  mucoid  is  present  in  each  case,  and  is 
present  to  a  greater  extent  in  the  white  tissue.  Except  for  the 
differences  in  content  of  collagen  and  elastin,  and  the  slight 
difference  in  mucoid,  the  constitution  of  the  two  varieties  is 
much  the  same. 

The  collagen,  or  gelatin,  and  the  elastin  and  mucoid  may  easily 
be  separated  from  these  tissues.  The  methods  for  such  a 


86 


GELATIN  AND  GLUE 


separation,  and  the  properties  of  these  proteins,  have  been 
described  in  a  previous  section. 

3.  The  Cartilage. — True  cartilage  is  not  found  in  any  of  the 
lower  animals,  except  in  the  internal  skeleton  of  the  cartilaginous 
fishes,  but  is  always  present  in  the  vertebrates.  The  so-called 
cartilage  of  the  cephalopods  and  the  anthropods  is  more  closely 
related  to  chitin.  Cartilage  is  usually  formed  as  cells  imbedded 
in  a  homogeneous  matrix  which  is  produced,  in  turn,  by  the 
cells.  As  the  age  of  the  animal  increases,  inorganic  salts  are 
often  deposited  and  a  bony  tissue  results.  This  does  not  always 
happen,  however,  for  the  cartilage  of  the  trachea,  and  especially 
the  larynx,  remains  unchanged  through  life. 

Cartilage  is  very  closely  related,  in  chemical  constitution,  to 
white  connective  tissue,  and  to  ossein.  Several  substances  have 
been  found  in  the  material.  Morner,1  on  investigating  the 
cartilages  of  full  grown  cattle,  reported  four  constituents  were 
obtained  by  a  partial  decomposition  of  the  matrix,  namely, 
collagen,  which  constituted  the  bulk  of  the  material,  mucoid, 
chondroitic  acid,  and  albuminoid.  The  chondromucoid  was 
very  similar  in  all  respects  to  the  tendomucoid  obtained  by 
Buerger  and  Gies,2  and  has  been  described.  Collagen  and 
chondroitic  acid  have  also  been  discussed  in  detail.  The  chon- 
droalbuminoid  was  an  albuminous  material  that  remained  after 
long  treatment  with  hot  water.  On  boiling  with  very  dilute 
potassium  hydroxide,  however,  it  went  easily  into  solution.  The 
amount  of  this  substance  present  in  collagen  is  very  small,  and 
has  been  believed  to  form  the  lining  of  the  Haversian  canals,  or 
little  tubes  in  the  cartilage  and  in  bone.  The  albuminoid 
obtained  from  cartilage  was  found,  in  fact,  to  resemble  very 
closely  the  albuminoid  from  bones.  The  elementary  composition 
is  given  below:3 


Carbon 

Hydro- 
gen 

Nitrogen 

Sulphur 

Oxygen 

Chondroalbuminoid..  .  . 
Osseoalbuminoid  

50.46 
50.16 

7.05 
7.03 

14.95 
16.17 

1.86 
1.18 

26.86 
25.46 

1  MORNER,  Skand.  Arch.  PhysioL,  1. 

2  BUERGER  and  GIES,  loc.  cit. 

3  HAWK  and  GIES,  Am.  J.  Physiol,  6  (1901),  388. 


CHEMISTRY  OF  GELATIN  87 

4.  The  Bones. — The  bones  are  made  up  of  cells  which  are 
enclosed  in  an  intercellular  matrix.  The  cells  have  received 
very  little  study,  but  have  been  shown  to  yield  no  gelatin  and  to 
contain  no  keratin.1  The  intercellular  substance  is  present  in 
great  excess  over  the  cellular,  and  is  composed  of  two  chief 
constituents:  an  organic  substance  which  is  known  as  ossein, 
and  a  heavy  inorganic  deposit  of  mineral  salts. 

The  ossein  may  be  readily  obtained  by  dissolving  out  the 
inorganic  salts  with  dilute  hydrochloric  acid.  It  comprises 
about  60  per  cent  of  the  dry  matter  of  bone,  40  per  cent  being 
inorganic  material.  The  ossein  is  not  a  definite  chemical  sub- 
stance, but  has  been  shown  to  consist  of  three  proteins :  collagen, 
osseomucoid,  and  ossalbuminoid.  The  latter  two  substances  are 
present  only*  in  small  amounts,  by  far  the  greater  portion  of  the 
ossein  consisting  of  collagen,  which  is  easily  converted  to  gelatin 
on  heating  with  water.  Collagen  and  osseomucoid  have  already 
been  described.  The  albuminoid  of  bones  is  nearly  identical 
with  that  obtained  from  cartilage,  and  has  been  described  in 
connection  with  that  substance. 

The  marrow  of  bones  contains  widely  varying  amounts  of 
proteins,  fats,  lecithin,  and  erythrocytes.  The  protein  portion 
consists  of  a  globulin,  a  nucleoprotein,  and  fibrinogen,  besides 
traces  of  albumin  and  proteose. 

The  inorganic  material  of  bones  is  chiefly  calcium  phosphate 
and  calcium  carbonate,  but  small  amounts  of  other  salts  are  also 
invariably  present  and  must  be  considered  as  a  necessary  part 
of  the  bone.  The  composition  of  the  mineral  portion  of  dry  bone 
is  quite  constant,  and  variations  found  in  different  parts  of  the 
body,  or  in  different  species  of  animals,  are  not  large.  The  water 
content  varies  greatly,  and  the  relation  of  the  organic  to  the 
inorganic  portion  likewise  varies,  especially  with  age,  the  greater 
amount  of  mineral  salts  being  found  in  the  older  animals,  but 
the  dry  bone  varies  but  little.  An  average  analysis  of  the  mineral 
portion  of  dry  bone  is  given  below: 

TABLE  27. — COMPOSITION  OF  INORGANIC  MATERIAL  OF  BoNE2 


PER 

PER 

CENT 

CENT 

Calcium  phosphate  

.  .  .    85.0     Calcium  fluoride  

0.3 

Calcium  carbonate  

...    10.  0     Calcium  chloride  

0.2 

Magnesium  phosphate  .  .  .  . 

...      1.5     Alkali  salts  

2.0 

1  SMITH,  Z.  Biol,  19  (1883). 

2  MATHEWS,  "Physiological  Chemistry,"  2nd  ed.,  New  York  (1916),  637. 


88  GELATIN  AND  GLUE 

Taggart1  gives  the  following  distribution  of  matter  in  fresh 
bone: 

TABLE  28. — COMPOSITION  OF  FRESH  BONE 

PER  PER 

CENT  CENT 

Water 51.0     Ossein,  etc 11.4 

Fat..  .    15.7     Mineral  matter...  .   21.9 


5.  Fish  Skins,  Scales,  Sounds,  etc. — Fish  refuse  has  long  been 
used  for  the  manufacture  of  glue.  Depending  upon  the  degree 
of  refinement  of  the  process,  and  the  care  with  which  the  different 
parts  of  the  fish  are  separated,  the  quality  of  the  product  will 
vary  greatly. 

The  pure  skin  of  the  fishes  is  quite  similar  in  most  respects  to 
that  of  the  higher  animals.  The  epidermal  layer  is,  of  course, 
free  from  hairy  growths,  but  the  sebaceous  or  fat  glands  are 
often  present  in  great  abundance,  and  the  scales  constitute  a 
special  development  of  the  outer  layer.  In  constitution,  the 
epidermis,  and  especially  the  scales,  often  contain  a  protein 
different  from  that  found  in  the  land  animals.  This  was  called 
ichthylepidin  by  Morner2  who  first  investigated  the  material. 
He  found  that  in  the  scales  of  some  varieties  of  the  Teleostean 
fishes  as  much  as  20  per  cent  was  present,  while  in  other  varieties 
of  the  Teleosts,  and  in  the  Ganoids,  it  was  entirely  absent.  Kera- 
tin also  appears  to  be  absent  in  some  cases  but  present  in  others. 
It  seems  that  the  hard  and  insoluble  portions  of  the  scales  and 
the  epidermis  are  for  the  most  part  a  combination  of  these  two 
proteins,  ichthylepidin  and  keratin,  in  some  cases  being  entirely 
the  one  and  in  other  cases  entirely  the  other.  Ichthylepidin 
may,  in  fact,  be  considered  as  a  modification  of  the  common 
horny  varieties  of  keratin  formed  in  the  skins  and  horns  of 
animals. 

The  epidermis  of  the  fishes  differs  from  that  of  the  land  animals 
in  containing  a  large  amount  of  collagen.  Morner  reported  80 
per  cent  of  collagen  in  fish  scales.  Green  and  Tower3  report 
that  more  than  52  per  cent  of  pure  gelatin  is  industrially  obtained 
from  the  scales  of  the  menhaden. 

1  TAGGART,  "Glue  Book." 

2  MORNER,  Z.  physiol  Chem.,  24  (1897),  125;  37  (1902),  88. 

3  GREEN  and  TOWER,  U.  S.  Fish  Commission,  Bull.  21  (1901),  97. 


CHEMISTRY  OF  GELATIN 


89 


The  inner  layer  or  corium  of  the  skin  of  the  fishes  is  more 
nearly  identical  to  that  of  land  animals,  since  in  both  cases  it 
consists  almost  entirely  of  collagen.  The  physical  structure  is 
somewhat  different,  as  in  the  fishes  the  layers  are  often  at  right 
angles  to  each  other,  and  somewhat  more  distinct,  giving  to  the 


FIG.  14. — Section  of  shark  skin,  vegetable  tanned.     50  diameters.     (Kindness  of 
John  Arthur  Wilson,  A.F.  Gallun  &  Sons,  Milwaukee.) 

skin  more  of  a  membranous  structure.  This  is  shown  clearly 
in  Fig.  14  which  shows  a  section  of  vegetable  tanned  shark  skin. 
The  collagen  in  either  layer  of  the  skin  of  the  fishes  is  converted 
into  gelatin  much  more  easily  than  the  collagen  in  the  land 
animals.  Heating  for  a  short  time  at  a  low  temperature  (60°C.) 
is  sufficient.  The  ichthylepidin  and  keratin  are  left  behind 
unattacked. 

The  bones  of  fishes  also  contain  collagen,  but  they  more  nearly 
resemble  the  cartilage  of  animals  than  the  bones,  and  are  thus 
rich  in  mucoids,  or  in  what  was  formerly  known  as  chondrin. 
In  the  manufacture  of  fish  glue  it  has  been  common  practice  to 
put  into  the  glue  pot  either  the  whole  fish,  or  the  entire  refuse 
from  fish  canneries,  salting  factories,  and  the  like.  Thus  no 
attempt  at  a  careful  separation  of  material  has  been  commonly 
made,  and  in  consequence  much  material  other  than  collagen 


90  GELATIN  AND  GLUE 

has  been  dissolved  and  the  quality  of  the  product  injured. 
Fatty  matter  is  usually  removed  from  the  glue  liquor  by  skimming 
processes,  as  fish  oils  find  valuable  use  in  other  fields.  The 
residue  left  in  the  kettles  is  dried  and  made  up  into  fertilizer. 

The  sounds,  or  air  bladders,  or  swimming  bladders,  as  they  are 
variously  called,  consist  almost  entirely  of  collagen,  and  have 
the  additional  advantage  of  being  exceptionally  free  from  other 
impurities  which  might  be  dissolved  out  upon  heating  with 
water.  They  also  differ  from  other  varieties  of  collagen  in  being 
more  readily  soluble  in  warm  water  than  any  other  type.  On 
account  of  this  unusual  purity  and  the  consequent  very  high 
'  quality  of  the  gelatin  obtained  from  them,  they  have  been  used 
for  a  great  many  years  in  the  preparation  of  what  is  known  as 
isinglass. 

Many  varieties  of  fish  have  been  used  in  the  preparation  of 
isinglass.  The  most  notable  of  these  is  the  sturgeon,  it  being  the 
first  fish  to  be  used  extensively  for  commercial  isinglass,  and 
its  product  is  still  the  standard  upon  which  all  others  are 
based.  It  is  made  up.  in  several  ways,  which  will  be  described 
later,  and  large  amounts  are  still  exported  from  Russia.  Catfish 
and  carp  also  contribute  to  the  Russian  product.  Many  other 
countries  produce  sounds,  usually  of  somewhat  inferior  quality 
to  the  Russian.  Cod  and  ling  sounds  are  obtained  from  Iceland, 
cod  sounds  from  Norway,  miscellaneous  types  from  Venezuela, 
Brazil,  Penang  and  Bombay.  In  America,  especially  in  the 
Canadian  waters,  sounds  are  obtained  mainly  from  the  hake,  cod, 
squeteague,  and  more  recently  the  tilefish. 


CHAPTER  III 

THE  PHYSICO-CHEMICAL  PROPERTIES  AND 
STRUCTURE  OF  GELATIN 

Life  is  so  completely  linked  to  the 
chemical  and  physical  properties  of 
proteins  that  the  knowledge  of  these 
properties  must  precede  the  attempt 
at  unraveling  the  dynamics  of  living 
matter.  Jacques  Loeb  (1921) 

PAGE 

I.  The  Physico-Chemical  Properties  of  Gelatin 91 

1.  The  Diffusion  of  Gelatin 91 

2.  The  Dialysis  of  Gelatin 95 

3.  The  Osmosis  of  Gelatin '.'..' 97 

4.  The  Vapor  Pressure 104 

5.  The  Boiling  Point  and  Freezing  Point 106 

6.  The  Molecular  Weight 107 

7.  The  Surface  Tension 114 

8.  The  Optical  Rotation 116 

9.  The  Index  of  Refraction 1 19 

10.  The  Gold  Number 121 

11.  The  Tyndall  Effect  and  Ultramicroscopy 123 

12.  The  Donnan  Equilibrium 128 

II.  The  Structure  of  Gelatin 132 

1.  The  Older  Theories  of  Gel  Structure 132 

2.  The  Recent  Theories  of  Gel  Structure ...    136 

3.  The  Theories  of  Sol  Structure,  and  the  Sol-Gel  Equilibrium.  .  .    145 

I.  THE  PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN 

1.  The  Diffusion  of  Gelatin. — Diffusioii  is  observed  in  solutions 
of  gelatin  much  the  same  as  in  crystalloidal  solutions  except  that 
the  rate  at  which  it  takes  place  is  much  slower  in  the  former 
instance.  If  a  gelatin  solution  is  placed  at  the  bottom  of  a  test 
tube  and  a  little  water  carefully  poured  over  the  gelatin,  the  latter 
will,  in  the  course  of  time,  gradually  permeate  the  water  and  a 
homogeneous  solution  will  result.  The  actual  mechanics  of  the 
diffusion  process  is  probably  much  the  same  in  the  two  cases, 
and  is  explained  by  the  laws  of  molecular  kinetics.  It  is  known 
that  the  diffusion  coefficients,  that  is,  the  rates  of  molecular 
migration,  are  greater  at  high  than  at  low  concentrations  in 

91 


92 


GELATIN  AND  GLUE 


molecularly  dispersed  systems,1  and  Zsigmondy2  has,  by  actual 
observation  with  his  ultramicroscope,  found  that  the  Brownian 
movement  of  colloid  suspensions  is  less  in  dilute  than  in  con- 
centrated solutions.  This  would  require  therefore  that  the 
molecules  in  the  one  case,  or  the  colloid  particles  in  the  other 
case,  must  move  gradually  from  an  environment  of  higher  to  one 
of  lower  concentration  in  order  that  the  energy  intensities  may 
remain  constant  throughout  the  entire  system. 

The  actual  coefficient  of  diffusion  may  be  calculated  from  the 
law  of  Fick:3 


where  ds  is  the  amount  of  diffusing  substance  which,  in  the  time 
dtj  passes  through  a  diffusion  cylinder  of  cross-section  q,  c  is  the 
concentration  at  the  point  x,  c  +  dc  is  the  same  quantity  at  the 
point  x  +  dx,  and  D  is  the  diffusion  coefficient,  a  constant 
expressive  of  the  rate  of  diffusibility  of  the  substance  under  in- 
vestigation. The  following  table  cited  from  Ostwald4  shows 
the  diffusion  coefficients  of  several  colloids  in  comparison  with 
certain  crystalloid  substances: 


Substance 

Temperature, 
°C. 

D 

Observer 

Sodium  chloride 

20 

1  04 

Voightlander1 

Magnesium  chloride  

20 

0.77 

Voightlander1 

Cane  sugar 

9 

0  31 

Graham  — 

Egg  albumin                .        .  . 

18 

0.059 

Stefan2 
Herzog3 

Ovomucoid  

18 

0.044 

Herzog3 

Emulsin                      

18 

0  036 

Herzog3 

Invertm 

18 

0  033 

Herzog3 

1  VOIGHTLANDER,  Z.  physik.  Chem.,  3  (1889),  329. 

2  GRAHAM— STEFAN,  Site.  Akad.  Wiss.  Wien.,  77,  II  (1879),  161. 

3  HERZOG,  Kolloid-Z.,  2  (1907),  1;  3  (1908),  83. 

As  a  result  of  an  investigation  upon  the  diffusion  velocity  of 

1  WM.  OSTWALD,  "Lehrbuch  der  allgemeine  Chemie,"  2nd  ed.  (1903),  686. 

2  R.  ZSIGMONDY,  Kolloid-Z.,  Jena  (1915),  111. 

3  A.  FICK,  Pogg.  Ann.,  94  (1855),  59. 

4  Wo.   OSTWALD — FISCHER,    "Handbook  of   Colloid  Chemistry,"  Phila- 
delphia (1915),  214. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN  93 

gold  hydrosols  of  different  sizes  Svedberg1  obtained  results  which 
Ostwald2  urges  are  indicative  that  the  diffusion  velocity  is 
approximately  inversely  proportional  to  the  size  of  the  particles, 
and  he  furthermore  points  out  that  such  a  deduction  is  entirely 
in  harmony  with  mathematical  expressions  formulated  by  Ein- 
stein3 and  Smoluchowski'  which  are  based  entirely  upon  mole- 
cular-kinetic considerations. 
Liesegang  Rings. — A  very  pretty  demonstration  of  the  diffusion 


FIG.  15. — Periodic  precipitation  of  silver  chromate  in  gelatin.     (R.  E.  Liesegang.) 

of  electrolytes  into  a  jelly,  known  as  the  Liesegang  ring  formation 
in  honor  of  its  discoverer,5  is  produced  by  allowing  a  drop  of  a 
solution  of  a  salt,  such  as  silver  nitrate,  to  rest  upon  a  slab  of 
jelly  in  which  has  been  dissolved  another  salt,  such  as  potassium 
bichromate,  with  which  the  former  salt  would  normally  produce 
an  insoluble  precipitate.  As  the  silver  ions  diffuse  outward  into 
the  jelly,  silver  chromate  will  be  precipitated,  but  instead  of  a 
uniform  precipitation  taking  place,  very  clearly  marked  rings  of 
precipitate  are  formed,  see  Fig.  15. 

1  THE  SVEDBERG,  Z.  physik.  Chem.,  67  (1909),  105. 

2  Wo.  OSTWALD,  lib.  cit. 

3  A.  EINSTEIN,  Ann.  physik.  (4),  21  (1905),  17,  549. 

4  M.  VON  SMOLUCHOWSKI,  ibid.,  21  (1906),  756. 

5  R.  E.  LIESEGANG,  Z.  anal.  Chem.,  50  (1910),  82;  Kolloid-Z.,  12  (1913), 
74;  269. 


94  GELATIN  AND  GLUE 

The  experiment  may  be  varied  greatly.  By  carrying  out  the 
work  in  a  test  tube  as  many  as  twenty  parallel  membranes  may 
be  formed.  The  phenomenon  varies,  however,  with  the  salt  used 
and  the  nature  of  the  jelly.1  For  example,  silver  nitrate  and 
potassium  bichromate  give  these  rings  in  gelatin  jelly  but  not  in 
agar,  while  lead  nitrate  and  potassium  chromate  give  them  in 
agar  but  not  in  gelatin.  Neither  of  these  combinations  give 
rings  in  silicic  acid,  though  some  others  do.  Sodium  chloride 
does  not  produce  rings  of  precipitate  in  gels  with  silver  nitrate, 
but  instead  a  continuous  band  results,  as  also  results  by  the  use 
of  lead  nitrate  and  potassium  chromate  in  gelatin. 

Microscopic  examination  shows  that  in  most  cases  the  rings 
contain  a  large  number  of  small,  and  the  clear  spaces  a  small 
number  of  large  crystals  or  crystalline  aggregates.  A  striking 
microscopic  illustration  is  shown  by  the  action  of  cadmium 
sulphide  in  silicic  acid  gel,  which  exhibits  no  clear  spaces  at  all, 
but  instead  a  succession  of  alternately  pink  and  yellow  bands. 
The  two  shades  are  known  to  be  due  to  differences  in  the  size  of 
the  particles,  and  either  may  be  obtained  by  the  precipitation 
of  aqueous  solutions  of  different  concentrations. 

Wm.  Ostwald2  offered  an  explanation  for  the  Liesegang  ring 
formation  based  upon  the  assumption  of  metastable  supersatura- 
tion.  As  the  silver  nitrate  diffuses  into  the  bichromate  gelatin, 
silver  chromate  is  formed  but  remains  in  solution  in  a  super- 
saturated condition  until  the  upper  limit  of  metastability  is 
reached.  At  this  point  silver  chromate  is  precipitated  and  the 
supersaturated  solution  adjacent  is  likewise  deposited  reinforcing 
the  first  deposition. 

Hatschek  discredited  the  conception  of  a  highly  supersaturated 
and  metastable  solution  by  disseminating  crystalline  lead  iodide 
in  an  agar  gel  containing  also  potassium  iodide.  On  placing  a 
solution  of  lead  nitrate  in  contact  with  this  gel,  the  rings  formed 
in  the  usual  manner,  but  the  presence  of  the  crystalline  nuclei  of 
lead  iodide  should  have  made  supersaturation  impossible. 

Bradford3  explains  the  rings  as  due  to  an  adsorption  of  one  of 
the  reacting  solutes  by  the  layer  of  precipitate,  resulting  in 
parallel  zones  practically  free  from  it.  This  theory  possesses 
the  advantages  of  simplicity  but  we  should  like  to  have  more 

1  E.  HATSCHEK,  2nd  Report  on  Colloid  Chemistry  (1919),  21. 

2  WM.  OSTWALD,  "Lehrbuch  Allg.  Chemie,"  2nd  ed.,  vol.  2,  778. 

3  S.  C.  BRADFORD,  Biochem.  J.,  10  (1916),  169;  11  (1917),  14. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN  95 

direct  evidence  that  adsorption  actually  does  take  place  in 
certain  instances  and  not  in  others  before  accepting  it.  Also  one 
would  be  led  to  inquire  why  adsorption  of  a  given  solute  by  a 
given  precipitate  should  take  place  in  one  type  of  gel  and  not  in 
another.  Holmes1  has  proposed  a  theory  along  somewhat 
similar  lines. 

Hatschek  regards  Freundlich's  suggestion  that  the  formation  of 
periodic  strata  may  be  an  instance  of  the  coagulation  by  electro- 
lytes of  a  suspensoid  sol  as  the  most  important  hypothesis 
yet  advanced  to  account  for  the  Liesegang  ring  formation.  If 
coagulation  by  electrolytes  is  a  deciding  factor,  then  the  forma- 
tion of  strata  must  be  dependent  in  large  measure  upon  the 
protective  influence  of  the  gel.  In  the  cases  cited,  the  protective 
effect  of  gelatin  and  agar  is  about  as  100  to  2,  and  that  of 
silicic  acid  is  negligible.  According  to  these  data  we  should  not 
expect  precipitation  of  the  rings  to  be  similar  in  the  three  gels. 

The  formation  of  banded  agates  and  other  minerals  is  generally 
ascribed  to  similar  diffusional  phenomena  as  described  above, 
but  Bechhold2  claims  that  diffusion  is  not  necessarily  involved 
since  somewhat  similar  structures  may  be  produced  in  jellies  by 
crystallization  of  water  or  sodium  phosphate  in  them. 

2.  The  Dialysis  of  Gelatin. — It  was  early  observed  by  Thomas 
Graham  that  those  substances  which  diffuse  easily  through  water 
or  other  solution  also  pass  freely  through  parchment  paper  or 
animal  membranes,  while,  on  the  other  hand,  the  substances 
which  diffuse  very  slowly  are  restrained  by  such  membranes. 
As  will  be  pointed  out  in  the  following  chapter,  this  separation 
formed  the  basis  of  the  distinction  between  crystalloids  and 
colloids,  and  marked  the  beginning  of  the  chemistry  of  colloids. 

There  are  two  explanations  which  are  generally  used  to  account 
for  the  fact  that  certain  membranes  are  permeable  to  substances 
in  the  molecular  state  of  subdivision  while  not  to  colloidally 
dispersed  substances.  The  older  of  these  is  sometimes  spoken 
of  as  the  sieve  theory.  This,  as  the  name  applies,  conceives  the 
membrane  under  consideration  as  consisting  of  a  large  number 
of  small  capillary  pores  of  such  dimensions  that  the  small  crystal- 
loidal  molecules  may  pass  freely  through,  while  the  larger  col- 
loidal aggregates  are  sufficiently  large  to  be  held  back.  There 
is  a  great  deal  of  evidence  in  favor  of  this  conception.  For 

1  H.  HOLMES,  /.  Am.  Chem.  Soc.,  40  (1918),  1187. 

2  BECHHOLD,  "Colloids  in  Biology  and  Medicine"  (1919),  261. 


96  GELATIN  AND  GLUE 

example,  gold  sols  may  be  prepared  of  various  sizes  from  the 
order  of  molecular  dispersoids  to  suspensions.  The  smaller  of 
these  pass  readily  through  parchment  paper  while  the  larger 
sizes  are  restrained.  The  principle  of  ultrafiltration  makes  use 
of  the  same  conception.  Porous  porcelain  cylinders  may  be 
prepared  of  varying  degrees  of  pore  dimension,  and  upon  fil- 
tering colloid  sols  through  these,  varying  degrees  of  separation 
are  obtained,  just  as  the  passing  of  sand  through  a  series  of 
sieves  will  separate  the  material  into  varying  sized  grains. 

The  other  theory  of  semipermeability  rests  upon  the  belief  that 
the  membrane  is  permeable  only  to  those  substances  which  may 
be  dissolved  by  it.  This  is  most  easily  understood  by  the  con- 
sideration of  a  two-phase  system  in  which  one  of  the  phases,  at 
least,  is  a  pure  liquid,  such  as  a  solution  of  sugar  in  water.  A 
collodion  membrane,  for  example,  will  readily  dissolve  water, 
but  not  sugar.  So,  if  such  a  membrane  be  permitted  to  separate 
a  solution  of  sugar  in  water  from  pure  water,  the  latter,  by  being 
soluble  in  the  membrane,  may  pass  freely  in  either  direction, 
while  the  sugar  molecules  may  not  pass  the  barrier.  But  since 
the  water  is  the  more  concentrated  oh  the  side  of  the  pure  solvent, 
the  membrane  will  become  supersaturated  with  water  in  respect 
to  the  solution,  and  equilibrium  will  necessitate  that  under  these 
conditions  more  water  will  pass  from  the  membrane  to  the 
solution  in  any  given  time  than  from  the  solution  to  the  mem- 
brane. That  is,  a  passage  of  water  will  take  place  from  the 
pure  solvent  to  the  solution  side  of  the  membrane.  In  an  entirely 
similar  way  a  rubber  film  may  function  as  a  semipermeable 
membrane  for  solvents  like  benzene,  pyridine,  etc.,  which  are 
soluble  in  rubber.  Likewise  a  film  of  water  may  be  used  as  a 
semipermeable  membrane  to  separate  a  mixture  of  hydrogen 
which  is  not  soluble  in  the  water,  and  ammonia  gas  which  is 
soluble.1  In  the  dialysis  of  colloids,  whereby  they  are  separated 
from  crystalloids,  the  sieve  conception  is  usually  regarded  as  most 
probable,  while  in  the  case  of  membranes  which  permit  only  of 
the  passage  of  a  pure  liquid,  the  solution  theory  is  generally 
accepted. 

A  number  of  different  types  of  membrane  have  been  used  in 
dialysis.  Ordinary  parchment  paper  is  very  satisfactory  and  is 
strong.  Special  diffusion  thimbles  are  made  of  this  material  by 
most  filter  paper  houses.  Fish  bladders  or  animal  bladders  are 

1  L.  KAHLENBERG,  J.  Phys.  Chem.,  10  (1906),  141. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN  97 

sometimes  used.  For  investigational  work  collodion  membranes 
have  been  found  very  adaptable.  These  are  made  by  coating 
the  interior  of  a  flask  or  tube  of  any  desired  size  with  a  10  per 
cent  solution  of  nitrocellulose  in  equal  parts  of  alcohol  and  ether.1 
The  flask  is  slowly  rotated  as  the  material  is  allowed  to  run  out, 
and  a  current  of  air  passed  in  until  the  film  is  dry.  By  immersing 
in  hot  water  the  film  easily  separates  from  the  glass  wall,  and 
may  be  drawn  out.  Membranes  made  in  this  way  make  excel- 
lent containers  for  use  in  dialyzing  due  to  their  very  ready  per- 
meability to  crystalloids  and  their  large  surface  exposed;  and 
have  been  used  in  comparative  osmotic  pressure  investigations 
with  marked  success.2 

Gelatin  may  also  be  dialyzed  by  permitting  the  gelatin  jelly 
to  function  as  its  own  membrane.  To  do  this  with  success  a 
rather  stiff  jelly  is  made,  cut  into  thin  slices,  and  suspended  in 
water  which  is  not  warmer  than  10°C.  The  water  must  be 
frequently  changed,  or  continuously  changing. 

3.  Osmosis  of  Gelatin. — The  concept  of  the  free  diffusion  of 
molecularly  or  otherwise  dispersed  substances  from  a  condition  of 
greater  to  one  of  lesser  concentration  of  the  solute,  as  described 
above,  necessitates  the  assumption  of  a  force  which  is  directive 
in  character  and,  if  opposed,  would  be  measurable  by  the  develop- 
ment of  a  pressure.  Such  an  opposition  to  the  movement  of  the 
dissolved  particles,  while  scarcely  affecting  the  movements  of 
the  solvent  is  obtained  by  the  interposition  of  a  semipermeable 
membrane  between  the  solution  and  the  solvent.  The  dissolved 
molecules  are  not  able  to  penetrate  the  membrane,  but  the  force 
which  is  accountable  for  their  diffusion  is  reflected,  under  these 
circumstances,  in  the  only  alteration  in  the  system  which  is  com- 
patible with  the  restoration  of  an  equilibrium,  namely,  the 
entrance  of  more  of  the  solvent  into  the  solution,  thereby  diluting 
the  solution  and  diminishing  the  potential  difference  between 
solvent  and  solution.  Nernst,3  Stieglitz,1  and  others  regard  the 
osmotic  pressure  as  the  force  producing  diffusion,  but  van  Laar, 5 

1  G.  LILLIE,  Am.  J.  PhysioL,  20  (1907),  133. 

2  Cf.  J.  LOEB,  /.  Gen.  PhysioL,  1  (1918-19),  717;  2  (1919-20),  87;  173;  255; 
273;  387. 

3  W.  NERNST,  " Theoretical  Chemistry,"  London  (1911). 

4J.  STIEGLITZ,  "Qualitative  Chemical  Analysis,"  New  York,  Vol.  1 
(1916),  p.  9. 

5  J.  VAN  LAAR,  "Vortrage  iiber  d.  thermodynam.  Potential  urw.  Braun- 
schweig" (1906). 


98  GELATIN  AND  GLUE 

Wo.  Ostwald,1  and  others  regard  the  semiperrneable  membrane 
as  necessary  before  true  osmotic  pressure  may  exist.  In  either 
case,  however,  and  irrespective  of  the  conception  assumed  as  to 
the  exact  mechanism  by  which  the  membrane  functions,  the 
presence  of  such  a  membrane  is  necessary  before  such  a  pressure 
may  be  measured.  Now  if  a  column  of  water  or  mercury  be 
placed  upon  the  solution  side  of  the  system,  the  solvent  will 
enter  the  solution  only  until  the  forces  producing  such  movement 
are  exactly  counterbalanced. by  the  hydrostatic  pressure  of  the 
column,  and  a  measure  of  this  hydrostatic  pressure  is  therefore 
a  measure  of  the  osmotic  pressure  of  the  system. 

The  existence  of  an  osmotic  pressure  produced  by  protein 
solutions  has  been  variously  affirmed  and  denied.  There  is  no 
difficulty  experienced  in  obtaining  a  development  of  such  a 
pressure  by  using  an  ordinary  protein,  but  as  the  protein  is  sub- 
jected to  exhaustive  methods  of  purification,  the  osmotic  pres- 
sure obtained  constantly  decreases,  and  Reid2  has  succeeded  in 
obtaining  a  preparation  of  egg-albumin  which  exhibits  no  measur- 
able osmotic  pressure.  On  the  other  hand  Reid  found,  after 
prolonged  dialysis,  osmotic  pressures  of  haemoglobin  which  were 
perfectly  constant,  and  indicated  a  molecular  weight3  of  about 
48,000.  The  reason  for  these  variations  in  osmotic  pressure  has 
been  asserted  by  Roaf,4  and  by  Bar  croft  and  Hill5  to  be  due  to  a 
polymerization  of  the  molecule  which  takes  place  upon  the 
elimination  of  ionogenic  impurities  or  combinations.  This  view 
is  substantiated  by  the  findings  of  Roaf  that  in  distilled  water 
haemoglobin  shows  a  molecular  weight  (by  osmotic  pressure) 
of  about  32,000  while  in  a  solution  of  sodium  carbonate,  which 
would  bring  about  further  ionization,  a  molecular  weight  of  only 
16,000  is  obtained. 

The  work  of  Lillie,6  and  especially  the  brilliant  researches  of 
Loeb,7  have  conclusively  demonstrated  that  gelatin  exhibits  a 
perfectly  definite  osmotic  pressure  under  any  precisely  specified 
conditions.  With  hydrochloric  and  sulphuric  acids  Loeb  ob- 

1  Wo.  OSTWALD — FISCHER,   "Handbook  of  Colloid  Chemistry,"  Phila- 
delphia (1915),  232. 

2  E.  REID,  /.  PhysioL,  31  (1904),  438;  33  (1905),  12. 

3  See  page  107  for  a  consideration  of  molecular  weight  determinations. 

4  ROAF,  Proc.  Am.  Phil  Soc.,  J.  PhysioL,  38  (1909),  1. 

5  BARCROFT  and  HILL,  /.  PhysioL,  39  (1910),  411. 

6  LILLIE,  Am.  J.  PhysioL,  20  (1907),  127. 

7  J.  LOEB,  J.  Gen.  PhysioL,  1  (1918-19),  483;  559;  3  (1920-21),  691. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN 


99 


tained  the  following  results  upon  a  1  per  cent  solution  of  originally 
isoelectric  gelatin : 


Osmotic  pressure  in 

Osmotic  pressure  in 

mm.  water 

mm.  water 

pH  of  solution 

pHof 
solution 

Gelatin 

Gelatin 

Gelatin 

Gelatin 

chloride 

sulphate 

chloride 

sulphate 

4.7 

29 

.   33 

2.85 

360 

164 

4.56 

124 

70 

2.52 

303 

125 

4.31 

202 

110 

2.13 

198 

95 

4.03 

322 

160 

1.99 

162 

85 

3.85 

375 

185 

1.79 

110 

70 

3.33 

443 

205 

1.57 

90 

61 

3.25 

442 

200 

These  data  illustrate  two  points  of  great  interest.  In  the  first 
place  the  osmotic  pressure  is  found  to  vary  enormously  at  differ- 
ent hydrogen-ion  concentrations.  In  the  above  experiments  the 
free  acid  had  been  washed  out  as  far  as  could  be  determined,  and 
the  varying  osmotic  pressures  developed  may  be  attributed, 
according  to  Loeb,  only  to  the  varying  amounts  of  gelatin 
chloride  and  gelatin  sulphate  produced.  The  maximum  produc- 
tion of  these  salts  is  reached  at  a  pH  of  about  3.4,  which  cor- 
responds to  the  maximal  development  of  osmotic  pressure. 

A  second  observation  is  that  the  maximum  osmotic  pressure 
obtained  with  sulphuric  acid  is  only  205  mm.  as  contrasted  with 
443  mm.  in  the  case  of  the  hydrochloric  acid.  After  making 
corrections  for  the  pressure  developed  by  isoelectric  gelatin,  the 
osmotic  pressures  become: 


Gelatin  chloride. 
Gelatin  sulphate. 


413  mm. 
180  mm. 


In  a  similar  way  it  was  shown  that  on  the  alkaline  side  of  the 
isoelectric  point,  gelatin,  combined  with  monovalent  cations, 
showed  a  maximal  pressure  of  about  400  mm.,  while  when  com- 
bined with  divalent  cations  it  attained  only  160  mm.  By 
correcting  as  before,  the  osmotic  pressure  became : 


Li,  Na,  K,  NH4  gelatinate, 370  mm. 

Ca,  Ba  gelatinate 130  mm. 


100  GELATIN  AND  GLUE 

If  these  differences  in  the  maximal  osmotic  pressures  are  to  be 
explained  in  accordance  with  the  theory  of  van't  Hoff  and  the 
laws  of  classical  chemistry,  it  is  necessary  to  assume  a  correspond- 
ing difference  in  the  number  of  particles  in  solution.  This  ques- 
tion will  receive  more  detailed  attention  in  Chap.  V. 

Non-electrolytes  appear  to  have  no  effect  upon  the  osmotic 
pressure  of  gelatin.  Inorganic  salts  act,  however,  in  a  very 
similar  way  to  the  acids  and  bases,  but  with  this  reservation: 
when  the  gelatin  is  in  the  form  of  an  anion,  as  in  sodium  gelatin- 
ate,  only  the  cation  of  the  added  salt  is  effective  e.g.,  the  addition 
of  calcium  chloride  would  influence  the  osmotic  pressure  of  the 
gelatin  only  by  virtue  of  the  calcium  ion.  A  lowering  would 
take  place.  Sodium  sulphate  would  be  ineffective.  If,  on 
the  other  hand,  the  gelatin  were  in  the  form  of  a  cation,  as  in 
gelatin  chloride,  then  only  the  anion  of  an  added  salt  would 
influence  the  osmotic  pressure,  e.g.,  sodium  sulphate  would 
lower  this  pressure,  while  calcium  chloride  would  be  without 
effect.  The  results  of  Hofmeister,1  Pauli,2  Lillie,3  and  others 
who  have  investigated  this  effect  differ  from  those  of  Loeb 
both  in  order  and  degree.  According  to  Lillie,  the  depressing 
effect  of  ions  upon  the  osmotic  pressure  of  gelatin  follows  the 
order:  For  cations:  alkali  metals  <  alkaline  earths  <  heavy 
metals.  For  anions :  CNS <  1< Br < N03 < Cl< F < SO4  < PO4. 

Loeb  ascribes  the  differences  as  due  to  the  failure  of  the  earlier 
investigators  to  measure  the  hydrogen  ion  concentration  of  their 
solutions.  Thus  when  a  buffer  salt,  like  sodium  acetate,  is 
added  to  a  gelatin  chloride  solution  of  pH  3.0  the  gelatin  solution 
is  brought  nearer  to  that  of  the  isoelectric  point,  and  this  was 
interpreted  by  the  earlier  workers  as  the  effect  of  the  acetate 
anion.  If  the  hydrogen  ion  concentration  is  taken  into  con- 
sideration it  is  found  that  'sodium  acetate  acts  like  sodium  chlo- 
ride.4 As  Loeb's  reports  upon  this  subject  are  of  comparatively 
recent  date  there  has  hardly  been  time  as  yet  to  ascertain  to  what 
degree  his  findings  will  replace  the  long  accepted  explanations  of 
Hofmeister  and  Pauli.  It  seems  certain,  however,  that  Loeb 
has  discovered  a  serious  error  in  the  earlier  investigations,  but 

1  HOFMEISTER,  Arch,  exptl.  Path.  Pharm.,  24  (1888),  247. 

2  PAULI,  Beitr.  physiol.  path.  Chem.,  3    (1903),  224;  Fortsch.  Naturwiss. 
Forschung,  4  (1912),  237. 

3  LILLIE,  Am.  J.  Physiol.,  20  (1907),  127. 

4  J.  LOEB,  /.  Gen.  Physiol.,  3  (1920-21),  407-8. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         101 

the  theory  which  he  advances  to  account  for  the  new  findings 
(which  will  be  discussed  in  Chap.  V)  will  doubtless  meet  with 
persistent  opposition  from  the  hands  of  many  colloid  chemists. 

The  development  of  a  definite  osmotic  pressure  by  colloids  is 
deducible  from  an  altogether  different  line  of  argument.  When  a 
protein  is  allowed  to  stand  with  an  acid  or  a  base  or  under 
certain  conditions  a  salt,  a  definite  chemical  reaction  is  found  to 
occur,1  which  eventually  reaches  a  definite  equilibrium.  If 
no  osmotic  pressure  were  developed  in  the  system,  the  transfor- 
mation of  a  given  mass  of  the  components  would,  according  to 
Robertson,  be  dependent  only  upon  the  temperature,  and  not  at 
all  upon  the  concentration  of  the  reacting  components.  The 
reaction  would,  therefore,  proceed  to  completion,  and  no  true 
equilibria  could  be  obtained.  But  it  has  been  shown  many 
times  that  protein  substances,  especially  in  the  form  of  their 
salts,  do  react  with  various  other  substances  with  the  develop- 
ment of  definite  equilibria.  This  fact  argues  against  the  theory 
of  Duclaux2  that  "  colloids  must  be  considered  as  having,  in 
water,  an  absolute  insolubility/'  and  favors  the  laws  of  Avogadro 
and  van't  Hoff  that,  in  the  attainment  of  such  equilibria,  the 
colloid  must  be  distributed  throughout  the  solution  in  molecu- 
lar dispersion.  Due,  however,  to  the  tendency  of  colloids  to 
polymerize  when  out  of  the  influence  of  electrolytes,  it  seems 
probable  that  real  suspensions  may,  under  certain  conditions, 
be  obtained  which  will  not  reveal  an  osmotic  pressure,  and 
accordingly  will  not  follow  the  law  of  Avogadro. 

From  the  abundant  literature  upon  the  osmotic  pressure  of 
protein,  every  conceivable  inconsistency  of  data  has  resulted. 
As  examples  of  these  the  following  striking  illustrations  may  be 
taken : 

Reid3  found  the  osmotic  pressure  of  ovalbumin,  under  the  same 
conditions  and  prepared  in  the  same  way,  to  vary  betwen  0.00 
and  15.71  mm.  of  mercury. 

Lillie4  found  the  osmotic  pressure  of  gelatin  to  increase,  and  of 
egg-albumin  to  decrease,  on  shaking. 

1T.  B.  ROBERTSON,  Z.  Chem.  Ind.  Kolloide,  2  (1908),  49;  "Physical 
Chemistry  of  the  Proteins,"  New  York  (1918),  340. 

2  DUCLAUX,    "Researches  sur  les  substances   Colloidals,"   Dissertation, 
Paris  (1904),  100. 

3  REID,  J.  Physiol.,  31  (1904),  439;  33  (1905),  12. 

4  LILLIE,  Am.  J.  Physiol.,  20  (1907),  127. 


102  GELATIN  AND  GLUE 

Biltz  and  von  Vegesack1  found  the  osmotic  pressure  of  congo- 
red  to  decrease,  and  of  iron  hydroxide  sol  to  increase,  with 
increasing  concentration. 

Bayliss2  found  the  osmotic  pressure  of  several  proteins  to 
increase  directly  with  the  absolute  temperature.  Moore  and 
Roaf3  found  the  osmotic  pressure  to  increase  much  faster  than 
the  absolute  temperature  in  gelatin  sols,  while  Duclaux  observed 
the  increase  to  be  slower  than  the  absolute  temperature  in  the 
case  of  iron  hydroxide  sol. 

Lillie4  affirms  that  all  salts  lower  the  osmotic  pressure  of 
gelatin. 

One  of  the  most  encouraging  developments  of  Loeb's  theories 
lies  in  their  ability  to  explain  and  adequately  to  account  for  the 
wide  diversity  and  apparently  chaotic  confliction  of  the  findings 
outlined  above.  When  measurements  are  made  in  the  presence 
of  the  excess  of  electrolyte,  which  has  almost  without  exception 
been  the  case,  the  results  obtainable  are  apparently  without 
order  or  reason;  but  just  as  soon  as  the  pure  gelatin  salt  is  once 
obtained,  then,  according  to  Loeb,  the  findings  become  well 
ordered,  readily  reproducible,  and,  above  all,  intelligible  and 
explainable  in  conformity  with  the  laws  of  classical  chemistry.5 

C.  R.  Smith6  has  taken  exception  to  some  of  the  conclusions 
reached  by  Loeb,  although  corroborating  his  experimental  find- 
ings. He  assumes  that  Loeb  used  an  unpurified  gelatin,  but 
this  seems  contrary  to  the  facts.7  In  his  own  experiments  Smith 
employs  an  ash  free  product  made  by  first  washing  out  from 
the  powdered  material  the  divalent  alkali  salts  using  a  10  per  cent 
sodium  chloride  solution  containing  about  5  c.c.  of  concentrated 
hydrochloric  acid  per  liter,  cooled  to  between  0  and  10°  C.,  and 
then  with  1  per  cent  salt  solution  without  acid.  The  concentra- 
tion of  the  salt  solution  is  then  diminished  until  finally  aerated- 
distilled  water  is  used.  When  the  washings  are  free  from  chloride, 
cold  90  per  cent  alcohol  is  poured  through  the  mass  until  it  has 

1  BILTZ  and  VON  VEGESACK,  Z.  physik.  Chem.,  68  (1909),  357;  73  (1910), 
481. 

2  BAYLISS,  Proc.  Roy.  Soc.  (London),  81  (1909),  269. 

3  MOORE  and  ROAF,  Biochem.  J.,  2  (1906),  34;  3  (1907),  55. 

4  LILLIE,  loc.  cit. 

p  For  a  further  discussion  see  Chap.  V. 

6  C.  R.  SMITH,  /.  Am.  Chem.  Soc.,  43  (1921),  1350. 

7  Miss  Field  has  confirmed  Loeb's  method  for  the  preparation  of  a  pure 
ash-free  gelatin.     /.  Am.  Chem.  Soc.,  43  (1921),  667. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         103 

shrunk  nearly  to  dryness,  after  which  it  is  dried  with  an  electric 
fan. 

Smith  concurs  with  Loeb  in  many  points.  He  finds  that 
isohydric  solutions  of  acids  or  of  bases  of  the  same  valency 
produce  the  same  osmotic  pressure,  the  value  increasing  on  both 
sides  of  the  isoelectric  point,  but  being  only  about  a  third  as 
great  for  bivalent  as  for  monovalent  electrolytes.  The  maxima 
occur  in  all  cases,  however,  at  the  same  pH.  These  values, 
according  to  Smith's  data,  which  were  calculated  only,  lie  at 
about  3.2  and  10.3,  for  a  concentration  of  0.5  g.  per  100  c.c.,  but 
the  hydrogen  ion  concentration  of  maximum  osmotic  pressure 
was  found  to  vary  with  the  concentration  of  the  gelatin.  By 
direct  measurement  Loeb  found  the  maximum  osmotic  pressure 
to  occur  at  pH  3.4.  Smith  is  led  to  believe  that  salt  ions  do  not 
combine  with  gelatin,  but  rather  increase  the  absorption  of  acids 
or  alkalies.  The  decrease  in  osmotic  pressure  and  swelling 
which  they  produce  is  explained  as  due  to  a  decrease  in  the  ioniza- 
tion  of  the  acids  or  alkalies  combined  with  the  gelatin. 

The  Determination  of  Osmotic  Pressure. — The  technique  of 
osmotic  pressure  determinations  requires  the  utmost  skill  and 
painstaking  care.  In  making  such  determinations  upon  mole- 
cular dispersoids  and  electrolytes  it  is,  of  course,  necessary  to 
employ  a  semipermeable  membrane  which  will  permit  of  the  free 
passage  of  the  solvent,  but  withhold  completely  the  molecules 
or  ions  of  the  solute.  Such  membranes  are  found  in  the  chemi- 
cally precipitated  films  such  as  copper  ferrocyanide,  and  the 
like.  But  when  it  is  desired  to  obtain  the  osmotic  pressure  of  a 
colloid  it  is  very  important  that  the  membrane  be  permeable, 
not  only  to  the  pure  solvent,  but  also  to  any  molecular  or  iono- 
genic  impurities  that  may  be  present.  When  such  membranes 
have  not  been  used  it  has  been  necessary  to  make  corrections  for 
the  pressure  developed  by  the  impurity,  and  such  a  correction  is 
usually,  at  best,  only  an  approximate  estimation. 

Parchment  paper  is  permeable  to  the  smaller  sized  molecules 
and  ions,  and  is  especially  easy  of  manipulation.  The  most 
careful  workers  have,  however,  usually  employed  collodion 
membranes.  The  preparation  of  these  has  been  described  on 
page  97. 

For  osmotic  pressure  determinations  a  50  c.c.  Erlenmeyer  shaped  sack  is 
very  satisfactory.  The  washed  sack  is  filled  with  the  colloid  sol,  and  a 
two  holed  rubber  stopper  inserted  in  the  neck  and  fastened  securely  with 


104  GELATIN  AND  GLUE 

several  turns  of  a  rubber  band.  In  one  of  the  holes  of  the  stopper  is  placed 
a  short  glass  tube,  extending  from  the  bottom  of  the  stopper  to  about  an 
inch  above  it.  In  the  other  is  inserted  a  long  glass  tube,  extending  from  the 
bottom  of  the  stopper  to  a  height  of  perhaps  24  inches.  The  large  tube  is 
left  open.  The  colloid  sol  is  introduced  through  tne  short  tube  until  the 
liquid  overflows;  and  this  tube  is  then  sealed  with  a  short  piece  of  rubber 
tubing,  closed  at  one  end.  The  sack  is  lowered  into  a  large  beaker  containing 
the  pure  solvent,  and  the  height  to  which  the  liquid  rises  in  the  long  tube 
noted  immediately,  and  at  the  end  of  a  few  hours,  at  which  time  it  should 
reach  its  maximum.  The  difference  in  millimeters  represents  directly  the 
osmotic  pressure  of  the  colloid  sol  in  millimeters  of  water  (or  other  solvent). 
Other  more  involved  types  of  manometer  may  be  used,  and  the  pressure  may 
be  measured  against  mercury,  but  the  procedure  as  outlined  has  been  found 
very  easy  of  manipulation,  and  capable  of  giving  excellent  and  readily 
duplicable  results. 

4.  The  Vapor  Pressure. — The  vapor  pressure  of  a  liquid  is  due 
to  an  equilibrium  which  exists  between  the  molecules  of  the 
liquid  and  those  of  a  space  above  it  which  is  saturated  with  the 
vapor  of  the  liquid.  The  molecules  of  any  given  liquid  at  any 
given  temperature  tend  to  project  themselves  into  the  space 
above  at  a  given  rate.  As  soon  as  this  space  is  saturated  an 
equilibrium  exists  due  to  the  fact  that  molecules  from  the  vapor 
are  entering  the  liquid  at  the  same  rate  as  those  from  the  liquid 
are  going  into  the  vapor  phase.  Dissolved  substances  are  found 
ordinarily  to  exert  no  vapor  pressure^  It  must  therefore  follow 
that  if  a  substance  is  dissolved  in  a  liquid, — there  being,  in  that 
case,  a  decreas~e  in  the  number  of  solvent  molecules  in  a  unit 
area  of  surface,  or  volume  of  solution, — the  number  of  molecules 
of  solvent  entering  the  vapor  phase  in  unit  period  of  time  must 
be  decreased.  It  must  also  follow  that  a  smaller  number  of 
molecules  in  the  vapor  state  will  suffice  to  return  to  the  solution, 
in  unit  period  of  time,  the  smaller  number  which  are  entering 
the  vapor  state.  In  other  words,  the  vapor  pressure  of  a  liquid 
is  lowered  by  the  addition  to  it  of  dissolved,  or  otherwise  dis- 
persed, molecules,  and  van't  Hoff  has  demonstrated  that  the 
lowering  of  the  vapor  pressure  of  a  liquid  is  in  direct  proportion 
to  the  number  of  molecules  or  ions  added. 

It  has  long  ago  been  shown,  however,  that  colloids  have  very 
little  effect  on  the  vapor  pressure  of  liquids.  Some  workers  have 
obtained  colloids  which  showed  no  change  whatsoever,1  while 
many  others  have  reported  various  reductions.  Liideking2  was 

1  A.  SMITS,  Z.  physik.  Chem.,  45  (1903),  608. 

2  C.  LUDEKING,  Ann.  Physik.  Chem.,  35  (1888),  552. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN        105 

unable  to  obtain  a  definite  lowering  in  vapor  pressure  with  gela- 
tin, but  Guthrie1  has  reported  positive  results.  Tammann2 
found  a  slight  reduction  in  vapor  pressure  due  to  gelatin,  but  the 
concentration  seemed  to  have  no  influence  on  the  results. 

The  reason  for  the  anomalous  behavior  of  gelatin  and  other 
colloids  in  failing  to  reduce  the  vapor  pressure  of  liquids  in  which 
they  are  dispersed  lies  undoubtedly  in  the  size  of  the  dispersed 
particles.  It  must  here  be  recalled  that  liquids  are  regarded, 
according  to  the  generally  accepted  theories,  as  consisting  of 
particles  (molecules,  ions,  or  aggregates  of  these)  which  are 
widely  separated  from  each  other,  and  are  moving  freely  about 
in  the  system.  When  dissolved  molecules  or  ions  are  added, 
these,  on  mixing  with  the  former,  produce  a  dilution  of  the 
solvents  molecules,  in  proportion  to  the  number  of  molecules 
added.  The  size  here  seems  to  have  practically  no  influence. 
But,  according  to  Millikan,3  a  gram-molecule  of  a  substance 
when  molecularly  dispersed  contains  about  6  X  1023  molecules. 
By  calculating  from  a  figure  of  this  order,  Perrin4  obtained  a 
" molecular  weight"  for  gutta-percha  of  about  3  X  1010. 

The  significance  of  these  values  is  made  evident  by  the  pro- 
vision of  van't  Hoff's  theory  which  would  require  that  thirty 
billion  grams  of  gutta-percha, — since  in  that  amount  there  are 
the  same  number  of  molecules  as  are  contained  in  one  gram 
molecule  of  any  other  substance, — must  be  dissolved  in  a  liter 
of  water  in  order  that  the  same  dilution  of  solvent  molecules, 
and  consequent  lowering  of  the  vapor  pressure,  might  ensue  as 
would  be  obtained  by  the  addition  of,  for  example,  60  g.  of  urea, 
or  342  g.  of  sugar.  It  becomes  strikingly  apparent  that  the 
addition  of  a  colloid,  in  any  amount  which  could  reasonably  be 
used,  would  have  an  almost  negligible  influence  on  the  vapor 
pressure.  Even  if  the  molecular  weight  were  placed  as  low  as 
2,433,  which  is  the  value  assigned  to  gelatin  by  Hofmeister,5  and 
is  not  properly  comparable  for  this  purpose,  a  gelatin  sol  would 
exert  but  a  fortieth  of  the  lowering  that  would  be  produced  by 
an  equal  weight  of  urea. 

1  F.  GUTHRIE,  Phil.  Mag,  (5),  2  (1876),  219. 

2  G.  TAMMANN,  Mem.  Acad.  St.  Petersburg  (7)  35. 

3  MILLIKAN,  Proc.  Nat.  Acad.  Sci.,  3  (1917),  314. 

4  J.  PERRIN,  Compt.  rend.,  147  (1908),  475. 

5  See  "Allen's  Commercial  Organic  Analysis,"  4th  ed.,  vol.    8    (1912), 
586. 


106  GELATIN  AND  GLUE 

5.  The  Boiling  Point  and  Freezing  Point. — As  the  temperature 
of  a  liquid  rises,  the  number  of  molecules  that  will  be  ejected 
from  it  into  the  vapor  space  above  will  continually  increase,  on 
account  of  the  added  kinetic  velocity  of  the  molecules  imparted 
by  the  increasing  temperature,  and,  if  the  pressure  above  the 
liquid  is  permitted  to  remain  constant,  a  temperature  is  eventu- 
ally reached  where  the  vapor  pressure  of  the  liquid  is  equal  to  the 
external  pressure.  This  is  called  the  boiling  point  of  the  liquid. 
It  is  obvious  therefore  that  any  condition  which  affects  the  vapor 
pressure  of  the  liquid  must,  a  priori,  affect  the  boiling  point.  If 
the  vapor  pressure  is  lowered,  as  by  the  dissolving  in  it  of  a  non- 
volatile substance,  then  the  boiling  point  of  the  solution  must  be 
raised. 

As  the  temperature  is  lowered  to  the  freezing  point,  which  is 
defined  as  the  temperature  at  which,  under  a  given  pressure,  the 
liquid  and  solid  phase  may  exist  together,  the  vapor  pressure 
decreases  and,  when  that  point  is  reached,  the  vapor  pressure 
of  the  liquid  and  that  of  the  solid  must  be  identical.  But  in  the 
presence  of  a  dissolved  substance,  which  lowers  the  vapor  pres- 
sure, an  even  lower  temperature  must  be  reached  before  the 
pressures  exerted  by  the  two  phases  are  equal.  In  other  words, 
a  non-volatile  dissolved  substance  lowers  the  freezing  point  of  a 
liquid.  Van't  Hoff  has  shown,  moreover,  that  just  as  the  lower-- 
ing of  the  vapor  pressure  is  directly  proportional  to  the  number 
of  dissolved  molecules,  so  also  are  the  elevation  of  the  boiling 
point  and  the  lowering  of  the  freezing  point  directly  proportional 
to  this  number. 

In  view  of  the  above  proportionality  it  is  obvious  that  colloids 
can  effect  but  little  change  in  the  boiling  points  or  freezing  points 
of  liquids  in  which  they  are  dispersed.  Many  attempts  have 
been  made  however  to  determine  the  effect  of  various  colloids 
upon  these  points,  and  the  values  obtained  have  often  been  used 
in  calculating  a  " molecular  weight."  In  illustration  may  be 
cited  the  work  of  Friedenthal,1  who  found  a  small  but  definite 
depression  of  the  freezing  point  in  solutions  of  soluble  starch; 
Sabenejew  and  Alexandrow,2  who  found  egg  albumin  to  have  a 
molecular  weight  of  14,270;  Krafft  and  Sturtz,3  who  found  the 

1  H.  FRIEDENTHAL,  Zent.  Physiol.,  12  (1899),  849. 

2  A.  SABENEJEW  and  N.  ALEXANDROW,  J.  Russ.  Phys.  Chem.  Soc.,  21  (1889), 
397. 

3  F.' KRAFFT  and  A.  STURTZ,  Ber.,  29  (1896),  1328. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         107 

molecular  weight  of  sodium  stearate  to  be  about  1,500  at  16 
per  cent,  and  nearly  infinity  at  27  per  cent  concentration; 
Robertson  and  Burnett,1  who  found  alkaline  caseinates  to  show 
a  molecular  weight  of  1,400  to  17,600;  Liideking,2  who  found 
that  even  a  40  per  cent  solution  of  gelatin  failed  appreciably  to 
affect  the  boiling  point;  and  Krafft  and  Wiglow,3  whose  findings 
confirmed  those  of  Liideking.  One  fact  stands  out  conspicuously 
in  the  work  of  Robertson  and  Burnett,  namely,  that  the  depres- 
sions in  the  freezing  points  stand  in  direct  proportion  to  the  con- 
centration of  combined  base.  As  the  authors  point  out,  this  must 
mean  that,  if  this  relation  held  at  all  concentrations,  then  "at 
zero  concentration  of  combined  base,  if  casein  wer.e  soluble  under 
such  conditions,  the  freezing  point  depression  due  to  dissolved 
casein  would  be  zero.  In  other  words  the  possibility  is  indicated 
that  base-  and  acid-free  protein  may  exert  an  immeasurably 
small  osmotic  pressure."  Robertson  attributes  this  to  a  poly- 
merization of  the  protein  as  the  uncombined  protein  is  set  free. 

6.  The  Molecular  Weight. — There  is  one  feature  of  the  colloid 
conception  which  is  almost  invariably  overlooked  by  investigators 
upon  the  subject,  and  which,  in  the  opinion  of  the  author,  is  of 
the  utmost  importance  to  a  proper  understanding  of  colloids,  and 
especially  of  the  proteins.  The  concept  of  colloids  has  been  based 
almost  entirely  upon  the  size  of  the  particles  in  suspension.  For 
example,  gold,  which  we  know  possesses  a  true  molecular  weight 
of  197.2  may  readily  be  obtained  in  a  colloid  state  of  suspension: 
that  is  a  large  number  of  molecules  are  caused  by  some  force  to 
flock  together  into  one  "  particle,"  and  the  size  of  these  particles, 
which  is,  of  course,  determined  by  the  absolute  number  of 
molecules  composing  them,  is  the  criterion  for  the  colloid  state. 
It  is  well  known  that  colloidal  gold  may  easily  be  obtained  in  a 
number  of  different  "degrees  of  dispersion,"  that  is,  a  relatively 
small  or  a  relatively  large  number  of  molecules  may  be  aggregated 
together  in  each  particle.  Now  it  may  be  possible  to  obtain 
the  relative  weight  of  these  particles  by  osmotic  pressure,  or 
vapor  pressure  methods,  but  no  one  would  affirm  that  any  value 
so  obtained  represented  correctly  the  molecular  weight.  We 
know  that  arsenic  trisulphide  has  a  molecular  weight  of  246.1. 
A  colloid  particle  of  this  substance  may  consist,  for  example,  of 

1  T.  B.  ROBERTSON  and  T.  BURNETT,  J.  Biol.  Chem.,  6  (1909),  105. 

2  C.  LUDEKING,  Ann.  chim.  phijs.,  36  (1888),  552. 

3  F.  KRAFFT  and  H.  WIGLOW,  Ber.,  28  (1895),  2566. 


J 


108  GELATIN  AND  GLUE 

100  molecules.     Then  the  particle  weight  would  be  24,61Q,  but 
under  no  condition  would  this  be  regarded  as  a  molecular  weight. 

In  the  case  of  proteins  quite  a  different  condition  obtains. 
There  is  no  doubt  but  that  many  proteins  contain  molecules  of 
such  complexity  that  one  individual  may  possess  a  true  molecular 
weight  of  several  thousand,  and  be  of  such  size  that  a  molecu- 
larly  dispersed  solution  will  lie  in  the  field  of  the  colloids.  This 
point  has  been  overlooked.  Wo.  Ostwald  justly  deprecates  the 
use  of  osmotic  pressure  and  vapor  pressure  methods  in  the  calcu- 
lation of  so-called  molecular  weights  of  colloids,  apparently 
basing  his  argument  upon  the  theory  that  all  colloidal  particles 
are  groups  of  polymerized  molecules.  In  the  great  majority  of 
cases  his  argument  is  doubtless  correct,  for  just  as  gold  and  arsenic 
sulphide  molecules  may  be  caused  to  undergo  an  extensive  poly- 
merization, so  also,  and  in  reality  much  more  easily,  may  protein 
molecules  be  caused  to  combine  into  large  aggregates.  The 
indications  in  the  work  of  Robertson  and  others  are  that,  when 
unionized,  the  proteins  are  highly  polymerized  aggregates  of 
molecules,  but  when  ionized  may  be  obtained  in  a  molecular 
degree  of  dispersion,  although  they  are,  even  then,  of  colloid 
dimensions. 

Bearing  in  mind  these  reservations  molecular  weight  deter- 
minations may  be  of  value  in  many  ways,  but  it  will  be  evident 
at  once  that  such  determinations,  when  made  by  different 
means,  should  not  be  expected  to  give  results  that  are  comparable 
with  each  other.  Osmotic  pressure,  freezing  point,  and  boiling 
point  estimations,  being  dependent  upon  the  number  of  particles 
in  solution,  will  give  values  approaching  the  mass  of  the  colloid 
complex,  and  this  has  been  shown  to  vary  greatly  under  different 
conditions.  By  determination  of  the  combining  capacity,  an 
equivalent  weight  will  be  measured. 

The  Osmotic  Pressure  Method. — Osmotic  pressure  is  dependent 
for  its  existence  upon  the  presence  of  molecules  or  otherwise 
dispersed  particles  in  the  solution,  and  has  been  found  to  be,  in 
the  case  of  molecular  dispersoids,  directly  proportional  to  the 
number  of  molecules  in  the  solution,  and  to  the  absolute  tem- 
perature. In  other  words,  the  behavior  of  solutions  is  in  con- 
formity to  the  gas  laws,  and  the  osmotic  pressure  of  a  substance 
in  solution  is  the  same  as  the  pressure  which  it  would  exert  if  it 
were  in  the  form  of  a  gas  at  the  same  concentration  and  tem- 
perature. These  laws  have  no  concern  over  the  size  of  the  par- 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN        109 

tides,  nor  their  degree  of  hydration.  This  being  the  case,  it  is  at 
once  apparent  that  the  molecular  weight  or  the  average  relative 
weight  of  the  particles,  may  be  calculated  if  the  osmotic  pressure 
is  known.  Thus 

/22.4  X  760 
M  =  I  - 


where  M  is  the  molecular  weight;  22.4  the  osmotic  pressure  of 
one  gram-molecule  of  any  molecularly  dispersed  substance  at  0 
degrees;  760,  the  pressure  in  millimeters  of  mercury  of  the  atmos- 
phere at  sea-level;  273,  the  absolute  temperature  of  zero  degrees 
Centigrade;  C,  the  concentration  of  the  solute  in  per  cent;  T 
the  absolute  temperature  of  the  observation;  and  P  the  osmotic 
pressure  observed. 

This  formula  has  frequently  been  used  in  an  estimation  of  the 
so-called  molecular  weight  of  proteins  and  other  colloids.  The 
justifiability  of  such  an  application  has  been  seriously  questioned 
by  Wo.  Ostwald,1  von  Smoluchowski,2  and  others.  Ostwald 
maintains  that,  in  the  case  of  colloids,  the  degree  of  dispersion 
and  the  degree  of  hydration  of  the  dispersed  phase  must  be 
considered.  M.  von  Smoluchowski  has  developed  a  formula 
based  upon  the  molecular-kinetic  theory  which  concludes  that 
"the  osmotic  pressures  of  two  equally  concentrated  but  differ- 
ently dispersed  phases  are  inversely  proportional  to  the  cubes 
of  the  radii  of  their  particles."  The  argument  of  both  of  these 
investigators  is  based  largely  upon  the  observed  and  measured 
Brownian  movement,  which  is  found  to  be  greater  in  the  more 
highly  dispersed  systems.  It  would  surely  seem  to  be  an  a  priori 
necessity,  and  based  upon  the  most  fundamental  principles, 
that  the  more  rapidly  the  particles  in  a  solution  or  suspension  are 
moving,  the  greater  must  be  the  osmotic  pressure  resulting  from 
such  movement. 

Experimental  evidence  of  the  soundness  of  this  reasoning  is 
not  lacking.  Bayliss3  has  found  in  experiments  upon  congo-red 
that  those  factors  which  produce  a  decrease  in  degree  of  disper- 
sion, as  shaking,  aging,  addition  of  electrolytes,  etc.,  decrease 
the  osmotic  pressure,  while  other  factors  which  increase  the 
degree  of  dispersion,  also  increase  the  osmotic  pressure.  Duc- 

1  Wo.  OSWALD,  lib.  cit. 

2  M.  VON  SMOLUCHOWSKI,  Boltzmann-Festschrift,  Leipzig  (1904),  626. 

3  W.  BAYLISS,  Prod.  Roy.  Soc.   (London),  81  (1909),  269;  Kolloid-Z.,  6 
(1910),  23. 


110  GELATIN  AND  GLUE 

laux1  observed  that  the  osmotic  pressure  of  the  highly  dispersed 
red  gold  hydrosol  was  considerably  greater  than  that  of  the  more 
coarsely  dispersed  blue  variety.  Ostwald2  found  that  the  osmotic 
pressure  and  the  swelling  of  gelatin  disks  paralleled  each  other 
under  the  influence  of  acids  and  alkalies,  even  to  details.  These 
observations,  when  taken  in  connection  with  the  great  disparity 
between  measurements  made  by  different  investigators,  lead,  if 
not  to  the  entire  rejection  of  the  osmotic  pressure  method  for 
so-called  molecular  weight  determinations  of  colloids,  at  least  to 
a  reserved  use  for  such  expressions.  They  are  of  undisputable 
use  in  comparative  studies,  but  a  dogmatic  acceptance  of  the 
absolute  values  reported  seems,  at  the  present  time,  to  be 
dangerous  and  unjustified.  For  this  reason  it  appears  more 
expedient  to  use  the  results  of  osmotic  pressure  determinations 
per  se,  rather  than  to  make  use  of  the  calculated  hypothetical 
molecular  weight. 

C.  R.  Smith3  has  observed  that  the  osmotic  pressure  of  a  gelatin 
solution  in  water  is  proportional  to  the  concentration,  and,  assum- 
ing the  applicability  of  the  gas  laws,  finds  a  molecular  weight 
for  the  gelatin  of  96,000. 

Loeb4  found  the  osmotic  pressure  due  to  the  gelatin  particles  of 
a  1  per  cent  solution  of  gelatin  phosphate  of  pH  3.60  to  be  about 
100  mm.  of  water.  Since  the  osmotic  pressure  of  1  gram  mole- 
cule is  about  250,000  mm.  of  water,  and  since  1  liter  of  a  1  per 
cent  solution  of  gelatin  contains  10  g.  of  gelatin,  Loeb  deduces 
that  the  molecular  weight  of  gelatin  should  be  expected  to  be  in 
the  neighborhood  of  25,000. 

The  Boiling  Point  Method. — By  employing  the  boiling  point 
method  as  previously  described  on  a  gelatin  that  had  been 
rendered  nearly  ash-free  (0.07  per  cent  ash)  by  prolonged  dialysis, 
Paal5  obtained  values  for  the  molecular  weight  of  gelatin  ranging 
from  878  to  960. 

The  Combining  Capacity  Method. — In  a  study  of  the  equi- 
librium between  hydrochloric  acid  and  gelatin,  Procter6  found  that 
the  gelatin  entered  into  chemical  combination  with  the  chloride 

1  DUCLAUX,  Compt.  rend.,  148  (1909),  295. 

2  Wo.  OSTWALD,  lib.  tit. 

3  C.  R.  SMITH,  J.  Am.  Chem.  Soc.,  43  (1921),  1350. 

4  J.  LOEB,  /.  Gen.  Physiol,  3  (1921),  704. 

5  C.  PAAL,  Ber.,  25  (1892),  1202. 

6  H.  R.  PROCTER,  /.  Chem.  Soc.,  105  (1914),  313.     See  also  page  128. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         111 

ion,  and  that  this  combination  appeared  to  be  a  reversible  reac- 
tion following  the  Mass  Law,  and  conforming  to  the  Ostwald 
hydrolysis  formula  for  a  diacid  base.  That  is, 

x  x  100 

y  ~  x  +  ki       x  +  &2  X   M' 

where  y  is  the  proportion  of  unhydrolyzed  salt  to  the  total  base 
present;  x,  the  molecular  concentration  or  normality  of  the  equi- 
librium-acid; M,  the  molecular  weight  of  the  gelatin;  ki  the 
hydrolysis  constant  of  the  primary  dissociation;  and  fc2  the  hydro- 
lysis constant  of  the  secondary  dissociation.  As  there  are  three 
variables,  ki}  k?,  and  M,  Procter  used  a  method  of  approximation, 
arbitrarily  fixing  M,  calculating  ki  and  &2,  and  applying  to  the 
formula  above.  Only  when  the  value  of  M  is  very  nearly  accu- 
rate will  the  calculated  and  experimental  values  of  y  coincide 
throughout.  The  value  of  839  for  the  molecular  weight  of  the 
gelatin  was  thus  derived  which  appeared  to  conform  satisfac- 
torily to  the  provisions  of  the  formula.  Using  this  value  Procter 
calculated  the  " rational"  formula  of  gelatin  to  be 

CwHBTOiaNn, 

which  gave  a  percentage  composition  corresponding  very  well 
with  results  obtained  analytically: 


Theetetical 

Experimental 

c  

50.06 

V50.1 

H  
0  

N 

6.79 
24.  79 
18  36 

6.6 
25.0 
18.3 

.  rt 

It  will  be  observed  that  the  value  of  839  for  the  molecular 
weight  of  gelatin  corresponds  very  well  with  the  values  878  to 
960  obtained  by  Paal1  using  the  boiling  point  method.  It  would 
seem  probable  from  this  that  the  sol  of  Paal. was  molecularly 
dispersed,  and  that,  in  that  instance  at  least,  the  colloid  particle 
was  not  an  aggregate  of  molecules,  as  is  usually  the  case  in 
colloid  sols,  but  consisted  of  an  individual  gelatin  molecule. 

Working  on  the  assumption  that  gelatin  is  a  monoacid  rather 
than  a  diacid  base  in  its  reactions  with  dilute  hydrochloric  acid, 
Wilson,2  by  similar  methods  to  those  employed  by  Procter, 

1  C.  PAAL,  loc.  cit. 

2  J.  A.  WILSON,  J.  Am.  Leather  Chem.  Assn.,  12  (1917),  115. 


112  GELATIN  AND  GLUE 

found  gelatin  to  have  a  molecular  weight  (or  an  equivalent  weight 
multiple)  of  768,  and  gave  it  the  slightly  altered  formula 

C32H52Oi2Nio. 

In  support  of  this  view  Wilson  observed  that  in  the  tanning  of 
leather  by  the  sesquioxid  of  chromium,  from  3.2  to  3.5  per  cent 
of  the  latter  is  held  in  combination  by  the  hide.  Considering 
the  hide  as  collagen  with  the  molecular  weight  of  750  (gelatin- 
water),  the  smallest  amount  of  the  chromic  oxide  required  to 
convert  100  g.  of  collagen  into  the  chromium  salt  would  be: 

152  X  100 

Tx-750-    "3.38 grams, 

which  agrees  well  with  the  observed  amount  found  to  be  necessary. 

The  value  768  also  seems  highly  probable  from  the  conformity 
of  the  experimental  and  the  mathematical  curves  obtained  by 
plotting  the  c.c.  of  solution  absorbed  by  one  millimol  of  gelatin 
against  the  concentration  of  hydrogen  ion  in  the  external  solu- 
tion, when  the  molecular  weight  of  gelatin  is  assumed  to  be  768. 
These  curves  are  shown  on  pages  181—2. 

Thomas1  has  obtained  an  octachrome  collagen,  and  by  applying 
the  value  of  750  as  the  molecular  weight  of  collagen,  has  obtained 
the  value  of  93  as  the  combining  weight  of  collagen. 

Calculation  from  Amino  Acids. — If  the  attempt  is  made  to 
calculate  the  molecular  weight  of  gelatin  from  the  amino-acid 
content,  comparatively  high  results  are  obtained.  The  most 
reliable  amino-acid  determinations  that  have  been  made  are 
those  of  Dakin.2  If  the  percentages  of  nitrogen  represented  by 
the  several  amino-acids  are  to  be  altered  so  as  to  represent 
nitrogen  atoms  it  becomes  necessary  to  multiply  each  percent- 
age figure  by  some  factor. 

The  histidine  molecule  contains  three  atoms  of  nitrogen,  the 
arginine  four  and  the  lysine  two.  All  others  one  each.  The 
histidine  nitrogen  is  present  in  only  very  small  amounts  (0.9 
per  cent),  but  if  only  one  molecule  be  present  in  the  gelatin  com- 
plex, it  will  contain  three  nitrogen  atoms.  In  order  to  raise  the 
histidine  value  to  3,  it  is  necessary  to  multiply  the  percentage 
figure  by  3.33.  If  this  factor  be  used  throughout,  the  results 
shown  below  in  Col.  3  are  obtained.  By  eliminating  the  frac- 

1  A.  W.  THOMAS,  paper  presented  at  New  York  Meeting  of  Am.  Chem. 
Soc.,  Sept.  6  to  10,  1921. 

2  H.  D.  DAKIN,  /.  Biol  Chem.,  44  (1920),  499. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         113 


tions  and  dividing  by  the  number  of  nitrogen  atoms  per  molecule, 
the  number  of  molecules  of  each  amino-acid  present  are  found 
(Col.  5),  and  by  multiplying  these  by  the  number  of  nitrogen 
atoms  in  each  molecule  the  revised  number  of  nitrogen  atoms  is 
obtained.  This  number  is  found  to  be  305,  which  multiplied  by 
the  atomic  weight  of  nitrogen  and  divided  by  the  percentage  of 
nitrogen  in  gelatin  gives  the  molecular  weight  of  the  gelatin: 


Amino  acid 

Per  cent 
of  total 
nitrogen 

X  3.33 

N  atoms 
in 
molecule 

Number 
of 
molecules 

Product  of 
last  two 
columns 

Glycine  

25.5 

85.0 

1 

85 

85 

Alanine 

8  7 

29  0 

1 

29 

29 

Leucine  

7.1 

23.6 

1 

24 

24 

Serine 

0  4 

1  3 

1 

1 

Phenylalanine  
Proline 

1.4 
9  5 

4.7 
31  6 

5 
32 

5 
32 

Hydroxyproline.  .  .  . 
Aspartic  acid. 

14.1 
3  4 

47.0 
11.3 

47 
11 

47 
11 

Glutamic  acid  
Histidine       

5.8 
0.9 

19.3 
3.0 

3 

19 
1 

19 
3 

Arginine  

8.2 

27.3 

4 

7 

28 

Lysine  

5.9 

19.7 

2' 

10 

20 

Ammonia 

0  4 

1  3 

1 

1 

1 

Total  

91.3 

304.1 

305 

305  X  14  X  100 
18 


=  23,700. 


The  value  23,700  is  obtained  as  the  calculated  molecular 
weight  of  gelatin,  but  too  great  significance  should  not  be  placed 
upon  a  value  derived  in  this  manner. 

The  values  obtained  for  the  molecular  weight  of  gelatin  by 
the  various  methods  are  brought  together  below : 

Schutzenberger  and  Bourgeois1  (1876) 1,836 

Paal2  (1892) 878  to  960 

Berrar3  (1912) 823 

1  SCHUTZENBERGER  and  BOURGEOIS,  Jahresber.  Thier.  chem.  (1876),  30. 

2  PAAL,  loc.  cit. 

3  BERRAR,  Biochem.  Z.,  47  (1912),  189. 

8 


114  GELATIN  AND  GLUE 

Procter4  (1914) 839 

Biltz,  Bugge,  and  Mehler5  (1916) 5,500  to  31,000 

Wilson6  (1917) 768 

Lloyd7  (1920) .10,300 

Smith*  (1921) 96,000 

By  calculation  from  Dakin9  (1920) 23,700 

Loeb10  (1921) 25,000 

4  PROCTER,  loc.  cit. 

5  BILTZ,  BUGGE,  and  MEHLER,  Z.  physik.  Chem.,  91  (1916),  705. 

6  WILSON,  loc.  cit. 

7  LLOYD,  Biochem.  J.,  14  (1920),  166. 

8  SMITH,  loc.  cit. 

9  DAKIN,  loc.  cit. 
10  LOEB,  loc.  cit. 

7.  The  Surface  Tension. — All  liquids  appear  to  possess 
different  properties  at  the  surface  than  at  other  interior  points. 
For  example,  if  a  capillary  tube  of  glass  is  placed  upright  in 
water,  the  water  is  observed  to  rise  in  the  tube  to  a  point  higher 
than  the  surface  of  the  external  liquid,  and  the  surface  of  this 
water  in  the  tube  is  curved  downwards,  i.e.,  the  water  rises 
higher  adjacent  to  the  glass  than  in  the  centre  of  the  tube.  If  a 
tube  be  made  of  some  material  which  is  not  wetted  by  the  water, 
or  if  a  glass  tube  be  placed  in  mercury,  which  does  not  wet  glass, 
the  reverse  conditions  are  found  to  obtain.  The  surface  of  the 
liquid  in  the  tube  is  below  the  surface  outside,  and  the  meniscus 
is  convex.  Again,  if  a  needle  be  placed  carefully  upon  water  it 
may  be  caused  to  float.  These  phenomena  are  the  result  of  the 
existence,  at  the  surface  of  liquids,  of  a  film,  and  of  the  resistance 
of  this  film  to  being  broken.  The  strength  of  this  film  varies 
greatly  in  different  liquids,  and  a  measure  of  its  resistance  to 
breaking  is  known  as  the  surface  tension  of  the  liquid. 

Nearly  all  inorganic  salts  in  solution  raise  the  surface  tension 
V'  of  the  solvent,  while  colloids  of  the  suspensoid  class  are,  as  a  rule, 
without  effect.  Emulsoid  colloids,  on  the  other  hand,  usually 
lower  the  surface  tension.  Small  amounts  of  soaps  are  especially 
effective;  egg  albumin  is  less  so.  Gelatin  lowers  the  surface 
tension  of  water,  but  to  a  lesser  degree  than  the  others  named. 
Table  29,  taken  from  Quincke,1  illustrates  this  effect. 

The  surface  tension  of  liquids  decreases  with  rise  in  tempera- 
ture, but,  if  a  substance  which  lowers  surface  tension  is  dispersed 
in  the  liquid,  the  lowering  will  be  proportionately  greater  than. 

1  G.  QUINCKE,  Ann.  Physik.,  10  (1903),  507.     Cited  after  Ostwald., 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN  -       115 
TABLE  29. — SURFACE  TENSION  OF  COLLOID  SOLS  20°  C. 


Substance 

Specific  gravity 

Surface   tension 
against  air 

Water                                        

1  .  0000 

8  253 

Egg  albumin 

1  0384 

5  141 

Venetian  soap  (24  oo  per  cent)  
Isinglass 

0.9992 
1  0000 

2.672 
6  790 

Gelatin1  

1.0000 

7.272 

1  Bancroft  questions  the  accuracy  of  the  specific  gravity  data  indicated 
for  gelatin  sols. 

in  the  pure  solvent.     This  is  shown  by  the  following  table  from 
Zlobicki:1 


TABLE  30. — CHANGES  IN  SURFACE  TENSION  WITH  TEMPERATURE 


Temperature 

Water 

2  per  cent  gelatin  sol 

0.0 

7.69 

6.62 

11.3 

7.52 

6.21 

17.0 

7.43 

5.98 

24.5 

7.32 

5.70 

Of  especial  importance  in  a  study  of  gelatin  is  the  theorem  of 
Willard  Gibbs,2  which  states  that  substances  which  lower  the 
surface  tension  of  a  liquid  tend  to  become  more  highly  concen- 
trated in  the  surface  layer  than  at  other  parts  of  the  solution. 
This  theorem  applies,  however,  according  to  Bancroft  (personal 
communication)  only  to  true  solutions.  Ramsden,3  who  has 
studied  this  phenomenon  with  proteins,  concludes  that  the  forma- 
tion of  membrane-like  films  which  appear  upon  the  surface  of 
some  proteins,  such  as  milk  and  gelatin,  on  heating,  is  attribut- 
able to  this  effect.  It  seems  certain  that,  in  the  case  of  gelatin 
at  least,  another  important  factor  in  the  film  formation  is  a 
dehydration  occurring  at  the  surface.  The  combined  effect  of  a 

1  L.  ZLOBICKI,  Bull  Acad.  Sc.  Cracovie  (1906)  488.     Cited  after  Ostwald. 

2  W.  GIBBS,  Trans.  Conn.  Acad.  Arts.  Sci.,  3  (1874-1878). 

3  N.  RAMSDEN,  Arch.  Anat.  Physiol.   (1894),  517;  Z.  physik.  Chem.,  47 
(1904),  336. 


116  GELATIN  AND  GLUE 

surface  concentration,  a  high  temperature,  and  a  dehydration, 
are  entirely  adequate  to  account  for  the  heavy  films  formed  over 
a  heated  solution  of  gelatin  or  glue. 

That  the  formation  of  films  is  not  confined  to  the  surface 
exposed  to  the  air  only,  but  may  be  produced  also  at  a  liquid 
interface,  is  shown  by  the  experiments  of  Winkelblech,1  which 
have  been  extended  and  modified  by  a  number  of  later  investiga- 
tors. If  a  little  gelatin  sol  is  shaken  up  with  benzine,  chloroform, 
or  other  hydrocarbon,  a  large  number  of  droplets  are  produced 
at  the  liquid  interface  between  the  two  solvents.  These  droplets 
are  unusually  stable.  Long  standing,  long  washing,  treating 
with  N/10  alkali,  or  shaking  with  more  hydrocarbon  fail  to 
bring  about  a  coalescence.  But  if  alcohol  is  added  it  enters  the 
droplets,  probably  by  osmosis,  causing  them  to  swell  and  burst, 
and  the  broken  membranes  may  be  seen  floating  in  the  otherwise 
clear  solution.2  Much  significance  is  placed  upon  these  experi- 
ments by  Bancroft,3  who  uses  them  as  the  basis  of  a  theory  of 
emulsions  which  postulates  that  the  efficacy  of  an  emulsifying 
agent  is  attributable  to  the  formation  of  similar  membranes 
around  the  dispersed  phase  of  the  emulsion.4 

8.  The  Optical  Rotation. — When  polarized  light  is  allowed  to 
pass  through  the  solution  of  an  organic  substance  which  contains 
an  asymmetric  carbon  atom,  the  ray  of  light  is  rotated  in  one 
direction  or  the  other,  depending  upon  the  structure  of  the 
asymmetric  group,  and  the  extent  of  such  rotation,  is  known  as 
the  optical  rotation  of  the  solution.  The  specific  rotation  is 
designated  by  the  symbol  [o:]D,  and  refers  to  the  rotation  produced 
by  a  solution  of  unit  denisty  in  a  layer  of  unit  length.  That  is, 
the  observed  rotation  in  degrees  is  divided  by  the  length  of  the 
tube  in  which  the  solution  is  contained,  and  by  the  density  of 
the  solution  at  the  temperature  of  observation. 

Prior  to  1910  there  had  been  very  little  work  reported  upon 
the  optical  rotation  of  gelatin.  De  Bary,5  Kruger,6  and  Framm7 

1  WINKELBLECH,  Z.  angew.  Chem.,  19  (1906),  1953. 

2  T.  B.  ROBERTSON,  J.  Biol  Chem.,  4  (1908),  1. 

3  W.  D.  BANCROFT,  J.  Phys.  Chem.,  19  (1915),  297. 

4  See  page  214. 

5  DE  BARY,  Hoppe-Seyler's  "  Medizinisch-Chemische  Untersuchungen," 
1  (1866),  71. 

6  KRUGER,  Mayl's  "  Jahresberichte  iiber  die  Fortschritte  der  Tierchemie," 
(1889),  29. 

7  FRAMM,  Arch.  ges.  Physiol,  68  (1897),  144. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN        117 

had  shown  that  the  specific  rotary  power  of  gelatin  changes  with 
the  temperature,  and  that  prolonged  heating  at  100°  gives  a 
product,  known  as  0  gelatin,  which  produces  a  rotation  lower 
than  that  of  ordinary  gelatin.  Trunkel,1  in  1910  made  a  valuable 
contribution  to  the  subject  by  showing  (1)  that  the  specific 
rotation  of  a  gelatin  sol  is  practically  constant  between  30  and 
80°C.;  (2)  that  the  rotation  increases  considerably  to  the  left  as  the 
temperature  is  lowered  to  10°;  and  (3)  that  the  rotation  may  be 
brought  back  to  its  original  value  by  again  raising  the  tempera- 
ture. C.  R.  Smith2  has  recently  made  a  more  exhaustive  study 
of  the  subject,  and  especial  attention  will  be  given  to  the  extra- 
ordinarily suggestive  findings  of  this  investigator. 

By  using  a  high  grade  ossein  gelatin  Smith  found  that  above 
33°C.  a  perfectly  constant  rotation  was  attained  in  a  short  period 
of  time.  At  lower  temperatures  several  hours  must  be  allowed 
for  the  rotation  to  become  constant.  At  and  above  35°  the 
specific  rotary  power  is  practically  constant  for  all  gelatins  used, 
and  at  all  concentrations.  As  the  solution  becomes  cooler  than 
35°  the  specific  rotation  rapidly  drops  until,  at  15°  and  below,  it 
has  again  become  constant.  At  all  intermediate  temperatures 
the  specific  rotation  varies  between  these  two  values  as  the  tem- 
perature. From  a  large  number  of  averages,  [a]D  at  35°  and 
above,  calculated  on  a  moisture-  and  ash-free  basis,  is  taken  as 
-141°;  and  at  15°  and  below  [«]D  =  -313°. 

This  peculiar  behavior  of  the  gelatin  leads  to  the  belief  that 
there  are  two  distinct  forms  of  the  substance:  one,  which  Smith 
calls  sol  form  A,  is  stable  at  35°  and  above;  the  other,  gel  form 
B,  is  stable  at  15°  and  below.  At  any  other  temperature  there 
will  exist  an  equilibrium  of  the  two  forms,  or 

«  Sol  form  A  +±  Gel  form  B. 

In  support  of  the  postulation  of  the  existence  of  an  equilibrium 
of  two  distinct  types  of  gelatin  between  the  temperatures  of 
15  and  35°,  the  velocity  of  mutarotation,  i.e.,  the  velocity  of  the 
change  in  rotation,  was  studied,  and  found  to  conform  to  the 
usual  equations  for  a  bimolecular  reaction  in  which  the  two 
reacting  components  are  present  in  equivalent  proportions.  By 
applying  the  equation  dx/dt  =  k(a  —  x)2, — which,  when  inte- 

1  TRUNKEL,  Biochem.  Z.,  26  (1910),  493. 

-  C.  R.  SMITH,  J.  Am.  Chem.  Soc.,  41  (1919),  135;  J.  Ind.  Eng.  Chem., 
12  (1920),  878. 


118  GELATIN  AND  GLUE 

grated,  becomes  k  =l/t.x/a(a  —  x),  where  a  represents  the 
active  mass  of  each  component;  x,  the  quantity  transformed  in 
time  t;  and  k,  a  constant, — Smith  obtained  a  fairly  uniform  value 
for  k  at  any  given  temperature.  Further,  by  applying  the  mathe- 
matical expression  for  the  equilibrium,  (a  —  x)z/x  =  K, — where 
a  is  the  difference,  about  1.20,  between  the  rotations  produced 
by  one  gram  of  gelatin  at  35°  and  at  17°;  x  is  the  difference  in 
rotation  between  that  at  35°  and  at  a  specified  temperature;  and 
K  is  a  constant, — a  reasonably  constant  value  for  K  was  obtained 
at  any  specified  temperature. 

Of  especial  interest  is  the  relation  which  is  shown  between  the 
geling  power  and  the  amount  of  gel  form  B  present.  It  appears 
that  in  the  entire  absence  of  the  gel  form,  the  gelatin  may  not 
be  caused  to  gel  at  any  concentration.  If  -an  amount  of  the  gel 
form  from  0.55  to  1.00  per  cent  is  present,  then  gelation  will 
take  place.  It  is  practically  immaterial  whether  there  be  any 
sol  form  present.  At  or  below  15°,  where  the  gelatin  is  all  in 
the  gel  state,  then  0.55  g.  of  gelatin  made  up  to  100  c.c.  with 
water  will  produce  a  jelly.  If  the  temperature  is,  say  30°,  it 
will  require  at  least  10.0  g.  of  gelatin  in  100  c.c.  in  order  that 
there  may  be  0.55  to  1.00  g.  of  the  gel  form  present,  i.e.,  in  order 
that  gelation  may  occur.  If  the  temperature  is  35°,  then  all  of 
the  gelatin  will  be  iri  the  sol  state,  and  gelation  will  not  occur  at 
any  concentration.  Upon  the  basis  of  these  experiments  a  new 
conception  of  melting  point  is  established,  namely  the  maximum 
temperature  at  which  there  will  be  present  in  the  solution  0.55- 
1.00  g.  of  the  gel  form  B;  which  is  the  critical  amount  necessary 
for  gelation.  This  should  also  be  exactly  identical  with  the 
selling  point. 

Any  increase  in  the  proportion  of  gel  form  B  is  found  to  be 
indicated  simultaneously  by  an  increase  in  viscosity  and  (after 
the  substance  has  become  a  jelly)  jelly  consistency,  and  also  by 
an  increase  in  the  levorotation  of  the  material.  This  relationship 
has  led  to  an  adaptation  of  the  polariscopic  measurement  of 
optical  rotation  to  control  processes  in  the  manufacture  of 
gelatin  and  glue.  This  will  be  considered  in  Chap.  VIII. 

It  should  be  pointed  out  that  the  salts  which  proteins  form 
with  acids  or  bases  often  differ  very  considerably  in  their  rotatory 
power  from  the  uncombined  protein.  This  has  been  found  to  be 
true  in  the  case  of  a  large  number  of  proteins  but,  so  far  as  the 
author  is  aware,  there  has  been  no  study  made  upon  the  exact 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         119 

relations  between  the  pH  value  of  gelatin  and  the  rotation  which  it 
exhibits.  Until  this  has  been  done  the  use  of  the  specific  rotatory 
power  of  gelatins  or  glues  as  a  property  characteristic  only  of  the 
equilibrium  Sol  Form  A  +±  Gel  Form  B,  should  be  regarded  as 
suggestive,  but  hardly  as  conclusive  in  any  instance. 

9.  The  Index  of  Refraction. — Very  little  work  of  a  careful 
nature  has  been  reported  upon  the  refractive  index  of  gelatin. 
By  far  the  most  exhaustive  treatment  of  the  subject  was  made 
by  Walpole1  in  1913.  By  employing  a  Zeiss  immersion  refracto- 
meter  he  investigated  the  refractive  index  of  gelatin  in  both  the 
sol  and  gel  condition,  and  in  solution  in  pure  water,  acids,  bases, 
and  salts.  Except  for  a  few  'minor  variations  he  found  that  at 
any  given  temperature  the  refractive  index  of'  gelatin  is  a  linear 
function  of  the  concentration;  that  no  variation  is  noted  in  the 
refractive  index  on  passing  from  the  sol  to  the  gel  state;  that  the 
refractive  index  is  the  same  in  solutions  of  acids,  bases,  or  salts 
as  it  is  in  pure  water;  and  that  the  position  of  the  salt  or  ion  in 
the  so-called  lyotropic  series  has  no  influence  upon  the  refractive 
index.  He  reports  the  most  probable  value  of  the  refractive 
index  of  dry  ash-free  gelatin  in  1  per  cent  solution  in  pure  water 
at  17.5°C.  to  be  0.001824. 

In  their  essential  characteristics  these  conclusions  are  quite 
identical  with  those  obtained  t>y  Robertson2  upon  casein,  gliadin, 
serum  globulin,  and  other  proteins,  and  by  Reiss3  upon  serum 
albumin.  In  all  of  these  cases  the  refractive  indices  are  very 
accurately  proportional  to  the  concentration,  and  are  independ- 
ent of  the  nature  of  the  acid,  base,  or  salt  with  which  they 
are  combined.  Robertson  accordingly  writes  the  equation  for 
the  refractive  index  of  any  solution  of  the  proteins  investigated 
by  him: 

n  —  HI  =  a  X  c, 

where  n  is  the  refractive  index  of  the  solution;  n\,  that  of  the 
solvent;  c,  the  percentage  of  protein  in  the  solution;  and  a,  the 
specific  refractivity  of  the  protein,  which  represents  the  change 
in  the  refractive  index  of  the  solvent  which  is  brought  about  by 
dissolving  one  gram  of  the  protein  in  100  c.c.  The  equation 
may  be  more  advantageously  written: 

1  G.  WALPOLE,  Kolloid-Z.,  13  (1913),  241. 

2  T.  B.  ROBERTSON,  "The  Physical  Chemistry  of  the  Proteins,"  361. 

3  E.  REISS,  Arch,  exptl.  Path.  Pharm.,  51  (1903),  18. 


120 


GELATIN  AND  GLUE 
n  —  n\ 


That  the  equation  applies  equally  well  to  gelatin  is  evident 
from  the  following  table  taken  from  Walpole: 


TABLE    31. — THE    REFRACTIVE    INDEX    OF    GELATIN   SOLUTIONS 


c 

n 

n  —  n\ 

a 

0    (H20) 

1.33097  (=  nO 

0.25 

1.33142 

0.00045 

0.00180 

0.50 

•1.33183 

0.00086 

0.00172 

0.75 

1.33251 

0.00154 

0.00205 

1.00 

1.33274 

0.00177 

0.00177 

1.25 

1  .  33307 

0.00210 

0.00168 

1.50 

1.33373 

0.00276 

0.00184 

1.75 

1.33410 

0.00313 

0.00179 

2.00 

1.33456 

0.00359 

0.00179 

The  explanation  of  the  independence  of  the  index  of  refraction 
on  the  nature  of  the  solvent  probably  lies  in  the  fact  that  refrac- 
tivity  is  a  function  of  the  volume  occupied  by  the  particle. 
The  molecular  volume  is  nearly  equal  to  the  sum  of  the  atomic 
volumes,  and  since  the  ionic  volume  of  the  protein  ion  is  many 
hundred  times  the  atomic  volume  of  any  of  the  inorganic  ions, 
very  little  change  in  the  molecular  volume  could  result  from  any 
interchange  of  inorganic  atoms  in  the  protein  molecule.  Some 
change  in  volume  undoubtedly  takes  place  on  substituting,  say, 
a  potassium  or  a  chloride  ion  for  one  of  hydrogen,  but  this  is 
insufficient  to  make  itself  felt  by  the  known  methods  of  measure- 
ment of  refractive  index.  Fischer  would  explain  the  constancy 
of  the  index  of  refraction  of  gelatin-water  systems  as  dependent 
upon  the  fact  that,  in  medium  concentrations  of  the  system, 
hydrated  gelatin  has  about  the  same  index  as  gelatin  solution. 

The  author  has  been  unable  to  observe  a  definite  alteration  in 
the  refractive  index  of  gelatin  sols  upon  partial  hydrolysis  to 
proteoses,  which  shows  that  a  halving,  or  even  a  quartering  of 
the  original  gelatin  molecule  (which  has  probably  taken  place 
during  the  hydrolysis)  does  not  sufficiently  alter  the  molecular 
volume  to  find  expression  in  refractive  index  measurements. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         121 

This  agrees  with  Robertson's  findings  with  casein,  and  the  latter 
investigator  has  suggested  a  method1  for  the  determination  of 
the  comparative  activities  of  trypsin  solutions,  based  upon  the 
constancy  of  the  refractive  index  upon  hydrolysis  of  the  casein. 

The  influence  of  temperature  upon  the  refractive  indices  of 
proteins  is  very  slight.  If  allowance  is  made  for  the  alteration 
in  the  refractive  index  of  the  solvent  with  varying  temperature, 
the  variation  in  the  protein  alone  will  be  nearly  inappreciable 
between  20  and  40°C. 

On  account  of  the  simple  relation  obtaining  between  the 
refractive  index  of  proteins  and  their  concentration,  and  the 
constancy  of  this  value  irrespective  of  the  solvent  or  the  hydrogen 
ion  concentration  employed,  the  determination  has  been  applied 
to  a  number  of  proteins  as  a  basis  for  estimating  the  concentration 
of  the  protein  in  the  solution.  This  has  been  especially  success- 
ful with  blood-serum,  but  a  number  of  other  proteins,  as  casein, 
globulin,  albumin,  etc.,  have  also  been  examined  in  this  way. 

10.  The  Gold  Number. — Colloids  of  the  "suspensoid"  class, 
such  as  the  sols  of  arsenic  sulphide,  ferric  hydroxide,  etc.,  are 
very  easily  precipitated  from  the  colloidal  condition  by  the 
addition  of  electrolytes.  The  addition  of  an  emulsoid  colloid 
to  the  suspensoid  is,  however,  even  in  very  minute  quantities, 
capable  of  greatly  lessening  or  even  quite  inhibiting  precipitation 
upon  adding  the  electrolyte.2  It  was  shown  by  Zsigmondy3 
that  different  emulsoids  exhibited  different  degrees  of  effective- 
ness in  this  regard,  and  he  suggested  a  scheme,  based  upon  these 
differences,  of  determining  the  relative  colloidality  of  emulsoids; 
for  distinguishing  between  various  commercial  preparations; 
and  for  differentiating  between  proteins  which  cannot  readily  be 
distinguished  by  other  tests. 

Zsigmondy  proposed  that  4>he  number  of  milligrams  of  a 
colloidal  substance  which  just  fail  to  prevent  10  c.c.  of  a  bright 
red  gold  sol,  prepared  by  a  standard  method,  from  changing  into 
violet  upon  the  addition  of  1  c.c.  of  a  10  per  cent  solution  of 
sodium  chloride,  be  known  as  the  gold  number  of  that  colloid. 
In  order  that  comparable  results  may  always  be  obtained,  it  is 
necessary  always  to  carry  out  the  procedure  in  the  same  way. 

1  T.  B.  ROBERTSON,  J.  Biol.  Chem.,  12  (1912),  23. 

2  Bancroft  (personal  communication)  points  out  that  the  addition  of  even 
smaller  quantities  of  emulsoid  colloid  may  precipitate  the  suspensoid. 

3  ZSIGMONDY,  Z.  anal.  Chem.,  40  (1901),  697;  ZSIGMONDY  and   SCHULZ, 
Beitr.  physiol.  path.  Chem.,  3  (1903),  137. 


122  GELATIN  AND  GLUE 

The  gold  sol  may  be  prepared  as  follows:1  120  c.c.  of  water,  that  has  been 
distilled  through  a  silver  condensing  tube  into  a  500  c.c.  Jena  glass  beaker, 
are  heated,  and  2.5  c.e.  of  a  0.6  per  cent  solution  of  hydrogen  gold  chloride 
and  3  to  3.5  c.c.  of  0.18  N  potassium  carbonate  solution  of  the  highest 
possible  purity  are  added.  After  boiling,  and  while  still  hot,  3  to  5  c.c. 
of  dilute  formaldehyde,  made  by  adding  0.3  c.c.  of  formalin  to  100  c.c.  of 
water,  are  added.  A  bright  red  color  is  produced  in  a  short  time.  To 
determine  the  "gold  number"  small  quantities  of  the  colloid  are  introduced 
into  a  series  of  50  c.c.  Jena  beakers.  A  0.2  c.c.  pipette,  graduated  to  thou- 
sands of  a  cubic  centimeter,  is  used,  and  the  quantities  delivered  should,  for 
the  first  trial,  be  0.005,  0.01,  0.02,  0.05,  0.1  and  0.5  c.c.  Five  cubic  centi- 
meters of  the  gold  sol  are  then  introduced  into  thejbeakers,  mixed  with  a  Jena 
glass  rod,  and  allowed  to  stand  3  to  5  minutes.  Then  0.5  c.c.  of  a  solution  of 
sodium  chloride,  made  by  adding  100  g.  of  pure  sodium  chloride  to  900 
c.c.  of  water,  are  introduced  and  stirred  in.  The  concentration  of  emulsoid 
in  the  gold  sol  that  just,  retains  the  red  color,  and  in  the  next  one  which 
shows  a  change  to  a  blue  or  violet,  is  noted.  These  values,  multiplied  by  2, 
define  the  gold  number  for  that  colloid. 

Hatschek2  suggests  that  it  would  be  better  to  express  the 
"gold  number"  as  the  percentage  concentration  of  emulsoid  in 
the  gold  sol  which  just  prevents  the  color  change  when  1  c.c. 
of  normal  sodium  chloride  is  added  to  10  c.c.  of  gold  sol.  Thus, 
if  the  tube  with  0.2  c.c.  of  emulsoid  remains  unaltered  while  that 
with  0.1  c.c.  has  turned  blue,  the  emulsoid  concentrations  are, 
if  10  c.c.  of  gold  sol  and  1  c.c.  of  N  NaCl  solution  are  used: 

0.2/10.2  =  0.0196  X  original  emulsoid  concentration; 
0.1/10.1  =  0.0099  X  original  emulsoid  concentration. 

The  "gold  number"  in  percentage  lies  between  these  values. 

It  is  evident  that  the  smaller  the  value  of  the  "gold  number," 
the  more  effective  is  the  protective  action  of  the  emulsoid.  It  is 
especially  interesting  that  gelatin  possesses  the  lowest  "gold 
number"  of  any  colloid,  which  shows  that  its  protective  action 
is  very  great.  The  following  table  gives  the  "gold  number" 
of  a  number  of  colloids. 

Menz3  in  1909  showed  that  the  protective  action  of  gelatin 
increased  as  the  concentration  of  the  same  was  decreased. 
Elliott  and  Sheppard4  have  corroborated  the  findings  of  Menz, 
and  have  by  ultramicroscopic  studies  been  able  to  confirm  his 

1  S.  B.  SCHRYVER,  "Allan's  Commercial  Organic  Analysis,"  vol.  8,  p.  78. 

2  E.  HATSCHEK,  "Laboratory  Manual  of  Colloid  Chemistry,"  Philadel- 
phia (1920),  97. 

3  W.  MENZ,  Z.  physik.  Chem.,  68  (1909),  129. 

4  F.  A.  ELLIOTT  and  S.  E.  SHEPPARD,  J.  Ind.  Eng.  Chem.,  13  (1921),  699. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN        123 

theory  that  protective  action  is  dependent  solely  upon  the  con- 
centration of  amicrons  present  in  the  solution.  A  decrease  in 
total  concentration  of  gelatin  was  shown  to  result  in  an  actual 
increase  in  the  concentration  of  the  smaller  sized  particles.  An 
aging  of  the  solution  results  in  flocculation,  hence  a  decrease  in 
amicron  concentration,  and  a  decrease  in  protective  action. 

TABLE  32. — THE  GOLD  NUMBER  OF  PROTEINS  AND  OTHER  SUBSTANCES 


Substance 

Gold  number 
(Zsigmondy) 

Gelatin  

0  005-0  1 

Russian  glue  

0.005-0.01 

Isinglass  ,  

0  01  -0  02 

Caseinogen 

0  01 

Egg  globulin  
Amorphous  egg-albumin 

0.02  -0.05 
0  03  -0  06 

Ovomucoid  

0.04  -0.08 

Glycoprotein 

0  05  -0.1 

Fresh  egg-white  

0.08  -0.15 

Crystallized  egg-albumin       

20     -8.0 

Dextrin 

10      -20 

Potato  starch  

25 

Deutero-albumose 

00 

11.  The    Tyndall    Effect    and    Ultramicroscopy.— Nearly    all 

colloidal  solutions,  including  those  of  the  proteins,  show  a 
decided  opalescence.  Even  if  the  solutions  appear  to  be  per- 
fectly clear  to  ordinary  observation,  yet  if  they  are  so  placed 
that  they  may  be  observed  at  right  angles  to  a  beam  of  light 
which  passes  through  the  solution,  a  cone  of  light,  known  as  the 
Tyndall  effect,  is  usually  seen.  This  is  due  to  a  scattering  of 
the  light  by  the  particles  in  the  solution,  just  as  dust  particles  in 
the  air,  which  are  ordinarily  invisible,  are  made  evident  by  a  beam 
of  light  entering  an  otherwise  dark  room.  The  smallest  particles 
which  are  capable  of  dispersing  light  must  have  a  diameter, 
according  to  de  Bruyn,1  of  from  5  to  10^.  Robertson2  has 
endeavored  to  show  by  an  uncertain  calculation  that  the  mole- 
cule of  casein  is  about  2.4/j/x,  or  only  about  half  the  diameter  of 
the  smallest  particle  that  will  scatter  transmitted  light.  He 

1L.  DE  BRUYN,  Rec.  Trav.  chim.,  19  (1900),  236;  251. 

2  T.  B.  ROBERTSON,  "Physical  Chemistry  of  the  Proteins"  (1918),  343. 


124  GELATIN  AND  GLUE 

suggests  that  the  opalescence  of  protein  solutions  may  not, 
therefore,  be  ascribed  to  the  protein  molecules  themselves,  but 
may  perhaps  be  "attributable  to  the  peculiar  characteristics  of 
ionic  protein,"  as  for  example  the  property  of  the  protein  to 
become  hydrated,  or  possibly  the  net-structure,  which  he  assumes 
all  proteins  to  possess.  That  ionic  protein  is  not  necessary  is 
shown  however  from  the  fact  that  the  most  beautiful  opalescent 
colloid  systems  can  be  made  out  of  organic  solvents  with  nitro- 
cellulose or  soaps. 

Konovalov1  offers  another  explanation.  He  points  out  that 
dust  particles  may  act  as  nuclei,  in  solutions  of  low  osmotic 
pressures,  to  which  the  dissolved  protein  will  adhere,  and  an 
observed  opalescence  may  be  produced  by  a  comparatively 
small  number  of  such  aggregates. 

It  is  probably  true  that  one  or  another  of  the  above  explana- 
tions may  function  in  special  cases  where  a  protein  is  obtained 
in  a  true  molecular  state  of  dispersion,  but  in  the  great  majority 
of  protein  solutions  which  exhibit  opalescence  it  is  probable 
that  a  greater  or  lesser  degree  of  polymerization  has  taken 
place,  and  it  seems  highly  probably  that  the  aggregates  so  ob- 
tained may  be  of  sufficient  size  to  act  themselves  as  dispersers 
of  transmitted  light. 

Siedentopf  and  Zsigmondy2  have  applied  the  microscope  in  an 
ingenious  manner  to  solutions  which  exhibit  the  Tyndall  effect. 
A  powerful  beam  of  light  is  caused  to  traverse  in  a  horizontal 
direction  a  thin  portion  of  the  solution  and  a  high  power  micro- 
scope focused  orthogonally  upon  the  points  of  light  diffracted. 
By  this  simple  process  they  have  been  able  to  study  particles  in 
solution  that  had  hitherto  been  invisible,  and  by  careful  technique 
have  counted  the  number  of  such  submicrons  in  a  given  volume 
of  the  liquid.  The  kinetics  of  Brownian  Movement  could  be 
investigated  with  vastly  smaller  particles  than  it  had  previously 
been  possible  to  observe,  and,  probably  most  important,  the 
concept  of  colloid  chemistry  as  the  chemistry  of  special  dimensions 
was  established.  A  vast  array  of  such  colloidal  solutions  which 
are  homogeneous  as  far  as  the  ordinary  microscope  is  capable  of 
determining,  have  been  shown  to  be  physically  heterogeneous 
by  means  of  the  ultramicroscope. 

1  D.  KONOVALOV,  Ann.  Physik.,  10  (1903),  360. 

2  SIEDENTOPF  and  ZSIGMONDY,  Ann.  Physik.  (4),  10  (1903),  1. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         125 

Rayleigh1  has  shown  that  the  intensity  of  the  scattered  light 
is  inversely  proportional  to  the  fourth  power  of  the  wave  length 
and  the  scattering  by  each  particle  is  proportional  to  the  square 
of  the  volume  of  the  particle.  Light  scattered  in  this  way  is 
chiefly  blue.  The  particles  themselves  are  not  seen,  but  only 
the  light  which  is  diffracted  by  them.  Minute  traces  of  dust  or 
other  impurities  in  the  liquid  are  also  observed  in  this  way, 
and  great  precaution  must  be  taken  to  insure  against  their 
presence  when  observations  are  being  made,  as  otherwise  one 
would  obtain  misleading  results.  Tyndall  himself  employed 
the  method  to  determine  if  air  was  free  from  dust.  Ordinary 
water  shows  a  faint  Tyndall  cone,  but  when  pure  it  is  found  to  be 
optically  empty,  as  are  nearly  all  pure  liquids  and  true  solutions. 
Martin2  has  claimed  however  to  have  found  that  some  organic 
liquids,  even  when  absolutely  pure,  scatter  light  and  so  produce 
the  Tyndall  cone. 

Since  the  particles  are  themselves  not  discernible,  it  would 
appear  to  be  an  imposition  on  good  faith  to  endeavor  to  measure 
their  size,  but  this  may  be  done  indirectly  with  a  fair  degree  of 
confidence.  The  number  of  particles  in  a  known  volume  is 
determined  by  counting  under  the  ultramicroscope,  and  the 
mass  and  density  being  known  the  length  of  the  side  of  one 
particle  is  calculated  from  the  formula:3 

L=  \/M/SN 

where  L  is  the  length  of  one  side;  M,  the  mass  in  unit  volume;  S, 
the  specific  gravity;  and  N,  the  number  of  particles  in  unit 
volume.  This  formula,  however,  is  capable  of  expressing  only 
the  average  size,  and  it  is  very  probable  that  particles  of  greatly 
varying  sizes  may  be  present.4 

The  visibility  of  the  separate  particles  depends  upon  their 
size  and  the  relation  between  the  index  of  refraction  of  the 
particles  and  that  of  the  medium  in  which  they  are  dispersed. 

1  RAYLEIGH,  Phil.  Mag.  (4),  41  (1871),  107;  447. 

2  MARTIN,  /.  Phys.  Chem.,  24  (1920),  478. 

3  Cf.  G.  KING,  /.  Soc.  Chem.  Ind.,  38  (1919),  4T;  3rd  Report  on  Colloid 
Chemistry,  British  Assoc.  Adv.  Science  (1920),  31. 

4  A.  W.  Thomas  points  out  (personal  communication)  that  the  light  re- 
flections from  many  of  the  particles  are  too  small  to  .be  resolved  and  hence 
are  not  counted,  resulting  in  an  overestimation  of  the  size  of  the  particles. 
Thomas  regards  all  figures  for  the  sizes  of  colloidal  particles  given  in  the 
literature  as  quite  valueless. 


126  GELATIN  AND  GLUE 

Under  the  most  favorable  conditions  particles  as  small  as  3/t/x 
have  been  discerned,  but  if  the  index  of  refraction  of  the  two 
phases  is  identical  or  nearly  so  they  become  invisible,  even  if  of 
large  size.  A  common  method  for  identifying  precious  stones 
and  distinguishing  between  genuine  and  artificial  gems  is  to 
place  the  crystal  in  a  liquid  which  has  an  index  of  refraction 
identical  with  that  of  the  genuine  stone.  If  the  sample  under 
examination  is  genuine  it  will  not  then  be  visible,  while  the 
imitation  stone  will  be  readily  seen  in  such  a  solution. 

One  of  the  difficulties  in  observing  emulsoids  under  the  ultra- 
microscope  is  due  to  the  nearly  identical  index  of  refraction  of 
the  two  phases.  The  actual  intensity  of  the  scattered  light  is 
likewise  dependent  upon  the  index  of  refraction.  According 
to  Lord  Rayleigh:1 


where  I8  is  the  intensity  of  the  diffracted  light;  ju,  the  refractive 
index  of  the  medium;  and  AH,  that  of  the  dispersoid.  Thus  as  the 
indices  of  refraction  of  the  two  phases  approach  each  other  the 
intensity  of  the  scattered  light  diminishes  until  at  the  point  of 
exact  coincidence  it  becomes  zero.  Suspensoids  of  metals  as 
gold,  silver,  etc.,  dispersed  in  water  make  therefore  the  ideal 
conditions  for  examination,  while  hydrated  emulsoids  are  much 
less  easily  observed.  In  these  cases,  however,  the  colors  due 
to  the  selective  reflection  of  the  metals  is  dominant,  and  the 
true  color  of  the  Tyndall  blue  is  not  observable.  In  fact  sus- 
pensoids  of  silver  may  be  obtained  which  look  red  in  the  ultra- 
microscope. 

The  mechanics  of  the  ultramicroscope  and  the  technique  of  its  operation 
should  be  thoroughly  in  hand  before  the  making  of  dependable  observations 
is  attempted.2  An  extended  discussion  of  these  considerations  would  be 
out  of  place  in  a  book  of  this  kind,  but  the  essential  features  may  be  briefly 
stated. 

The  slit  ultramicroscope  is  the  most  generally  used  type  of  instrument, 
and  is  so  called  because  the  illuminating  beam,  entering  the  quartz  windows 
of  the  observation  cell,  is  controlled  by  means  of  a  bilateral  micrometer  slit. 
This  is  so  adjusted,  in  the  Zeiss  instrument,  that  the  image  is  about  KG 
the  dimensions  of  the  slit.  A  definite  area  of  the  beam  is  observed  by  the 
use  of  a  micrometer  eyepiece,  and  the  depth  made  less  than  the  depth  of 
normal  vision,  so  that  the  product  of  depth  and  area  gives  the  volume  of 

1  RAYLEIGH,  loc.  cit. 

2C°  E,  F.  BURTON,  "The  Physical  Properties  of  Colloidal  Solutions" 
(1916),  28. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         127 

the  solution  actually  observed.  This  is  necessary  where  countings  are  to  be 
made.  The  solution  should  be  so  dilute  that  only  four  or  five  particles  are 
seen  at  one  time  in  this  volume,  and  a  number  of  instantaneous  counts 
made.  This  is  required  on  account  of  the  rapid  Brownian  movement  taking 
place. 

In  the  cardoid  ultramicroscope  a  more  intense  illumination  makes  possible 
the  visualization  of  smaller  particles  than  are  seen  in  the  slit  type.  An 
immersion  form  of  instrument  has  also  been  perfected  by  Zsigmondy,1 
and  by  its  use  particles  as  small  as  3^M  have  been  observed. 

King2  found  that  solutions  of  peptone,  starch,  gelatin,  agar- 
agar,  and  dextrin  were  all  optically  empty,  due  probably  to  the 
similarity  in  the  indices  of  refraction  of  the  two  phases.  In  a 
solution  of  aqueous  alcohol,  Bancroft3  reports  that  it  is  possible 
to  obtain  gelatin  in  the  form  of  a  net  structure  made  up  of 
definite  globules  rather  than  filaments,  and  Bachmann4  reports 
that  it  is  impossible  to  distinguish  between  sponge  and  honey- 
comb structures  in  gels  by  means  of  the  ultramicroscope.  Elliott5 
has  reported  however  that  no  difficulty  should  be  experienced 
in  observing  the  colloid  particles  of  a  gelatin  sol  in  very  dilute 
solutions.  The  author  has  found  that  gelatin  sols  which  were 
hydrated  to  the  minimum  degree — (obtained  by  bringing  gelatin 
to  its  isoelectric  point,  pH  4.7)  and  which  when  cooled  to  15°C. 
readily  precipitated — were  easily  observed  under  the  ultra- 
microscope  (at  15°  the  particles  had  attained  microscopic  dimen- 
tions)  while  under  conditions  of  greater  hydration  the  scattering 
of  light  rapidly  became  less  pronounced,  and  could  not  be  per- 
ceived at  pH  3.5.  This  simple  observation  doubtless  explains 
why  some  investigators  have  succeeded  and  others  have  failed 
to  detect  the  presence  of  colloid  emulsoid  particles  with  the 
ultramicroscope.  The  degree  of  hydration  of  the  particles 
examined  is  a  critical  factor. 

Menz6  observed  that  a  strong  Tyndall  effect  was  produced  in 
solutions  of  gelatin  of  from  1.0  to  0.1  per  cent  concentration, 
that  a  faint  but  non-resolvable  light  cone  was  apparent  at 
concentrations  of  0.01  per  cent;  and  that  at  0.001  per  cent  con- 
centration the  light  cone  was  barely  visible.  He  concluded, 
first,  that  protective  action  was  due  only  to  the  very  small 

ZSIGMONDY,  Kolloid-Z.,  14  (1914),  281. 

G.  KING,  loc.  cit. 

W.  D.  BANCROFT,  "Applied  Colloid  Chemistry"  (1921),  241. 

BACHMANN,  Kolloid-Z.,  23  (1918),  89. 

F.  ELLIOTT,  60th  Gen.  Meeting  Am.  Chem.  Soc.,  Chicago,  1920. 

MENZ,  Z.  physik.  Chem.,  66  (1909),  129. 


128  GELATIN  AND  GLUE 

amicroscopic  particles,  larger  particles  having  no  effect,  and 
second,  that  the  dispersion  in  aqueous  solutions  depended  upon 
the  concentration  after  warming,  simple  dilution  with  cold  water 
having  no  effect  whatsoever.  A  similar  effect  has  been  observed 
as  regards  structure  (see  below). 

12.  The  Donnan  Equilibrium.— Donnan's  membrane  equilib- 
rium1 is  established  when  a  membrane  separates  two  ionized 
solutions,  one  of  which  has  one  ion  for  which  the  membrane  is 
impermeable,  while  all  of  the  other  ions  are  readily  diffusible 
through  the  membrane.  This  nondiffusible  ion  may  be  either  a 
colloid  or  a  crystalloid,  but  protein  salts  in  electrolytic  solutions 
constitute  a  typical  case.  A  collodion  membrane  is  suitable  for 
the  separation,  it  being  impermeable  to  the  protein  ions  but 
easily  permeable  to  the  ordinary  ions  of  inorganic  electrolytes. 

When  a  gelatin  chloride  solution,  for  example,  is  separated 
from  a  solution  of  dilute  hydrochloric  acid  by  a  collodion  mem- 
brane, the  distribution  of  the  ions  of  the  acid,  after  equilibrium 
is  reached,  is  not  the  same  on  the  two  sides  of  the  membrane. 
It  is  found  to  be  higher  on  the  side  free  from  gelatin  ions.  This 
was  shown  by  Donnan  to  be  a  necessary  consequence  of  the 
second  law  of  thermodynamics,  and  Procter2  has  deduced  that 
the  relative  distribution  of  the  acid  on  the  inside  and  the  outside 
of  a  solid  block  of  gelatin  chloride  in  equilibrium  with  hydro- 
chloric acid  is  determined  by  the  equation: 

x*  =y(y  +  z), 

where  x  is  the  concentration  of  H  ions  (and  of  Cl  ions)  outside 
the  gelatin,  y  the  concentration  of  the  H  ions  (and  the  Cl  ions) 
of  the  uncombined  HC1  within  the  gelatin,  and  z  the  concentra- 
tion of  Cl  ions  in  combination  with  the  gelatin  ions,  x  is  obvi- 
ously greater  than  y,  which  necessitates  the  concentration  of 
free  HC1  on  the  outside  being  greater  than  on  the  inside  of  the  gel. 

Although  Procter  studied  only  the  solid  gel  equilibrium,  Loeb3 
has  found  the  same  condition  to  apply  when  a  solution  of  gelatin 
chloride  is  separated  from  pure  water  by  a  collodion  membrane. 
The  Donnan  equilibrium  has  been  made  use  of  in  the  develop- 
ment of  theories  of  structure,  swelling,  osmotic  forces,  etc.,  by 

1  F.  G.  DONNAN,  Z.  Electrochem.,  17  (1911),  572;  DONNAN  and  HARRIS,  /. 
Chem.  Soc.,  99  (1911),  1554;  DONNAN  and  GARNER,  ibid.,  115  (1919),  1313. 

2  H.  R.  PROCTER.  /.  Chem.  Soc.,  105  (1914),  313;  PROCTER  and  WILSON, 
ibid.,rW9  (1916),  307. 

3  J.  LOEB,  J.  Gen.  PhysioL,  3  (1920-21),  247. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         129 

Procter,1  by  Loeb,2  and  by  Miss  Lloyd.3  These  theories  are 
described  elsewhere  in  this  text,  but  it  is  desired  to  point  out 
here  some  of  the  mathematical  relationships  that  have  been  found 
between  this  equilibrium  and  the  various  effects  of  hydrogen 
ion  concentration  and  valence  upon  the  properties  of  proteins. 

The  difference  in  the  concentration  of  acids  on  the  two  sides 
of  the  membrane  must  lead  to  a  difference  in  the  potential  at 
the  surface  of  the  membrane.  This  P.D.  (potential  difference) 
may  be  calculated  on  the  basis  of  Nernst's  formula  for  concen- 
tration cells,  which  at  24°C.  becomes: 

P.D.  =  0.059  log  ^> 

C2 

where  C\  is  the  H  ion  concentration  inside  the  gelatin  solution 
and  Cz  the  H  ion  concentration  in  the  outside  solution.  And 

C 
since  log  -^  is  pH  inside  the  gelatin  solution  minus  pH  outside 

^2 

the  gelatin  solution,  it  follows  that 

P.D.  =  0.059  (pH  inside  -  pH  outside), 

the  P.D.  being  expressed  in  volts. 

Loeb4  has  tested  the  validity  of  this  postulation  by  a  long 
series  of  convincing  experiments.  The  potential  differences  were 
measured  by  a  Compton  electrometer.  The  gelatin  solution  was 
placed  inside  a  collodion  bag  closed  with  a  rubber  stopper  through 
which  a  funnel  was  introduced,  and  the  electrode  dipped  into  the 
solution  which  was  caused  to  rise  a  little  way  into  the  funnel. 
The  collodion  bag  was  dipped  into  water  or  other  electrolytic 
solution  and  the  second  electrode  introduced  into  this.  In 
practically  all  cases  Loeb  found  that  the  P.D.  calculated  by 
multiplying  the  values  of  pH  inside  minus  pH  outside  by  59 
agreed  very  satisfactorily  with  the  observed  P.D.  (in  millivolts). 
Loeb  furthermore  demonstrated  that  the  variation  in  P.D.  with 
changes  in  hydrogen  ion  concentration,  and  with  variation  in  the 
valence  of  the  combined  ion,  was  in  all  cases  studied  by  him 
practically  parallel  with  the  changes  that  were  induced  by  the 

1  See  page  136. 

2  See  page  157. 

3  See  pages  167-8. 

4  J.  LOEB,  /.  Gen.  Physiol.,  3  (1920-21),  557;  667;  691;>!827;  4  (1921), 
73;  97. 


130  GELATIN  AND  GLUE 

same  forces  in  the  swelling,  the  viscosity,  and  the  osmotic  pressure 
of  the  solution.1 
If  the  equation 

za  =  y(y  +  z) 
is  written  in  the  form 

y  m:'j* 

x      y  +  z 

then  -  =  the  ratio  of  H  ion  concentration  inside  over  the  H  ion 
x 

concentration  outside ;  and  — -r —  =  the  ratio  of  the  Cl  ion  con- 
centration outside  over  the  Cl  ion  concentration  inside.  And 
since 

log  -  =  pH  inside  -  pH  outside, 
and 

log  — -   -  =  pCl  outside  -  pCl  inside, 

it  follows  that 

pH  inside  —  pH  outside  =  pCl  outside  —  pCl  inside. 

Upon  putting  this  to  the  test  Loeb  found  that  the  values  for  pH 
inside  minus  pH  outside  were,  for  the  same  solution  at  the  point 
of  equilibrium,  equal  to  the  value  pCl  outside  minus  pCl  inside. 

The  peculiar  variations  in  the  osmotic  pressure  of  gelatin 
solutions  upon  changing  the  pH  or  the  valency  of  the  combined 
ion  were  also  found  to  be  not  only  qualitatively  but  almost 
quantitatively  explainable  and  calculable  by  means  of  the 
Donnan  equilibrium. 

If  y  be  the  H  and  Cl  ion  concentrations  of  the  free  HC1  inside 
a  solution  of  gelatin  chloride,  z  the  Cl  ion  concentration  of  com- 
bined Cl  ions  within  the  gelatin  solution,  and  a  the  concentration 
of  gelatin  ions  and  unionized  molecules,  then  the  osmotic  pressure 
of  the  solution  will  be 

2y  +  z  +  a. 

When  the  measurement  is  made  by  observing  the  height  in 
millimeters  to  which  the  solution  will  rise  in  a  tube  upon  placing 
the  gelatin  solution,  contained  in  a  collodion  bag,  into  water, 
the  observed  osmotic  pressure  will  be  less  than  the  above  by 

1  It  is  still  necessary  to  explain  the  phenomena  which  take  place  in 
unionized  systems,  as,  for  example,  the  swelling  of  rubber  or  polymerized 
isoprene  in,  e.g.,  carbon  disulphide. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         131 

that  component  due  to  the  pressure  of  the  ions  in  the  outside 
solution,  or,  letting  x  be  the  concentration  of  H  ions  outside,  and 
Po  the  observed  osmotic  pressure, 

Po  =  2y  +  z  +  a  -  2x. 

y  is  calculated  from  the  pH  inside,  x  from  pH  outside,  and  z  from 
Donnan's  equilibrium  equation, 

^  =  (x  +  y)(x  —  y)- 

y 

Since  the  theoretical  osmotic  pressure  of  a  gram  molecular 
solution  in  terms  of  mm.  of  water  is  expressed  by 


22.4  X  760  X  13.6  X          =  2.5  X  105, 

then 

(2y  +  z  +  a-  2x)  X  2.5 

gives  the  calculated  osmotic  pressure  of  the  gelatin  solution 
expressed  in  terms  of  10~5N. 

The  above  equation  satisfies  a  monovalent  ion  condition.  If 
the  anion  of  the  gelatin  salt  is  divalent,  the  equilibrium  equation 
becomes  one  of  the  third  degree,  and  is  calculated  from  the  values: 

3          z_3 
2  y  "h  2      2X' 

Very  good  agreement  between  the  observed  values  for  the 
osmotic  pressure  of  gelatin  solutions,  and  those  calculated  by  the 
above  formulas  have  been  reported  by  Loeb. 

The  value  of  a,  the  actual  osmotic  pressure  due  to  the  gelatin 
ions  and  molecules,  was  obtained  by  subtracting  the  pressure 
calculated  for  the  inorganic  ions: 

^Gelatin  =  PO  -  [(2y  +  z  -  2x)  X  2.5  mm.  H2O]. 

Although  the  probability  of  error  is  large,  Loeb  concludes  that 
the  osmotic  pressure  due  to  the  gelatin  particles  in  a  1  per  cent 
solution  of  gelatin  phosphate  of  pH  3.60  is  about  100  mm.  H2O. 
In  order  to  explain  also  the  viscosity  variations  upon  changes 
in  pH  or  other  influences,  Loeb  finds  it  necessary  to  discard 
the  older  theories  of  structure  and  to  propose  a  new  hypothesis 
which  will  be  known  as  the  occlusion  theory.1  By  assuming  that 
gelatin  solutions  contain  isolated  ions  and  molecules  of  gelatin 
together  with  pieces  of  solid  submicroscopic  particles  capable  of 
occluding  water,  Loeb  precedes  to  demonstrate  that  the  amount 

1  The  occlusion  theory  is  described  on  page  157. 


132  GELATIN  AND  GLUE 

of  water  that  will  be  occluded  under  any  given  set  of  conditions, 
and  the  ratio  of  the  solid  to  the  ionic  and  molecular  particles, 
will  be  controlled  likewise  by  the  Donnan  equilibrium.  The 
effective  volume  of  the  gelatin  in  solution  will  accordingly  vary 
as  the  water  occluded,  and  the  viscosity  will  vary  as  this  effective 
volume  of  gelatin.  The  osmotic  pressure,  on  the  other  hand,  is 
shown  to  vary  reciprocally  as  the  viscosity.  That  is,  the  larger 
the  proportion  of  gelatin  in  the  form  of  solid  particles  occluding 
water,  the  greater  will  be  the  viscosity,  but  the  smaller  will  be  the 
osmotic  pressure,  since  the  latter  is  dependent  upon  the  ionic  and 
molecularly  sized  particles,  but  if  these  ions  and  molecules  are 
caused  to  increase  in  number  at  the  expense  of  the  solid  particles, 
the  viscosity  will  drop  while  the  osmotic  pressure  will  rise. 

II.  THE  STRUCTURE  OF  GELATIN 

1.  The  Older  Theories  of  Gel  Structure.— The  earliest  recorded 
studies  of  the  structure  of  gels  were  conducted  by  Frankenheim  in 
1835  and  von  Nageli  in  1858.  Although  the  ultramicroscope  had 
not  then  been  invented  they  concluded  from  a  study  of  the 
properties  of  gels  that  these  consisted  of  a  loosely  bound  network 
or  aggregation  of  molecules  or  solid  particles  of  submicroscopic 
size.  This  early  work  was  placed  upon  a  sounder  basis  by  the 
long  series  of  optical  observations  of  Blitschli1  between  1892  and 
1900.  From  this  time  on  to  the  present,  an  emulsion  theory  of 
gels  has  been,  with  numerous  modifications,  the  most  generally 
accepted  theory  to  account  for  the  characteristic  properties  of 
these  substances. 

Van  Bemmlen2  was  one  of  the  early  champions  of  the  concep- 
tion of  a  structure  in  colloidal  gels.  He  states  in  connection  with 
investigations  upon  silicic  acids  that  the  semiliquid  particles 
of  the  colloid  arrange  themselves  with  the  water  molecules  to 
form  "a  cell-like  structure  of  definite  form,"  and  that  these  cells 
"hang  together  at  certain  points,  so  forming  a  network."  The 
water  is  then  retained  partly  by  the  cells  themselves  and  partly 
in  the  interstices  between  the  cells.  He  finds  that  all  of  the 
properties  of  the  colloids  are  in  harmony  with  the  view  that  the 
hydrogel  is  a  cell-like  structure  or  network. 

1  BUTSCHLI,   "  Uutersuchungen  iiber  mikroskopische  Schaume  und  das 
Protoplasma,"  Leipsig,  1892;  " Untersuchungen  iiber  Strukturen,"  Leipsig, 
1900. 

2  J.  M.  VAN  BEMMELEN,  Z.  anorg.  Chem.,  13  (1896),  233;  18  (1898),  14. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         133 

Hardy1  does  not  think  it  probable  that  any  structure  exists  in 
gelatin  so  long  as  it  is  in  the  form  of  a  solution,  nor  even  in  the 
form  of  an  ordinary  gel,  but  after  it  has  been  made  irreversible 
by  some  hardening  agent,  as  formaldehyde,  mercuric  chloride, 
or  osmic  acid,  it  then  possesses  a  net  structure,  and  bears  a 
close  resemblance  to  the  finer  structures  of  protoplasm.  The 
nature  of  the  structure  produced  he  found  to  vary  with  the 
type  of  hardening  agent  employed,  the  concentration,  and  the 
temperature. 

In  1901  Quincke2  postulated  a  theory  of  the  structure  of 
colloidal  solutions  according  to  which  such  solutions  are  not 
homogeneous  like  water,  but  consist  of  a  mixture  of  two  phases, 
one  rich  in  colloid  and  the  other  poor  in  colloid.  A  surface 
tension  develops  at  the  surface  of  contact  of  these  two  phases, 
with  the  result  that  the  richly  colloidal  solution  forms  into  cells 
which  may  be  either  full  of  the  colloid-poor  solution,  or  of  uniform 
composition.  The  cells  so  formed  may  be  isolated,  or  they  may 
form  chain-like  threads  or  masses  which  cling  together. 

Garrett,3  in  a  study  of  the  changes  in  viscosity  obtaining  in 
solutions  of  typical  colloids,  i.e.,  gelatin,  albumin,  and  silicic 
acid,  upon  variation  of  the  temperature  and  concentration, 
obtained  results  which  led  him  to  accept  Quincke's  theory  and 
to  extend  somewhat  and  enlarge  upon  it.  He  found  that 
"  although  the  logarithmic  decrement  of  a  disk  oscillating  in 
water  or  other  homogeneous  fluid  is  constant,  yet  in  a  gelatin 
solution  the  decrement  varied  considerably,  even  when  the 
temperature  and  concentration  remained  unchanged."  In 
general  the  decrement  increased  with  the  time  during  which  the 
disk  had  been  immersed  in  the  solution.  In  the  case  of  solutions  - 
slowly  cooled  to  the  temperature  under  observation,  the  logarith- 
mic decrement  was  a  linear  function  of  the  time.  There  did  not  r 
appear  to  be  any  definite  maximum  of  decrement,  but  a  fixed  ^ 
minimum  was  shown  to  exist,  i.e.j  the  decrement — as  found  by 
interpolation — at  the  moment  when  the  plate  was  introduced  into 
the  solution.  This  value  was  the  only  one  which  remained  the 
same  from  day  to  day  and  with  various  solutions  of  the  same 
strength.  When  the  plate  was  taken  out  of  the  solution,  well 
washed  with  hot  water,  cooled,  and  reintroduced,  the  same 


1  HARDY,  Am.  J.  Physiol,  24  (1899),  158. 

2  QUINCKE,  Sitz.  kon.  preuss.  Akad.  Wiss., 

3  H.  GARRE-TT,  Phil.  Mag.  (6),  6  (1903),  374. 


2  QUINCKE,  Situ.  kon.  preuss.  Akad.  Wiss.,  Berlin  (1901),  858.  / 


134  GELATIN  AND  GLUE 

11  anf angsdekrement "  was  obtained.  Furthermore,  the  decre- 
ment was  found,  below  a  certain  temperature,  to  increase 
continuously  whether  the  plate  was  washed  or  not;  in  dilute 
solutions  (1  to  3  per  cent)  the  decrement  observed  after  a  few 
large  vibrations  was  smaller  than  when  observed  with  only  small 
swings;  and  ft  gelatin,  i.e.,  gelatin  that  had  been  boiled  for  some 
time,  did  not  show  these  characteristics. 

Garrett  explains  the  phenomena  observed  upon  Quincke's 
theory.  He  assumes  that  "the  colloidal  cells  which  have  fixed 
themselves  upon  the  disk  are  carried  by  it  through  the  liquid 
and  come  in  contact  with  new  cells,  so  that  the  mass  of  cells 
hanging  onto  the  plate  increases,  and  with  it,  the  resistance  to 
vibration,  or,  in  other  words,  the  logarithmic  decrement.  If 
the  oscillation  is  too  large  the  cell  walls  will  be  drawn  out  and 
become  thinner;  the  thickness  of  the  wall  may  become  less  than 
twice  the  distance  of  the  action  of  molecular  force  (21)  in  which 
case  the  surface  tension  becomes  less.  The  thickness  of  the  cell 
wall  may  even  become  zero,  and  the  cells  then  tear  themselves 
entirely  away.  Hence  after  a  large  oscillation  the  damping  will 
t  be  less  than  after  a  small  one.  Continued  boiling  destroys  the 
/  cells  and  a  gelatin  solution  then  behaves  as  a  homogeneous  fluid." 

By  experiments  upon  the  diffusion  of  solutions  through  gelatin 
jelly,  Bechhold  and  Ziegler1  also  came  to  favor  a  net  structure 
theory.  They  caused  a  precipitate  to  be  formed  in  the  interior 
of  a  strip  of  gelatin  by  allowing  the  proper  reagents  to  diffuse 
into  it  from  opposite  sides,  and  noted  the  nature  of  the  permea- 
bility of  the  films  so  produced.  They  concluded  that  the  jelly 
acts  as  a  network  of  gelatin  with  pores  filled  with  water,  through 
which  alone  the  diffusion  takes  place.  The  role  of  the  precipitate 
is  merely  the  filling  of  these  pores  and  the  consequent  prevention, 
partial  or  complete,  of  the  diffusion. 

Sutherland2  in  1906  added  a  new  conception  to  the  structure 
of  colloids.  He  affirmed  that  "the  characteristic  of  the  colloid 
state  is  that  the  molecules  cease  to  have  a  separate  existance; 
they  link  on  to  one  another  by  means  of  the  atomic  electric 
charges,  thus  forming  the  meshes  so  characteristic  of  colloids. 
Each  particle  in  a  suspension  (of  globulin)  might  therefore  be 
called  a  molecule,  but  with  no  advantage.  In  each  such  particle, 
however,  a  certain  pattern  is  repeated  in  three-dimensional 

1  H.  BECHHOLD  and  J.  ZIEGLER,  Ann.  Physik.  (IV),  20  (1906),  900. 

2  N.  SUTHERLAND,  Proc.  Roy.  Soc.  (London),  79B  (1907),  130. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         135 

space."  This  pattern,  Sutherland  denotes  as  a  semplar.  The 
basis  of  this  theory  rests  upon  the  conception  of  neutralized 
valencies  within  the  molecule.  Thus  in  NH3  the  valence  of  the 
N  is  5,  as  in  NH4C1,  these  five  consisting  of  four  negative  and  one 
positive  valence.  In  the  compound  NH3  three  of  the  negative 
valencies  are  combined  with  the  three  positive  valencies  of  the 
hydrogen,  and  the  remaining  positive  and  negative  valence  are 
combined  with  each  other  forming  a  doublet.  Such  doublets 
Sutherland  believes  are  the  cause  of  cohesion  and  rigidity.  Now 
if  each  doublet  in  the  compound  NH3  is  broken  by  some  force, 
and  the  positive  valence  of  one  molecule  is  caused  to  combine  with 
the  negative  valence  of  its  neighbor,  then  each  molecule  of  NH3 
becomes  a  semplar,  and  the  whole  system  becomes  a  mesh  of 
such  semplars.  The  actual  linking  up  of  molecules  in  such 
manner  is  however,  according  to  Sutherland,  usually  confined  to 
groups,  each  group  containing  a  limited  number  of  molecules. 
If  this  number,  m,  is  small  and  definite,  as  in  water  (dihydrol, 
(H2O)2)  and  ice  (trihydrol,  (H2O)3),  the  material  will  be  crystal- 
loid, but  when  m  becomes  large  and  indefinite,  then  the  substance 
is  in  the  colloidal  state. 

This  theory  Sutherland  applies  to  proteins  (globulin)  by 
assuming  solutions  of  such  to  contain  a  large  and  indefinite 
number  of  such  " semplar"  combinations.  The  action  of  acids 
or  other  solvents  upon  these  is  to  break  down  the  doublets 
permitting  of  a  salt  formation. 

Wo.  Ostwald1  is  of  the  opinion,  with  Garrett,  that  gelatin, 
even  in  solution,  possesses  a  structure.     His  explanation  of  the ; 
influence  of  heat  upon  gelatin  solutions  is  based  on  the  supposi- 
tion that  the  structure  is  partially  destroyed  by  heating.     He  has 
further  observed  that  the  influence  of  added  salts  upon  the  swell- 
ing of  gelatin  plates  in  water  is  similar  in  character  to  their 
influence  upon  the  viscosity  of  gelatin  solutions,  "that  the  degree 
of  swelling  runs  closely  parallel  with  the  diminuation  in  viscosity  1 
of  solutions,  as  determined  by  von  Schroeder,  since  the  concen-  1 
trations  of  salts  at  which  maxima  of  swelling  occur  are  nearly 
identical  with  those  at  which  mimima  in  the  viscosity  of  solutions 
are  observed."     Ostwald  concludes  from  this  that  the  passage 
of  gelatin  into  solution  does  not  destroy  the  structure  of  the  gel 
but  that  this  structure  persists  in  solution. 

1  Wo.  OSTWALD,  Arch..ges.  Physiol,  109  (1905),  277;  111  (1906),  581. 


136  GELATIN  AND  GLUE 

2.  The  Recent  Theories  of  Gel  Structure.  Procter's  Theory  — 
Procter1  pictures  a  compound  of  gelatin  with  an  acid  as  a  coher- 
ent mass  from  which  the  gelatin  cannot  diffuse  or  separate,  and 
which  in  its  essential  characteristics  behaves  like  a  single  large 
and  complex  molecule.  He  visualizes  it  as  a  felted  mass  of 
amino-acid  chains  held  to  each  other  by  attractions  which 
possibly  attach  only  their  ends,  but  freely  admit  of  the  passage 
of  liquid  between  them.  Such  a  structure  explains,  according 
to  Procter,  the  osmotic  effects  which  result  in  a  swelling  of  gelatin 
when  immersed  in  water  or  dilute  acids.  The  latter  have  free 
passage  through  the  gelatin,  but  the  anions  of  the  acid  combine 
with  the  gelatin  cations  forming  a  salt  which,  although  it  may  be 
highly  ionized,  is  prevented  from  diffusing  out  into  the  solution 
on  account  of  the  immobility  of  the  gelatin  ions  and  the 
consequent  electrostatic  limitation  of  the  outward  diffusion  of 
the  anions.  Since  they  cannot  move  outward,  the  osmotic 
forces  are  compensated  only  by  the  inward  movement  of  water 
resulting  in  swelling. 

Robertson's  Theory. — Robertson2  is  impelled  to  believe  that 
the  type  of  viscosity  which  solutions  of  proteins  exhibit  may  in 
some  manner  owe  its  existance  to  a  structure  rather  than  an 
internal  friction  which  merely  hinders  molecular  and  ionic 
motion.  He  suggests  that  "a  netlike  structure,  such  as  a  tennis 
net,  will  offer  no  hinderance  to  the  passage  through  it  of  a 
quickly  moving  body  which  is  smaller  than  its  meshes,  other  than 
that  which  is  due  to  the  fact  that  the  material  which  composes 
the  net  occupies  a  small  fraction  of  the  area  which  the  body  must 
traverse,  but  to  any  force  which  involves  deformation  of  the 
structure,  for  instance,  a  force  which  seeks  to  drag  it  through  a 
small  tube,  it  will  offer  a  very  considerable  resistance.  On  the 
other  hand  the  resistance  which  is  offered  to  a  small  moving  body 
by  a  viscous  liquid  (viscous,  that  is.  in  the  ordinary  sense)  is 
accurately  measured  by  the  resistance  which  the  liquid  offers  to 
\j  passage  through  a  tube."  The  direct  methods  for  determining 
viscosity,  as  the  passage  through  a  capillary  tube  or  the  rotation 
within  the  liquid  of  a  suspended  disk,  are  all  such  as  would  involve 
a  deformation  of  any  existing  structure,  and  consequently  are 
not  competent  as  means  for  distinguishing  betweenjbrue  internal 
friction  and  viscosity  due  to  a  structure.  Trie  indirect  method  of 

XH.  R.  PROCTER,  "Collegium"  (1915). 

2  T.  B.  ROBERTSON,  "Physical  Chemistry  of  the  Proteins"  (1918),  325. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         137 

measuring  viscosity  by  determining  the  conductivity  reveals 
however  only  that  viscosity  that  is  due  to  true  internal  friction, 
and  when  this  method  is  applied  to  proteins,  the  presence  of  the 
protein  is  found  to  leave  the  viscosity  of  the  solvent  practically 
unaltered. 

That  a  net  structure  is  not  confined  to  protein  or  colloid  sub- 
stances, but  may  in  fact  be  present  in  most  or  all  solutions  of 
electrolytes  is  urged  by  Robertson  from  the  fact  that  in  watery 
solutions  the  hydrogen  and  hydroxyl  ions  are  the  most  rapidly 
moving,  while  in  other  solvents  this  may  not  be  the  case,  but 
instead  those  ions  which  by  their  combination  give  rise  to  the 
solvent.  That  viscosity  measurements  of  crystalloidal  electro- 
lytes have  not  so  far  revealed  the  presence  of  a  net  structure 
within  them  is  tentatively  attributed  by  him  to  "the  tenuity  of 
the  net  and  to  the  fineness  of  its  framework;  to  revert  to  the 
analogy  employed  above,  a  net  of  the  finest  and  most  flexible 
silk  will  readily  pass  without  appreciable  resistance  through  a 
tube  which  would  offer  a  considerable  resistance  to  the  passage  / 
of  a  net  of  coarse  thread."  The  structure  of  the  protein  molecule  \ 
becomes  observable  by  viscosity  measurements  only  through  the  / 
enormously  greater  size  of  its  molecules.  \ 

Loeb1  observed  the  enormous  increases  in  viscosity  which 
resulted  from  merely  keeping  a  gelatin  in  a  refrigerator  for  a 
number  of  hours  and  then  warming  to  24° C.  for  the  measure-  / 
ment.     Immediately  after  melting,  his  viscosity  determinations  I 
showed  values  ranging  from  86  to  102,  while  after  leaving  in  the  ) 
refrigerator  for  one  hour  and  then  warming,  they  had  increased^ 
to  130  and  143,  and  after  18  hours  in  the  refrigerator  they  had  J 
risen  to  180  and  250.     However,  by  first  heating  to  50°  and  ( 
cooling  to  24°,  the  viscosity  again  became  normal.     These  data  ) 
are  shown  in  the  following  table. 

If  the  structure  is  responsible  for  the  viscosity,  as  seems  pro- 
bable, these  results  can  only  mean  that  the  standing  at  a  low 
temperature  in  some  way  favors  the  development  of  the  structure, 
while  heating  to  a  high  temperature  tends  to  destroy  it. 

McBain's  Theory. — McBain  and  his  collaborators2  have  studied 

1  J.  LOEB   J.  Gen.  PhysioL,  1  (1919),  495. 

2  C/.  J.  W.  McBAiN  and  C.  S.  SALMON,  /.  Am.  Chem.  Soc.,  42  (1920), 
426;    J.    W.    McBAiN,  3rd.   Report  on  Colloid  Chemistry,  British  Assoc. 
Adv.  Science  (1920),  2;  and  M.  E.  LAING  and  J.  W.  McBAiN,  J.  Chem.  Soc., 
117  (1920),  1506. 


138 


GELATIN  AND  GLUE 


the  structure  of  soap  solutions  in  connection  with  the  develop- 
ment of  their  micelle  theory1  which  they  apply  to  all  colloidal 
electrolytes.  As  the  evidence  in  the  case  of  these  soap  gels  is 
much  more  direct  than  has  been  found  for  proteins,  and  as  there 
is  much  reason  to  believe  that  the  structure  relationships  are 

TABLE  33. — VARIATION  IN  VISCOSITY  OF  GELATIN  SOLUTIONS  ON  STANDING 


Treatment,    all    measure- 
ments made  at  24°C. 

Na 
gelatinate 

K 

gelatinate 

Mg 
gelatinate 

Ca 
gelatinate 

Immediately  after  melting.. 

99.0 

102.0 

88 

86.0 

After  1  hour  in  refrigerator. 

135.0 

143.0 

138 

130.0 

After  18  hours  in  refriger- 

ator   

180.0 

250.0 

240 

200.0 

After  18  hours  in  refriger- 

ator and  being  kept  at  24° 

for  2  hours  

170.0 

210.0 

194 

173.0 

After  18  hours  in  refriger- 

ator, heating  to  50°,  and 

cooling  to  24°              .... 

94.5 

95.5 

86 

83.5 

very  similar  in  the  two  cases,  a  description  of  McBain's  findings 
will  not  be  out  of  place.  His  work  has  led  him  towards  the 
opinion  that  "in  a  gel  there  exist  well  developed  strings  of  long 
molecules  forming  an  exceedingly  fine  filamentous  structure 
which  accounts  for  the  elasticity  of  gels  and  also  for  the  fact 
that  they  exhibit  more  or  less  clearly  oriented  properties  such, 
for  instance,  as  the  lenticular,  fairly  definitely  oriented,  form  of 
bubbles  generated  within  gels." 

McBain  believes  that  the  same  forces  are  in  play  as  account 
for  the  phenomena  of  crystalline  liquids  and  liquid  crystals. 
Such  a  conception  would  serve  to  explain,  for  example,  the 
incipient  structure  which  most  sols  develop  on  standing  and 
which  is  such  as  prevents  definite  measurements  of  viscosity 
from  being  taken  independent  of  age  and  rate  of  shear. 

In  a  study  of  vanadium  pentoxide  sols  which  had  been  aged  for 
many  years,  Freundlich  found  that  at  the  boundaries  and 
throughout  the  sol,  whenever  the  sol  was  set  in  motion,  it  was 
anisotropic,  and  exhibited  all  the  behavior  of  a  crystalline  liquid. 
From  Vorlander's  observations  that  long  molecules  are  required 

1  Vide  page  264. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         139 

for  the  formation  of  liquid  crystals,  it  is  believed  that  the  behavior 
of  Freundlich's  sols  is  also  attributable  to  this  condition. 

These  strings  of  molecules  in  colloid  sols  may,  according  to 
McBain,  be  microns  or  millimeters  in  length  "in  other  words 
they  consist  of  innumerable  molecules  placed  lengthwise  and 
their  formation  would,  of  course,  be  ascribed  to  residual  affinity." 
This  links  up  Sutherland's  "semplar"  theory  of  the  breaking  of 
doublets  with  the  later  conceptions  of  structure. 

In  one  instance  McBain  was  able  to  observe  and  photograph  a 
cloudy  gel  of  sodium  oleate  which,  "in  addition  to  curd  fibers, 
contained  an  exceedingly  fine  and  delicate  filamentous  network. 
The  quartz  surfaces  in  contact  with  these  gels  often  exhibit 
indefinitely  long  and  exceedingly  fine  filaments  just  on  the  limits 
of  visibility  and  just  capable  of  being  photographed  with  long 
exposures.  Their  regularity  of  form  and  texture  is  astounding. 
They  simulate  living  matter  in  their  appearance,  they  may  take 
the  form  of  a  simple  sine  wave,  or  of  regular  waves  with  higher 
harmonic  series  superimposed  on  them.  Any  one  part  of  such  a 
filament  is  identical  in  form  and  structure  with  any  other  part. 
They  are  probably  derived  from  originally  straight  just  resolvable 
translucent  tubes  containing  very  regularly  spaced  whitish  dots 
or  lengths." 

In  the  case  of  sodium  soaps  the  curds  were  invariably  found  to 
consist  of  fine  fibers.  These  might  be  many  centimeters  in 
length  but  in  thickness  are  never  greater  than  one  micron.  Most 
of  them  are  even  of  ultramicroscopic  diameter,  and  the  thicker 
ones  are  probably  parallel  bundles  of  finer  ones.  "These  fibers 
are  often  so  fine  that  shorter  or  unattached  fibers  exhibit  Brown- 
ian  movement  when  the  medium  is  not  too  viscous.  These  curd 
fibers  constitute  the  only  mechanical  structural  element  of 
sodium  soap  curds  and  represent  the  stable  condition  of  such 
curds  even  after  the  lapse  of  years." 

The  effect  of  temperature  on  viscosity  is  readily  explainable 
upon  the  assumption  that  the  films  increase  in  number  or  length, 
with  decreasing  temperature.  That  such  takes  place  in  soap' 
curds  is  affirmed  by  McBain,  who  finds  that  "at  the  temperature 
of  initial  solidification  only  a  few  fibers  are  formed,  the  bulk  of 
the  soap  remaining  in  the  solution  which  therefore  exhibits  a 
practically  undiminished  vapor  pressure  and  conductivity.  As 
the  temperature  is  lowered  the  solubility  of  the  curd  fibers 
rapidly  diminishes  until  the  enmeshed  liquid  consists  chiefly  of 


140  GELATIN  AND  GLUE 

water,  and  its  vapor  pressure  and  conductivity  behave  accord- 
ingly. Throughout  this  range  of  temperature  the  stable  con- 
dition of  the  soap  solution  is  the  formation  of  the  appropriate 
amount  of  curd  fibers  with  enmeshed  gel.  The  definite  solu- 
bility of  the  curd  fibers  at  any  one  temperature  is  evinced  by 
the  fact  that  the  conductivity  of  a  well  aged  curd  is  approximately 
independent  of  the  concentration  of  the  original  soap." 

In  the  later  work  of  McBain  and  his  co workers  they  urge  that 
a  gel  and  sol  are  identical  except  for  differences  in  mechanical 
properties.  They  contend  that  the  exact  coincidence  of  osmotic 
activity,  electromotive  force,  and  conductivity  alike  prove  that 
the  chemical  equilibria  are  identical  in  sol  and  gel.  And  they 
insist  further  that  "since  the  conductivity  of  concentrated  gel 
and  sol  is  thus  identical,  the  hypothesis  of  a  closed  cellular, 
spongy,  or  honeycomb  structure  or  other  similar  structure  is 
disproved,  and  even  a  similar  structure  with  partly  open  pores  is 
rendered  extremely  unlikely."  They  find,  however,  that  there 
is  a  distinct  tendency  for  the  colloid  gels  to  form  long  strings  of 
molecules  or  colloidal  particles,  and  attribute  the  elasticity  of 
gels  to  the  exceedingly  fine,  filamentous  structure  so  produced. 
Each  of  these  innumerable  threads  consisting  of  colloidal  particles 
stuck  together  would  be  capable  of  exhibiting  mechanical  elas- 
ticity. On  account  of  the  amicroscopic  size  of  the  particles  and 
of  the  threads  any  displacement  of  them  in  the  liquid  would 
meet  with  such  great  frictional  resistance  that  the  property  of 
elasticity  would  be  transmitted  to  the  whole  mass  of  gel.  This 
conception  of  gel  structure  may  account  for  the  various  charac- 
teristic properties  which  they  exhibit,  and  especially  for  the 
more  or  less  clearly  oriented  properties,  as  the  lenticular  form 
of  bubbles  generated  within  gels,  and  the  phenomenon  of  syneresis. 
"Thus  if  there  is  an  orienting  force  between  the  particles,  there 
must  necessarily  be  in  that  force  a  component  of  attraction,  and 
hence  the  gel  structure  of  oriented  particles  must  exhibit  a 
distinct  tendency  to  shrink.  Even  if  this  attractive  force  is 
only  feeble,  it  must  in  course  of  time  produce  syneresis,  since  in 
dilute  gels  it  is  opposed  only  by  the  viscosity  of  a  fluid.  The 
swelling  of  gelatin  salts  is  not  in  conflict  with  this  view,  because 
the  ionic  micelle  of  gelatin  and  proteins,  unlike  that  of  soap, 
does  not  become  crystalloidal  in  dilute  solution,  and  so  continues 
to  be  retained  within  the  gel." 

The  only  possible  conceptions  of  the  nature  of  the  colloidal 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         141 

particles  which  link  together  to  form  the  gel  structure  are,  accord- 
ing to  McBain's  views,  neutral  colloid  or  ionic  micelle.  It  may 
even  be  that  all  of  the  former  is  included  in  the  ionic  micelle,  in 
which  case  the  conducting  particles  are  identical  with  those 
which  by  orientation  give  to  the  gel  its  structure. 

Bachmann1  had  previously  found  evidence  that  when  soap 
solutions  set  to  a  jelly  long  threads  are  formed.  It  seemed 
probable  to  him  that  these  threads  eventually  passed  over  into 
or  rearranged  themselves  into  a  net  structure. 

Bancroft's  Theory. — Bancroft2  considers  that  in  gelatinous 
precipitates  we  may  have  either  a  honeycomb  or  a  sponge 
structure.  Viscous  drops  may  partially  coalesce  to  form  fila- 
ments or  films,  or  spherical  drops  of  water  may  become  coated 
with  the  gelatinous  material.  The  latter  condition  would  give 
rise  to  the  honeycomb  structure,  while  the  former  condition 
would  produce  an  interlacing  or  sponge  structure,  each  phase 
being  continuous.  The  latter  conception  seems  to  be  the  one 
more  generally  accepted.3  Bancroft  has  remarked  that  the 
fact  that  ultrafiltration  may  be  carried  on  through  a  gelatin  or 
collodion  membrane  argues  strongly  in  favor  of  a  porous  structure 
in  those  cases,  but  this  does  not  follow  for  a  continuous  membrane 
that  was  semipermeable  in  the  sense  of  being  able  to  dissolve  the 
solvent  or  medium  of  dispersion  would  likewise  behave  as  an 
ultra-filter. 

Bancroft4  regards  a  dilute  colloidal  solution  of  gelatin  in  water 
as  consisting  essentially  of  turbid  drops  of  a  gelatin-rich  phase 
dispersed  in  water.  On  increasing  the  concentration  of  gelatin  a 
tendency  will  become  manifest  for  the  separate  drops  to  coalesce 
to  form  larger  drops,  or,  if  too  viscous  to  do  this,  they  may  only 
partially  coalesce,  forming  threads  or  a  "  chain  of  beads."  It 
is  not  even  necessary,  in  the  latter  case,  that  the  droplets  should  \ 
be  in  actual  contact.  Where  the  droplets  are  of  different  sizes, 
the  small  ones  will  tend  to  group  themselves  about  the  larger 
ones.  As  the  concentration  of  the  gelatin  becomes  greater, 
loose  chains  may  be  formed  which  will  finally  pass  into  a  net  or 

1  BACHMANN,  Kolloid-Z.,  11  (1912),  145. 

2  W.  D.  BANCROFT,   "Applied  Colloid  Chemistry,"  New  York  (1921), 
239. 

3  Cf.  QUINCKE,  Ann.  Physik.,  10  (1903),  482;  14  (1904),  489;  ZSIGMONDY, 
Z.  anorg.  Chem.,  71  (1911),  356;  BACHMANN,  ibid.,  73  (1912),  125;  FLADE, 
ibid.,  82  (1913),  173. 

4W.  D.  BANCROFT,  lib.  cit.,  242. 


142  GELATIN  AND  GLUE 

sponge  structure.  In  such  a  system  both  phases  will  be  con- 
tinuous, and  an  interlacing  of  the  phases  exist.  By  still  greater 
increase  in  the  gelatin  concentration,  water  may  become  dis- 
persed as  droplets  in  a  gelatin-rich  phase,  which  is  the  conclusion 
arrived  at  by  Hardy1  from  experiments  on  the  compression  of 
jellies.  We  have  however  no  data  which  permits  us  to  speculate 
upon  the  extent  to  which  increases  in  concentration  of  the  gelatin 
result  in  a  filling  up  of  the  solution  with  chains  or  filaments  of  the 
same  diameter  or  upon  the  increase  in  diameter  which  these 
filaments  undergo. 

Bancroft  believes  that  the  previous  history  of  a  jelly  will 
manifest  itself  in  the  rate  and  amount  of  swelling  only  in  case  the 
walls  of  the  porous  cells  are  fairly  rigid  and  do  not  unite  when 
brought  into  contact,  as  by  the  drying  out  of  the  cell.  If  the 
walls  are  not  rigid  they  may  collapse  to  such  an  extent  that 
the  pores  will  almost  completely  disappear,  and  in  that  case  the 
swelling  will  be  independent  of  the  previous  history  of  the  jelly. 
While  not  entirely  satisfactory,  this  explanation  is  the  best  we 
have  seen  to  account  for  the  differences  observed  in  the  swelling 
of  gelatin  which  has  been  made  from  solutions  of  different  con- 
centrations. It  will  be  recalled  that  if  gelatin  solutions  are  made 
at  concentrations  of  say  10,  20,  and  30  per  cent,  and  allowed  to 
dry  out  to  a  uniform  concentration  of  say  90  per  cent,  on  immer- 
sion in  water  the  three  will  take  up  the  liquid  in  different  amounts : 
that  made  from  the  10  per  cent  solution  will  take  up  water 
rapidly  until  its  concentration  is  again  10  per  cent,  while  that 
made  from  the  30  per  cent  solution  will  take  up  much  less  water, 
i.e.,  it  will  go  rapidly  to  a  30  per  cent  gel  but  only  slowly  beyond 
this  point.  By  Bancroft's  hypothesis  this  would  mean  that  the 
cross-section  of  the  filaments  were  different  in  the  several  cases, 
depending  upon  the  concentration  of  the  heated  solution.  It 
would  appear  that  this  cross-section  increased  with  the  concen- 
tration, making  the  cell  wall  more  rigid.  This  rigidity  in  turn 
permits  of  less  expansion  and  consequently  of  less  water  adsorp- 
tion per  unit  of  mass  than  if  the  walls  were  thinner  and  capable 
af  occupying  a  greater  volume.  It  may  be  mentioned  that  the 
ultramicroscope  is  incompetent  as  a  means  for  differentiating 
between  a  sponge  structure  and  a  honeycomb  structure  in 
J$  gelatin  jellies.2 

1  HARDY,  Z,  physik.  Chem.,  33  (1900),  326. 
2BACHMANN,  Kolloid-Z.,  23  (1918),  89. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         143 

Other  Theories  of  Gel  Structure. — The  experiments  of  von 
Schroeder1  upon  the  different  degrees  of  swelling  observed  in 
water  and  in  saturated  water  vapor  also  favor  a  theory  of  cellular 
structure.  He  found  that  gelatin  immersed  in  water  imbibed 
1000  per  cent  of  water,  and  when  placed  in  an  atmosphere  of 
saturated  water  vapor,  only  400  per  cent.  This  must  mean 
that  the  vapor  pressure  of  water  in  gelatin  is  higher  than  the 
vapor  pressure  of  pure  water,  for  water  distills  from  the  gelatin 
to  the  vapor  phase. 

This  curious  anomaly  may  be  accounted  for  upon  the  grounds 
of  a  cellular  structure.  If  a  sponge,  for  example,  is  placed  in  an 
atmosphere  of  water  vapor,  the  cellular  structure  will  absorb  a 
certain  amount  of  water  upon  its  surface,  but  water  will  not 
condense  in  the  interior  of  the  cells.  If  placed  in  liquid 
water,  however,  the  water  will  not  only  be  absorbed  upon  the 
surface,  but  will  also  fill  the  cells,  and  if  the  sponge  is  then  taken 
from  the  water,  and  placed  in  the  vapor  phase,  water  will  distill 
from  the  curved  capillary  pores  and  surfaces  of  the  sponge  to 
the  plane  surfaces  in  the  containing  vessel,  which  is  what  von 
Schroeder  observed  in  the  case  of  swollen  gelatin.2 

Thompson3  and  Wilson4  have  also  urged  the  necessity  of 
assuming  a  three  dimensional  network,  in  any  continuous  mass 
of  gelatin,  made  up  of  chains  of  atoms  forming  a  network  with 
interstices  very  much  larger  than  the  simple  molecules  or  ions, 
although  still  too  small  to  be  detected  microscopically.  This 
viewpoint  is  a  necessary  assumption  in  Wilson's  theory  of  the 
action  of  acids  upon  gelatin.5 

The  existence  of  a  crystalline  structure  in  gelatin  gels  has  been 
urged  by  Bradford6  and  tohers.  Bradford  claims  to  have 
obtained  gelatin  crystals  by  the  following  procedure.  A  small 
amount  of  a  high  grade  commercial  gelatin  was  heated  to  boiling 
in  water  to  which  a  trace  of  mercuric  chloride  had  been  added. 
This  was  then  filtered  and  allowed  to  stand  in  covered  crystalliz- 
ing dishes.  On  the  thirtieth  day  the  residue  showed,  under  the 

1  VON  SCHROEDER,  Z.  physiol  Chem.,  46  (1903),  109.     Vide  also  page  167. 

2  Wolff  and  Buchner  and  D.  J.  Lloyd  have  failed  to  confirm  the  findings 
of  von  Schroeder.  See  pages  167-8. 

3  F.  C.  THOMPSON,  /.  Soc.  Leather  Trades'  Chem.,  3  (1919),  209. 

4  J.  A.  WILSON,  J.  Am.  Leather  Chem.  Assn.  (1920),  374. 

5  Vide  page  179. 

6S.  C.  BRADFORD,  Biochem.  J.,  14  (1920),  91. 


144  GELATIN  AND  GLUE 

microscope,  numerous  single  spherites  of  0.25  toO.28/*  in  diameter, 
and  many  clusters  of  these  spherites. 

Scherrer1  has  studied  gelatin,  among  other  organic  and  colloid 
substances,  by  means  of  the  Rontgen  photograph,  and  has  been 
able  to  find  no  trace  of  interference  figures  arranged  in  a  manner 
characteristic  of  the  space  lattice,  which,  if  present,  would  indi- 
cate crystallinity.  Some  other  gels,  as  silicic  acid  and  stannic 
acid  gels,  exhibited  well  marked  crystalline  interference  figures 
in  addition  to  the  characteristics  of  amorphous  substances. 
Scherrer  concludes  that  the  latter  represent  substances  that  are 
at  the  point  of  crystallizing.  The  gelatin  (and  other  protein  gels) 
consists,  therefore,  of  colloid  particles  which  are  composed 
either  of  individual  molecules,  or  of  groups  of  molecules  that  are 
irregularly  orientated. 

Fischer2  considers  that  there  are  three  possibilities:  at  high 
temperatures  a  true  solution  of  gelatin  in  water;  below  this, 
(liquid)  hydrated  gelatin  in  solution  of  gelatin  (the  so-called  sol) 
and  if  the  temperature  is  reduced  sufficiently  and  enough  solvent 
is  present,  this  liquid  hydrated  gelatin  becomes  solid  hydrated 
gelatin  (crystalline)  in  gelatin  solution.  If  the  solvent  is  limited 
a  reversal  in  type  of  system  occurs  to  liquid  or  solid  hydrated 
gelatin  holding  within  itself  gelatin  solution. 

From  data  obtained  in  a  study  of  the  swelling  of  gelatin  in  acid 
and  alkali,3  Miss  Lloyd4  concludes  that  gelatin  gel  consists  of  a 
solid  and  a  liquid  phase,  both  of  which  are  continuous.  In  the 
isoelectric  condition  the  gelatin  becomes  "precipitated  at  numer- 
ous crystallization  centers:  the  solid  drops  will  run  together  to 
form  a  framework,  but  since  there  can  be  no  osmotic  forces  in  the 
system  the  framework  will  contract  under  the  action  of  its  own 
surface  forces,  and  the  internal  phase  will  be  squeezed  out. 
Therefore  the  gel  state  as  a  two-phase  system  cannot  exist  as  a 
stable  system  at  the  isoelectric  point."  In  strongly  acid  or 
alkaline  solution  the  gel  form  cannot  exist,  and  this  is  taken  by 
Miss  Lloyd  to  signify  that  gelatin  salts  cannot  form  gels.  The 
process  of  gelation  is  therefore  pictured  as  follows:  "gelation 
will  only  occur  on  the  cooling  of  a  sol  which  contains  in  solution 
iso-electric  gelatin,  and  gelatin  salts  in  equilibrium  with  free 

1  P.  SCHERRER,  Nachr.  Kgl.  Ges.  Wiss.  Gottengen  (1819),  96. 

2  MARTIN  FISCHER,  personal  communication. 

3  See  pages  167-8. 

4  D.  J.  LLOYD,  Biochem.  /.,  14  (1920),  162. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         145 

electrolytes.     As  the  sol  is  cooled  the  insoluble  isoelectric  gelatin    / 
is  precipitated  in  a  state  of  suspended  crystallization  and  forms  a 
solid   framework   throughout   the   system.     The   more   soluble 
gelatin  salts  remain  in  solution,  and  by  their  osmotic  pressure 
keep  the  framework  extended.     Gels  therefore  are  two-phase 
systems,  the  solid  phase  consisting  of  isoelectric  gelatin,  the      \ 
liquid  of  gelatin  in  the  salt  form." 

3.  The  Theories  of  Sol  Structure  and  the  Sol-Gel  Equilibrium. 
Although  a  sponge  or  honeycomb  structure  may  exist  in  the 
case  of  gelatin  gels  it  seems  highly  improbable  that  such  a  struc- 
ture is  present  in  the  sol  condition.  That  some  kind  of  a  struc- 
ture is  present  however,  seems  indisputable  from  such  evidence 
as  the  influence  of  previous  history  of  a  sol  upon  the  viscosity 
or  upon  the  swelling  when  again  permitted  to  dry  out,  and  upon 
the  influence  of  long  standing,  especially  in  the  cold,  upon 
viscosity. 

The  conception  of  a  catenary  or  thread-like  structure  has  been 
suggested  by  the  author1  to  account  for  the  behavior  of  gelatin 
sols.  By  this  conception  the  individual  molecules  may  be 
regarded  as  the  separate  links  of  the  chain,  while  the  colloid 
aggregate  is  represented  as  the  catenary  thread.  An  alteration 
in  what  is  commonly  spoken  of  as  degree  of  dispersion  may  be 
regarded  either  as  an  alteration  in  the  length  or  the  number  of 
these  threads,  or  a  change  in  the  hydration  of  the  molecules,  or  a 
combination  of  these  influences.  —An  interesting  analogy  to  this 
condition  is  the  acid  agglutination  of  bacteria  which  Bordet2 
and  Arkwright3  have  separately  pointed  out  belongs  to  the  same 
class  of  reactions  as  the  coagulation  by  hydrogen  ions  or 
electrolytes  of  amphoteric  colloids.  If  the  analogy  is  correct, 
electrolytes  may  bring  about  coagulation  in  protein  sols  by  a 
flocculation  or  bunching  together  of  the  catenary  threads  into  aggre- 
gates of  such  size  that  the  influence  of  gravity  becomes  more 
effective  than  the  kinetic  effects  of  Brownian  movement.  The  exis- 
tence of  a  fibrous  or  filamentous  structure  in  such  precipitates  is 
beginning  to  be  recognized,  and  it  seems  that  the  analogy  is  a 
sound  one. 

None  of  the  lines  of  evidence  that  have  been  advanced  for  the 
existence  of  a  net  structure  in  gelatin  sols  are  inconsistent  with 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  61. 

2  BORDET,  Cent.  Bakt.,  64  (1910),  150. 

3  ARKWRIGHT,  Z.  Immunitdt,  22  (1914),  396. 

10 


146  GELATIN  AND  GLUE 

the  conception  of  a  catenary  structure,  while  an  intermingling 
of  these  threads  would  result  in  the  formation  of  a  mesh  which 
would  give  all  of  the  evidences  and  ultramicroscopic  appearance 
of  the  net  structure  which  is  generally  claimed  for  the  gel  state. 
Any  deformation  in  these  threads  of  molecules  would  likewise 
reveal  itself  in  viscosity  measurements  by  making  such  determina- 
tions abnormally  high,  and  by  producing  variations,  such  as 
were  observed  by  Garrett,  in  the  logarithmic  decrement  of  a 
disk  oscillating  in  a  medium  in  the  presence  of  such  threads. 

The  conception  of  the  mechanism  of  protein  ionization  is  also 
readily  accounted  for  by  this  hypothesis.  Robertson1  observed 
a  serious  inconsistency  in  the  net  structure  theory.  He  pointed 
out  that  the  abnormal  viscosity  of  proteins  is  usually  said  to  be 
attributable  to  the  net  structure  of  their  sols,  and  that  it  also 
appears  to  be  closely  related  to  the  ionization  of  the  protein. 
Therefore  the  net  structure  within  the  protein  sol  must  be  built 
up  of  protein  ions.  But  such  a  conclusion  is  altogether  incom- 
patible with  the  generally  accepted  view  of  ions  as  we  consider 
them  to  exist  in  solutions  of  electrolytes.  The  latter  are  regarded 
as  " mutually  independent  and  physically  discrete  bodies,"  but 
the  net  structure  conception  of  an  ion  "  appears  to  invite  a  dis- 
tinction between  the  mode  of  ionization  of  ordinary  electrolytes 
and  that  of  protein  salts."  But  a  catenary  aggregate  or  string 
of  molecules  may,  without  an  undue  stretch  of  the  imagination, 
be  regarded  as  capable,  by  opening  up  of  their  — CONH —  groups, 
or  by  reactions  involving  terminal  — NH2  or  — COOH  groups,  of 
forming  a  salt  and  of  ionization,  and  without  losing  its  identity 
act  in  every  way  as  an  independent  and  discrete  body,  capable  of 
developing  osmotic  pressure,  of  conducting  the  electric  current, 
and,  in  general,  of  carrying  out  the  reactions  associated  with 
ions  in  electrolytes. 

Since  heating  breaks  up  whatever  structure  may  exist  in 
gelatin  sols,  and  shaking  or  beating  tends  towards  the  same 
result,  it  seems  equally  reasonable  to  expect  that  long  standing, 
especially  at  low  temperatures,  would  produce  the  opposite 
effect,  with  the  formation  of  exceptionally  long  threads.  The 
abnormal  viscosities  obtained  by  such  a  procedure  seem  to 
indicate  that  this  is  the  case. 
'Gelatin  sols  appear  therefore  to  consist  of  slightly  hydrated 

lfT.  B.  ROBERTSON,  "The  Physical  Chemistry  of  the  Proteins"  (1918), 
325. 


*JUJL 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         147 

molecules  united  together  into  short  threads  resembling  strepto- 
cocci. These  threads1  are  probably  very  short,  but  should  be 
capable  of  exhibiting  mechanical  elasticity  roughly  proportional 
to  their  length.  Procter2  thinks  that  at  70°C.  the  solution 
becomes  probably  nearly  molecular. 

A  lengthening  of  these  threads  seems  to  take  place  as  the 
temperature  falls,  and  at  the  same  time  the  water  absorbing 
power  of  the  gelatin  increases.3  This  accounts  for  the  rapid 
increase  in  viscosity  with  drop  in  temperature.  At  temperatures 
above  40°C.  the  change  in  length  of  thread,  or  water  absorption, 
per  unit  change  in  temperature  is  small,  but  at  30°-20°  the 
change  is  very  great.  A  solid  jelly  will  result  only  when  the 
relative  volume  occupied  by  the  swollen  molecular  threads  has 
become  so  great  that  freedom  of  motion  is  lost,  and  the  adjacent 
heavily  swollen  aggregates  cohere.  The  rigidity  seems  to  depend 
on  the  relative  amount  of  free  solvent  in  the  interstices  of  these 
aggregates,  and  on  the  amount  of  solvent  that  has  been  taken 
up  by  the  gelatin  in  a  hydrated  or  imbibed  condition.  The 
resiliency  or  elasticity  is  probably  dependent  upon  the  length 
and  number  of  the  catinary  threads.  A  solution,  or  change  from 
the  gel  to  the  sol  form,  may  result  only  through  the  reversal  of 
these  processes,  that  is,  a  release  of  a  part  of  the  water  retained 
by  the  heavily  swollen  molecules,  and  a  partial  disintegration 
of  the  long  enmeshed  fibrils  of  the  gel.  Any  tendency  on  the 
part  of  the  fibrils  towards  an  orientation  would  imply  an  attrac- 
tive force  between  them  which  would  result  in  a  shrinkage.  This 
becomes  manifest  in  syseresis.  That  some  such  orienting  force 
does  exist  is  indicated  by  the  lenticular  form  of  bubbles  that 
are  generated  within  gels.  The  degree  of  swelling  that  may  be 
produced  in  cold  water  or  electrolyte  solutions  is  probably  deter- 
mined by  osmotic  forces,  as  described  by  Procter,  and  may  be 
controlled  by  observing  the  principle  of  the  Donnan  Equilibrium 
as  shown  by  Procter  and  by  Loeb. 

1  J.  Loeb  has  found  the  assumption  of  a  few  united  molecules  to  account 
for  the  differences  in  the  osmotic  pressures  of  calcium  and  sodium  gelatin- 
ates     («/.  Gen.  Physiol.,  1  (1919),  496.) 

2  H.  R.  PROCTER,  Report  of  the  Faraday  and  Physical  Societies  (1921),  41. 

3  Whether  or  not  this  is  real  hydration  is  undetermined.     H.  C.  Jones 
(J.  phys.  Chem.,  74  (1910),  325)  has  shown  that  the  hydration  of  molecules 
and  ions  increases  with  a  fall  in  temperature,  and  McBain  and  Salmon 
(J.  Chem.  Soc.f  119  (1921),  1374)  have  reported  an  increase  in  the  hydration 
of  soaps  upon  a  lowering  of  the  temperature. 


148  GELATIN  AND  GLUE 

The  property  of  elasticity  is,  in  the  fluid  state,  synonymous 
with  plasticity,  for,  on  account  of  the  amicroscopic  size  of  these 
particles  and  the  short  threads  characteristic  of  the  sol  state, 
any  displacement  of  them  in  the  fluid  would  meet  with  so  great 
a  frictional  resistance  that  the  property  of  elasticity,  or  plastic 
flow,  would  be  transmitted  to  the  whole  mass.  This  conception 
seems  to  be  in  agreement  with  the  theory  of  McBain  and  his 
collaborators1  with  respect  to  the  sol  and  gel  structure  in  soaps. 

The  transition  from  the  sol  to  the  gel  forms,  and  vice  versa,  is 
not,  therefore,  a  critical  change,  as  occurs,  for  example,  when  a 
crystal  of  benzol  changes  to  the  liquid  state.  Indeed  it  no  longer 
seems  pertinent  to  regard  the  gel  and  sol  forms  as  distinct  phases 
in  the  classical  sense,  but  rather  as  different  forms  of  the  same 
state.  Since  this  is  the  only  conclusion  it  seems  justifiable  to 
draw,  it  would  appear  difficult  to  conceive  that  a  concise  point 
of  transition  from  the  one  form  to  the  other  could  exist.  There 
is,  rather,  a  period  through  which  the  transition  is  especially 
marked  by  virtue  of  rapidly  changing  physical  properties. 

None  of  the  instruments  that  have  been  devised  for  measuring 
molecular  or  molecular  group  elasticity  (plasticity)  are  in  any 
sense  absolute:  e.g.,  there  is  a  sensitivity  coefficient  below  which 
they  cease  to  function.  In  other  words,  while  we  can  say  defi- 
nitely that  paint,  for  example,  is  a  plastic  solid  or  shows  prop- 
erties of  plastic  flow,  we  cannot  say  as  positively  that  water 
exhibits  no  such  properties.  All  that  we  may  say  is  that,  as  far 
as  the  most  delicate  sensitivity  of  our  instrument  reveals,  there 
is  no  indication  of  plastic  flow  in  water.  Theoretically  there  is 
no  reason  to  believe  that  liquid  water  (dihydrol)  should  not 
possess  intermolecular  elasticity. 

With  the  instrument  made  use  of  in  the  author's2  experiments 
on  plasticity  (the  MacMichael  viscosimeter)  the  highest  tem- 
perature at  which  evidences  of  plastic  flow  were  observed  (in 
25  per  cent  concentration)  was  about  34°C.  A  more  delicate 
instrument  might  show  this  property  at  a  higher  temperature. 
As  the  concentration  of  the  gelatin  solution  was  decreased,  the 
maximum  temperature  where  plastic  flow  was  first  observed 
became  lower,  e.g.,  about  33°  in  the  20  per  cent  concentration, 
and  29°  in  the  10  per  cent  concentration.  This  is  in  entire 
conformity  with  the  argument  presented  above.  For  while  it 

1  See  M.  E.  LAING  and  J.  W.  McBAiN,  /.  Chem.  Soc.,  117  (1920),  1506. 

2  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  44  (1922),  1313.     (See  page  207.) 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         149 

was  stated  that  the  plasticity  was  an  expression  of  interfibral 
elasticity,  and  that  elasticity  was  determined  by  the  length  of 
the  fibrils,  it  also  follows,  from  the  limited  sensitivity  of  our 
apparatus,  that  the  measureability  of  this  property  must  depend 
upon  the  actual  concentration  of  fibrils  in  the  solution.  And 
this  is  proportional  to  the  total  concentration  of  gelatin  in  the 
solution  at  any  given  temperature. 

Davis  and  Oakes1  have  reported  that  at  the  temperature  of 
38.03°C.  gelatin  sol  and  gel  can  exist  in  equilibrium,  while  this 
is  not  true  for  any  other  temperature.  That  is,  a  " seeded" 
solution  (one  to  which  a  little  gelatin  gel  had  been  added)  showed 
no  change  in  viscosity  with  time  at  the  temperature  of  38.03°. 
At  any  temperature  below  this  a  regular  increase  in  viscosity 
with  time  was  observed.  At  higher  temperatures  a  decrease 
occurred  until  the  viscosity  equalled  that  of  a  similar  unseeded 
portion  at  the  same  temperature.  Sheppard2  has  been  able  to 
very  closely  corroborate  this  value,  but  Loeb3  has  reported  that 
at  any  temperature  above  35°C.  the  viscosity  (of  a  2  per  cent 
solution  of  gelatin  chloride  of  pH  2.7)  decreases  on  standing. 

In  order  to  bring  more  data  to  bear  upon  this  point  a  series  of 
experiments  was  performed4  with  the  object  of  noting  the  changes 
in  viscosity  with  time,  of  gelatin  solutions  of  varying  pH  and  of 
varying  concentration.  The  data,  shown  in  part  in  Fig.  16, 
indicate  a  decrease  in  viscosity  with  time  at  every  pH  value 
tested,  from  2.0  to  9.4,  with  the  exception  of  the  sample  at  4.7 
in  which  case  there  was  no  change.  The  sample  at  4.8  was 
"seeded,"  but  no  alteration  in  the  slope  of  the  curve  was  observed. 
The  nearer  the  pH  of  the  samples  to  the  isoelectric  point,  the 
less  the  variation  in  viscosity  with  time.  There  was  in  no  case 
however  an  increase  in  viscosity  with  time. 

A  gelatin  that  had  been  purified  by  dialysis  was  also  subjected 
to  the  same  treatment,  and  although  the  curves  were  in  most 
instances  very  similar  to  the  previous  ones,  yet  at  pH  4.7  there 
was  a  slight  tendency  for  an  increase  in  viscosity  with  time  as 
indicated  by  the  dotted  curves  in  Fig.  16.  At  37.0°  the  curve 
was  again  horizontal. 

1  C.  E.  DAVIS  and  E.  T.  OAKES,  J.  Am.  Chem.  Soc.,  44  (1922),  464. 

2  S.  E.  SHEPPARD,  Discussion  at  62nd  Meeting,  Am.  Chem.  Soc.,  New 
York,  Sept.  6-10,  1921. 

3  J.  LOEB,  J.  Gen.  Physiol.,  4  (1921),  107. 

4  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  44  (1922),  1343. 


150 


GELATIN  AND  GLUE 


The  significance  of  these  data  is  now  apparent.  There  are 
obviously  many  factors  which  influence  the  effective  volume  of 
the  gelatin  in  the  solution.  Of  these  the  pH  seems  to  be  most 
important.  The  amount  and  nature  of  the  inorganic  ions  with 

146 


140 


T3 

§130 

0) 
(D 

c 


REGULAR  GELATIN 


DIAL  YZED  GELATIN 

Figures  on  Curves  Indicate  p#  Values 


9.4. 


I          2         3        4         5         6 

Time  in  Hours 

FIG.  16. — Change  in  viscosity  with  time  at 

35°C. 


7          8 
varying  pH,  2  per  cent  solution, 


which  the  gelatin  is  associated  is  another.  The  presence  of  the 
hydrolysis  products  of  gelatin  is  a  third  factor.  And  the  meas- 
ureability  of  these  influences  will  be  determined  by  the  concen- 
tration. At  low  temperatures,  e.g.,  25°,  the  tendency  in  the 
system  is  for  an  increase  in  the  size  of  the  molecular  aggregate. 
Hence  an  increase  in  viscosity  with  time.  At  high  temperatures, 
e.g.,  40°,  the  tendency  is  for  a  decrease  in  the  size  of  this  aggre- 
gate. Hence  a  decrease  in  the  viscosity  with  time.  At  any 
specific  temperature,  e.g.,  35°,  whether  the  aggregate  will  become 
larger  or  smaller  is  determined  by  the  pH  of  the  solution,  and  the 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         151 

presence  of  inorganic  ions  and  protein  hydrolysis  products. 
Under  any  given  set  of  conditions  there  will  be  some  temperature 
at  which  neither  increase  nor  decrease  will  occur.  .  This  point 
was  found  in  gelatins  studied  by  Davis  and  by  Sheppard  to  be  at 
about  38°  ;  in  gelatins  studies  by  Loeb  to  be  at  35°  (in  solution  of 
pH  2.7)  ;  and  in  gelatins  studied  by  the  author  to  be  at  35  and 
37°  (in  solution  of  pH  4.7). 

It  appears  that  at  elevated  temperatures  the  colloid  fibril 
consists  of  but  a  few  partially  hydrated  molecules  attached  to 
each  other,  and  floating  about  as  discrete  particles  in  the  solvent. 
An  increase  in  viscosity  with  time  would  signify  either  an  increase. 
in  the  size  (length)  of  the  threads  or  an  increased  degree  of  hydra-/ 
tion.  At  elevated  temperatures  the  equilibrium  is  evidently) 
rapidly  attained.  This  seems  to  be  due  to  the  relatively  small) 
changes  that  are  induced  in  the  particle  size  and  degree  of  hydra- 
tion  by  variations  in  temperature  at  elevated  temperatures,  and 
to  the  high  mobility  of  the  free  solvent.  But  as  the  temperature 
falls  the  amount  of  change  per  unit  drop  in  temperature  rapidly 
increases,  and  with  this  a  rapid  decrease  in  the  mobility  of  the 
solution  through  the  rapid  withdrawal  of  the  solvent  by  hydra- 
tion.  The  time  required  for  the  colloidal  molecule-fibrils  to 
reach  a  state  of  complete  equilibrium  with  the  solvent  is  con- 
sequently vastly  increased.  In  other  words,  the  solution  will 
show  an  increase  in  viscosity  with  time. 

Under  any  given  condition  of  temperature  and  hydrogen  ion 
concentration  there  will  be  a  definite  viscosity  which  the  system 
will  attain  at  equilibrium.  A  temperature  at  which  no  change 
in  viscosity  with  time  occurs  indicates  an  equilibrium  condi- 
tion, but  this  temperature  will  vary  with  different  hydrogen  ion 
concentrations  and  with  different  degrees  of  purity  of  the  sample. 
It  is  in  no  way  indicative  of  a  critical  temperature  between  the 
sol  and  gel  forms,  but  is  rather  only  a  point  on  a  continuous 
curve.  This  may  be  expressed  by  the  equation: 


where  r/pH  is  the  viscosity  at  equilibrium  at  any  given  pH, 
/(T)  is  some  function  of  the  temperature,  and  K  is  a  constant. 
On  account  of  the  length  of  time  required  to  attain  equilibrium, 
and  the  difficulty  of  eliminating  completely  all  other  influences, 
as  hydrolysis  due  to  the  prolonged  action  of  water,  electrolytes, 
or  bacteria,  the  exact  measurement  of  r;pH  is  uncertain,  except 


152  GELATIN  AND  GLUE 

where  the  conditons  have  been  met  for  the  existance  of  an 
equilibrium  immediately.  This  is  the  condition  encountered 
where  that  •  temperature  is  obtained  at  which  no  change  in 
viscosity  with  time  is  observed. 

The  author1  has  shown  that  the  gel  consistency  is  proportional 
to  the  undegraded  protein  present  in  a  gelatin  or  glue.  It 
follows,  therefore,  that  the  undegraded  gelatin  possesses  a  much 
larger  water-absorbing  capacity  than  the  proteoses  or  peptones. 
It  was  also  early  pointed  out2  that  the  viscosity  varied  with  the 
size  of  the  colloid  aggregate  in  the  solution.  The  present  theory 
demands  that  viscosity  vary  with  the  degree  of  hydration  (meas- 
ured by  the  rigidity  of  the  gel)  and  with  the  size  (length)  of  the 
colloid  fibril  (measured  by  the  elasticity  of  the  gel3).  This  is 
only  an  amplification  of  the  earlier  findings.  The  "  melting 
point"  was  shown4  to  be  determined  by  the  protein  content  and 
was  found  to  give  a  " grading"  lying  between  that  resulting  from 
measurements  of  gel  strength,  and  of  viscosity  at  high  tempera- 
tures (60°C.).  Since  it  has  been  shown  that  " melting  point" 
is  in  reality  only  a  transitional  period  between  the  sol  and  gel 
forms,  and  that  the  transition  involves  only  an  increase  in 
degree  of  hydration  and  a  lengthening  in  the  colloid  molecule- 
threads,  it  must  also  follow  that  any  measure  of  " melting  point" 
will  indicate  a  resultant  between  the  effects  of  hydration  and  of 
length  of  thread,  or,  differently  expressed,  a  resultant  between 
gel  strength  and  viscosity  at  high  temperatures,  which  is  exactly 
in  conformance  with  the  data  reported  in  an  early  paper. 

The  conclusions  of  Fischer5  state,  "  the  phenomena  of  hydration 
(swelling)  and  of  'solution '  while  frequently  associated  are  essenti- 
ally different.  Hydration  is  to  be  regarded  as  a  change  through 
which  the  protein  enters  into  physicochemical  combination  with 
its  solvent  (water);  'solution/  as  one  which  can  be  most  easily 
understood  at  the  present  time  as  the  expression  of  an  increase 
in  the  degree  of  dispersion  of  the  colloid."  This  is  in  satisfactory 
agreement  with  the  ideas  expressed  above,  for  although  we  do  not 
consider  that  a  true  solution  may  exist  at  low  temperatures  on 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  105. 
j      2  Idem.,  108. 

3  See  Apparatus  of  S.  E.  SHEPPARD,  /.  Ind.  Eng.  Chem.,  12  (1920),  1007. 

4  R.  H.  BOGUE,  loc.  cit.,  64. 

5  MARTIN  FISCHER  and  W.  D.  COFFMAN,  /.  Am.  Chem.  Soc.,  40  (1918), 
304;  MARTIN  FISCHER,  "Soaps  and  Proteins,"  New  York  (1921),  219. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         153 

account  of  the  heavy  hydration,  yet  the  change  in  a  jelly  upon 
conversion  to  a  liquid  involves  a  disintegration  of  the  colloid 
aggregates  (increase  in  degree  of  dispersion)  as  well  as  a  lessening 
in  the  degree  of  hydration. 

Specific  Influence  of  Electrolytes. — The  specific  effects  of  electro- 
lytes upon  the  sol-gel  equilibrium  were  studied  in  a  special 


.234-56789     10    II 
•buOsoeledric  Gelofm+HCI  orNaOH) 

^    -  A  H 

FIG.   17.— Influence  of  hydrogen  ion  concentration  on  the  swelling,  viscosity 
and  foam  of  gelatin. 

series  of  experiments.1  The  influence  of  pH  on  the  swelling, 
viscosity,  jelly  consistency,  foam,  turbidity,  and  alcohol  number 
was  investigated. 

It  was  found  that  the  maximum  viscosity  and  swelling  occurred 
at  a  pH  of  3.5  or  9.0,  and  the  maximum  jelly  consistency  at  a 
pH  of  4.0  to  4.5.  All  of  the  properties  studied,  with  the  exception 

1  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  44  (1922),  1343. 


154 


GELATIN  AND  GLUE 


of  turbidity  and  foam,  appear  to  have  their  minimum  values, 
and  these  two  properties  their  maximum  values,  at  or  near  the 
isoelectric  point.  If  acid  or  alcohol  are  present  in  excess  of  the 
optimum  specified,  the  values  of  the  properties  again  decline. 
Phosphoric  and  lactic  acids  were  found  to  behave  quite  similarly 
to  hydrochloric  acid,  but  sulfuric  acid  produced  a  diminution  in 


Very 
Turbid 

Turbid 
Clear 

Solid 

Sem'i- 
Solid 

Liquid 
30 

i_ 
-f  20 

3 

z 

1  10 

< 
0 

I 

\. 

/ 

/ 

JUR 

SID 

\, 

7Y 

\ 

s 

/ 

\ 

JEL 

LY6 

Tff£ 

NG1 

'H 

\ 

I 

CO/ 

1O  L 

NUh 

fffft 

9    / 

\ 

/ 

/ 

/ 

\ 

^ 

/ 

S 

pH(l5oelectric  Gelatin  +HCI  or  NaOHj 

FIG.  18. — Influence  of  hydrogen  ion  concentration  on  the  alcohol  number,  jelly 
strength,  and  turbidity  of  gelatin. 

the  swelling  and  viscosity.  These  results  are  all  similar  to  those 
reported  by  Loeb,  and  will  be  discussed  more  fully  in  Chap.  V. 
Curves  of  some  of  the  data  obtained  are  shown  in  Figs.  17  and  18. 
The  data  are  found  to  furnish  additional  evidence  in  favor  of  the 
sol-gel  equilibrium  that  has  been  described.  The  swelling  may 
be  taken  as  a  measure  of  the  hydration  and  this  is  found  to  be 
parallel  to  the  viscosity.  Any  increase  in  viscosity  must  be 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         155 

accountable  to  an  increase  in  the  effective  volume  of  the  gelatin 
in  the  solution.  This  volume  is  obviously  at  a  minimum  at  the^ 
isoelectric  point  which  signifies  that  hydration  is  least  at  that 
particular  pft.1  This  seems  to  be  due  to  the  fact  that  at  thatV 
hydrogen  ion  concentration  gelatin  is  unionized,  and  ions  appear 
to  be  capable  of  greater  hydration  than  unionized  molecules.2 
The  viscosity  of  isoelectric  gelatin  increases  upon  standing,  \ 
however,  at  a  greater  rate  than  at  any  other  pH,  and  this  appears 
to  be  due  to  the  very  marked  insolubility  of  the  gelatin  at  that 
pH,  for  the  tendency  of  the  gelatin  molecules  and  colloid  fibrils 
to  increase  in  size  (length)  is  so  decided  that  it  is  easily  observable 
under  the  ultramicroscope.  It  is  especially  significant  to  observe 
that  the  jelly  consistency  of  isoelectric  gelatin  (see  curves) 
becomes  very  low  at  that  pH  which  also  indicates  a  low  degree 
of  hydration/ 

The  increases  in  viscosity  observed  by  raising  or  lowering  the 
pH  from  the  isoelectric  point  are  probably  attributable  to  a     , 
variation  in  degree  of  hydration,  as  shown  by  the  parallelism  of    L 
the  viscosity  and  swelling  curves.-    The  sudden  drop  in  both    I 
viscosity  and  swelling  at  pH  above  9  or  below  3  seems  to  be  due 
to  a  "solution"  or  breaking  down  of  the  colloid  molecule-threads/ 
and    this    disintegration   is    accompanied    by    a    corresponding 
lessening  in  the  ability  of  the  smaller  aggregates  or  molecules  to 
take  up  water.     That  this  reasoning  is  correct  is  further  evi- 
denced by  the  known  inability  of  the  proteoses  and  peptones  to 
absorb  water  to  anything  like  the  degree  attained  by  the  gelatin 
aggregates. 

The  depressing  influence  of  inorganic  ions  on  the  swelling  and 
viscosity  of  the  gelatin  is  partly  attributable  to  the  withdrawal 
of  water  from  the  swollen  gelatin  by  these  ions.  And  since 
the  high  viscosities  are  due  to  the  heavily  swollen  gelatin  aggre- 
gates, any  decrease  in  the  degree  of  such  hydration  must  be 
reflected  by  a  drop  in  the  viscosity  of  the  solution.  Divalent 
ions  appear  to  be  capable  of  greater  hydration  than  monovalent 
ions  and  should  therefore  be  expected  to  be  capable  of  withdraw- 

1  That  the  presence  or  absence  of  ions  is  responsible  for  swelling,  etc. 
is  denied  by  Fischer.     He  believes  (personal  communication)  that  the  poly- 
merized amino-acid  (isoelectric  gelatin)  has  a  lower  hydration  capacity  than 
the  ordinary  salts  of  the  acid  (gelatin  salts  and  gelatinates).     That  is,  the 
free  acid  is  simply  a  poorer  solvent  for  the  water. 

2  H.  C.  JONES,  Am.  Chem.  J.,  34  (1905),  291. 


156  GELATIN  AND  GLUE 

ing  larger  amounts  of  water  from  the  gelatin  particles.     From 
the  experiments  of  Fischer,1  it  is  also  shown  that  divalent  base    / 
(soaps  and)  proteinates  dissolve  less  water  than  monovalent  ones.^/ 

The  turbidity  curves  indicate  that  the  greatest  opacity  results 
from  the  largest  aggregates  of  least  swollen  particles,  i  This 
maximum  of  opacity  occurs  at  the  isoelectric  point.  Any 
decrease  in  the  size  of  the  aggregates  or  increase  in  the  hydration 
results  in  greater  clarity  or  transparency  of  the  solution. 

The  foaming  qualities  appear  to  be  influenced  in  a  similar 
manner  to  the  turbidity,  the  maximum  of  foam  being  obtained 
at  the  isoelectric  point.  This  is  exactly  what  would  be  expected 
for,  since  the  foam  consists  of  bubbles  of  air  retained  by  a  con- 
tinuous film,  only  molecules  that  have  a  strong  tendency  to 
adhere  to  each  other  would  be  efficacious  in  film  formation.  At 
the  isoelectric  point  gelatin  molecules  show  their  maximum 
tendency  to  form  large  aggregates. 

The  alcohol  number  is  at  its  minimum  value  near  the  isoelectric 
point,  and  rises  rapidly  to  infinity  on  the  acid  side  and  somewhat 
less  rapidly  on  the  alkaline  side.  Since  the  alcohol  number  refers 
to  the  precipitability  of  gelatin  by  alcohol  it  would  be  expected 
that  the  larger  the  molecular  aggregate,  and  the  less  the  water 
content  of  the  aggregate,  the  more  readily  would  such  pre- 
cipitation be  brought  about.  This  is  especially  significant  in 
that  alcoholic  precioitation  of  proteins  probably  consists  essenti- 
ally of  a  dehydration,  or  extraction  of  water.  Therefore  in 
such  solutions  in  which  dehydration  is  already  high  and  there  is 
but  little  tendency  towards  hydration,  the  completion  of  the 
reaction  is  readily  brought  about  by  alcohol,  but  in  systems  that 
are  heavily  hydrated,  the  dehydrating  influence  of  added  alcohol 
may  be  insufficient  to  effect  precipitation. 

Mutarotation. — The  data  of  C.  R.  Smith2  on  mutarotation 
have  been  examined  critically  in  their  applications  to  the  sol-gel 
equilibrium.  The  change  in  specific  rotation,  or  mutarotation,  of 
gelatin  solutions  of  a  constant  concentration  upon  reduction  of 
the  temperature  from  35  to  15°C.  was  found  to  drop  off  very 
markedly  with  a  decreasing  jelly  consistency  of  the  gelatin  or 
glue  employed.  That  is,  the  mutarotation  was  highest  in  a 
(3  per  cent)  solution  of  a  gelatin  which  was  capable  of  gelling 
(at  15°C.)  at  a  concentration  of  about  0.56  per  cent,  and  very 

1  MARTIN  FISCHER,  lib.  cit.,  p.  14. 

2  C   R.  SMITH,  /.  Ind.  Eng.  Chem.,  12  (1920),  878. 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         157 

low  in  a  (3  per  cent)  solution  of  gelatin  which  would  gel  (at 
15°C.)  only  when  the  concentration  had  been  raised  to  2.00  per 
cent  or  higher.  To  have  a  more  concise  picture  of  the  exact 
relations  the  data  of  Smith  have  been  plotted,  the  ordinate 
representing  the  mutarotation,  (15°  -  35°)  and  the  abscissa 
the  minimum  amount  of  gelatin  required  to  produce  a  standard 
jelly  at  15°.  This  curve  is  shown  in  Fig.  99.  (See  page  413.) 

There  are  many  minor  discrepancies  observable,  but  these 
are  attributable  to  the  failure  of  the  method  employed  for 
measuring  jelly  consistency1  to  distinguish  between  rigidity 
(hydration)  and  elasticity  (length  of  colloid  fibril).  The  general 
tendency  of  the  curve  is,  however,  incontrovertable. 

Since  the  jellying  power  of  a  gelatin  solution  has  been  shown2 
to  be  proportional  to  the  content  of  unhydrolyzed  protein 
present,  it  follows  that  the  mutarotation  is  also  proportional  to 
the  protein  content.  But  the  specific  rotation  at  elevated  tem- 
peratures (above  35°C.)  does  not  vary  with  jelly  consistency. 
The  specific  rotation  at  low  temperatures  (below  15°C.)  does 
however  increase  (negatively)  with  increasing  power  of  jelly 
formation.  We  have  given*  evidence  which  indicates  that  the 
proteins  (gelatin)  are  capable  of  vastly  greater  hydration  than 
the  proteoses  and  peptones.  It  appears,  therefore,  necessary  to 
conclude  that  the  increase  in  mutarotation,  or  in  specific  rotation 
upon  reduction  in  temperature  (35°  to  15°C.)  must  be  dependent 
for  its  existence  upon  the  greatly  increased  hydration  which 
such  unhydrolyzed  proteins  are  found  to  undergo  upon  similar 
reductions  in  temperature. 

The  Occlusion  Theory. — Loeb3  has  recently  questioned  the 
whole  conception  of  hydration  in  the  older  sense  in  which  the 
term  was  used  by  Pauli,  at  least  in  so  far  as  it  applies  to  solutions 
of  the  proteins  (gelatin,  casein  and  crystalline  egg  albumin),  and 
finds  it  impossible  to  reconcile  the  results  of  his  experiments 
upon  the  viscosity  and  osmotic  pressure  of  such  solutions  with 
the  early  hydration  theory. 

To  study  the  question  Loeb  performed  a  long  series  of  experi- 
ments with  solutions  and  suspensions  of  gelatin.  He  found  that 
the  influence  of  electrolytes  on  the  viscosity  of  suspensions  of 

1  A  standard  viscidity  which  would  permit  a  bubble  of  air  to  rise  through  a 
tube  of  the  gelatin  sol-gel  at  an  arbitrarily  selected  rate. 

2  R.  H.  BOGUE,  loc.  cit. 

3  JACQUES  LOEB,  J.  Gen.  Physiol,  3  (1921),  827;  4  (1921),  73;  97. 


158  GELATIN  AND  GLUE 

powdered  particles  of  gelatin  in  water  was  similar  to  their  influence 
on  the  viscosity  of  solutions  of  the  gelatin  in  water.  He  found 
it  unnecessary  to  assume  that  the  high  viscosity  of  proteins  is 
due  to  the  existence  of  a  different  type  of  viscosity  from  that 
existing  in  crystalloids,  but  that  such  high  viscosities  could  be 
accounted  for  quantitatively  and  mathematically  on  the  assump- 
tion that  the  relative  volume  of  the  gelatin  in  solution  is  compara- 
tively high.  And  since  isoelectric  gelatin  is  not  ionized,  the 
large  volume  cannot  be  due  to  an  hydration  of  gelatin  ions.  Loeb 
therefore  postulates  that  the  high  volume  of  gelatin  solutions 
is  caused  by  the  existence  in  the  solution  of  "  submicroscopic 
pieces  of  solid  gelatin  occluding  water,  the  relative  quantity  of 
which  is  regulated  by  the  Donnan  equilibrium."  This  view  was 
supported  by  experiments  on  solutions  and  suspensions  of  casein 
chloride  and  gelatin  chloride  in  which  it  was  shown  that  viscosity 
was  due  chiefly  to  the  swelling  of  solid  particles,  occluding  quan- 
tities of  water  regulated  by  the  Donnan  equilibrium,  and  that 
the  breaking  up  of  these  solid  particles  into  smaller  particles,  no 
longer  capable  of  swelling,  diminished  the  viscosity. 

The  idea  is  advanced  that  proteins  form  true  solutions  in  water 
which  in  certain  instances  contain,  side  by  side  with  isolated  ions 
and  molecules,  submicroscopic  solid  particles  capable  of  occlud- 
ing water  whereby  the  relative  volume  and  the  viscosity  of  the 
solution  is  considerably  increased.  This  seems  to  account  for 
the  high  order  of  magnitude  of  the  viscosity  of  such  protein 
solutions,  and  also  for  the  similarity  of  the  influence  of  electro- 
lytes upon  the  viscosity  and  the  swelling  of  protein  particles. 
That  type  of  viscosity  which  is  due  to  the  isolated  ions  and 
molecules  is  of  a  low  order  of  magnitude,  as  that  of  crystalloids 
in  solution,  and  this  seems  to  predominate  in  solutions  of  crystal- 
line egg  albumin  and  in  metal  caseinates,  while  that  viscosity 
which  is  due  to  the  submicroscopic  solid  particles  is  very  high, 
and  predominates  -  in  solutions  of  gelatin  and  acid  salts  of 
casein. 

The  typical  influence  of  electrolytes  on  the  osmotic  pressure 
of  protein  solutions  is  explained  as  due  to  the  isolated  protein 
ions,  since  these  alone  are  capable  of  causing  a  Donnan  equilib- 
rium across  a  membrane  impermeable  to  the  protein  ions  but 
permeable  to  crystalloid  ions.  The  effect  of  electrolytes  on 
the  viscosity  of  protein  solutions  is,  on  the  other  hand,  due  to  the 
submicroscopic  solid  particles  with  their  occluded  water,  for  the 


PHYSICO-CHEMICAL  PROPERTIES  OF  GELATIN         159 

amount  of  water  occluded  by  them  (the  swelling)  is  also  regulated 
by  the  Donnan  equilibrium. 

Substances  such  as  glycine  or  crystalline  egg  albumin  should 
not  be  expected  to  show  variations  in  degree  of  hydration  with 
changes  in  pH  of  a  nature  parallel  to  those  variations  in  hydration 
resulting  from  similar  changes  in  pH  of  gelatin  solutions.  The 
author1  has  already  shown  many  differences  in  fundamental 
properties  between  true  gelatin,  proteose  and  peptone.  Thus 
the  swelling,  viscosity,  and  power  of  gelation  vary  directly  as  the 
gelatin  content  (of  a  commercial  gelatin  or  glue),  and  the  size 
of  the  gelatin  aggregate,  and  further  evidence2  has  been  given 
that  these  variables  are  controlled  to  a  large  extent  by  the  degree 
of  hydration  or  imbibition. 

In  order  to  understand  the  importance  which  Loeb  attaches 
to  his  argument  against  the  hydration  theory,  it  is  necessary 
to  emphasize  that  the  term  hydration  was  used  in  a  very  specific 
sense.  By  it  Loeb  referred  exclusively  to  the  hydration  con- 
cept postulated  by  Kohlrausch  and  extended  by  Pauli.3  Accord- 
ing to  this  conception  each  individual  protein  ion  is  surrounded 
by  an  enormous  shell  of  water  molecules,  while  the  non-ionized 
molecule  of  protein  has  no  or  little  of  such  a  shell.  If  this  theory 
were  correct,  the  variations  in  swelling,  viscosity,  osmotic  pres- 
sure, etc.,  should  follow  the  variations  in  degree  of  ionization 
of  the  protein.  But  Loeb  has  shown  by  conductivity  measure- 
ments that  this  is  not  the  case. 

The  sense  in  which  the  term  hydration  has  been  used  by  the 
author  is  that  adopted  by  Wo.  Ostwald  and  Martin  Fischer  to 
signify  only  the  taking  up  of  water  by  the  protein  ions,  molecules, 
or  particles,  and  without  any  necessary  implication  upon  the 
mechanism  of  such  combination.  The  study  on  the  sol-gel 
equilibrium  outlined  above  has  concerned  itself  with  an  explana- 
tion of  certain  characteristic  phenomena  which  has  necessitated 
the  postulation  of  hydration  in  the  above  sense.  Loeb  has 
however  confined  his  argument  for  the  most  part  to  a  considera- 
tion of  the  intermolecular  mechanism  by  which  such  combinations 
with  water  may  be  most  satisfactorily  accounted  for.  The  two 
points  of  view  are  in  no  sense  contradictory. 

1 R.  H.  BOGUE,  loc.  cit.,  105. 

2  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  43  (1921),  1764);  /.  Ind.  Eng.  Chem., 
14  (1922),  32. 

3  Personal  communication  from  Jacques  Loeb. 


CHAPTER  IV 
GELATIN  AS  A  LYOPHILIC  COLLOID 

Since  the  birth  of  the  Classical  physical 
chemistry  of  molecular  solutions  no  branch 
of  physics  or  chemistry  has  arisen  which  can 
be  compared  in  importance  with  that  of  col- 
loid chemistry.  Wo.  Ostwald.  (1917). 

PAGE 

1.  The  Colloid  Conception 160 

2.  The  Swelling,  Solution,  and  Gelation  of  Gelatin 164 

Physical  Equilibria 164 

Osmotic  Phenomena  in  Swelling 171 

Chemical  Phenomena  in  Swelling 173 

Influence  of  Salts  upon  Swelling  and  Solution 182 

3.  The  Viscosity  of  Gelatin 186 

General  Considerations 186 

Conditions  Affecting  Viscosity 187 

The  Theories  of  Viscosity 200 

The  Viscosity-Plasticity  Relationship 207 

4.  The  Theory  of  Emulsions 212 

I.  THE  COLLOID  CONCEPTION 

The  conception  of  the  term  colloid  was  first  introduced  into 
scientific  literature  by  Thomas  Graham1  in  1861.  This  illustrious 
English  chemist  first  pointed  out  that  while  ordinary  (inorganic) 
salts  (in  solution)  would  pass  easily  through  parchment  or 
animal  membranes,  many  organic  compounds  refused  to  do  so. 

As  the  substances  which  dialyzed  through  were  quite  generally 
of  the  crystallizable  type,  and  those  which  were  retained  were 
not  crystallizable,  he  called  the  former  crystalloids,  and  the 
latter  colloids,  from  colla,  meaning  glue.  Although  this  distinc- 
tion was  held  to  obtain  for  many  years,  yet  today  a  somewhat 
different  conception  of  the  terms  is  understood.  Some  sub- 
stances which  are  not  crystallizable,  are  found  to  pass  through 
certain  membranes,  and,  on  the  other  hand,  many  substances 
which  do  not  dialyze  have  been  in  recent  years  prepared  in  the 
crystalline  state.  Again,  it  was  long  believed  that  the  property 
of  dialysis  was  characteristic  of  the  composition  of  the  particular 

1  THOMAS  GRAHAM,  Trans.  Roy.  Soc.  London,  1861-1864. 

160 


GELATIN  AS  A  LYOPHILIC  COLLOID  161 

substance,  but  the  classical  researches  of  von  Weimarn,1  and 
Wo.  Ostwald2  have  demonstrated  that  as  a  matter  of  fact  nearly 
every  substance  may  be  prepared  in  the  colloid  state.  Ostwald 
and  von  Weimarn  regard  the  colloid  condition  as  a  universal 
state  of  matter.  Even  such  a  simple  inorganic  crystalloid  as 
sodium  chloride  may  easily  be  obtained  in  a  colloid  condition. 

Zsigmondy3  has  shown  that  nearly  all  solutions  which  exhibit 
the  characteristic  properties  of  colloids,  such  as  non-diffusibility, 
non-dialyzability,  high  viscosity,  etc.,  also  reveal  under  the 
ultramicroscope  the  presence  of  a  large  number  of  particles  that 
are  too  small  to  be  seen  by  the  ordinary  microscope,  but  yet  are 
vastly  larger  than  the  ordinary  molecules.  These  he  called 
amicrons.  As  soon  as  the  particles  become  microscopic  in  size, 
the  " solution"  exhibits  the  properties  of  an  ordinary  suspension 
or  emulsion,  the  particles  gradually  settle  out,  and  the  typical 
colloid  characteristics  are  lost.  Likewise,  when  the  particles 
attain  molecular  dimensions,  the  solution  again  loses  the  special 
properties  by  virtue  of  which  it  has  been  classified  as  colloidal. 

The  question  naturally  arises :  why  should  a  solution  in  which 
are  dispersed  particles  of  a  size,  between  molecular  and  micro- 
scopic exhibit  properties  so  vastly  different  than  other  solutions? 
The  explanation  lies  probably  in  the  surface  energies  involved. 
For  example,  one  solid  cubic  centimeter  has  a  total  surface  of  6 
sq.  cm.  If  this  is  subdivided  into  cubic  millimeters  the  number 
of  cubes  will  be  1,000,  and  the  total  surface  will  be  60  sq.  cm. 
On  repeating  the  process,  if  the  length  of  an  edge  of  the  cube  is 
made  one  micron  GHbOOO  mm.,  or  lju)  the  number  of  cubes  is 
increased  to  1012  (one  trillion),  and  the  total  surface  is  6  sq.  m. 
A  length  of  edge  of  a  milli-micron  (;K>ooo  micron  or  1/i/x) 
results  in  1021  cubes,  and  a  total  surface  of  6,000  sq.  m. 

It  is  well  known  that  the  surface  of  bodies,  even  of  ordinary 
dimensions,  exhibits  many  properties  and  peculiarities  which  are 
not  observed  at  interior  points.  Surface  condensation,  surface 
potential,  surface  tension,  and  the  like  are  common  manifesta- 
tions with  which  we  are  familiar.  It  is  not  unreasonable  there- 
fore to  expect  that  where  the  surface  is  increased  to  such  vast 


:VON  WEIMARN,  "Grundzuge  der  Dispersoidchemie,"  Dresden  (1911). 

2  Wo.    OSTWALD — FISCHER,     "A    Handbook    of    Colloid    Chemistry," 
Philadelphia  (1919). 

3  ZSIGMONDY— SPEAR,  "The  Chemistry  of  Colloids,"  New  York  (1917). 

11 


162 


GELATIN  AND  GLUE 


proportions  as  obtains  in  colloidal  solutions,  the  properties  of 
such  systems  should  also  be  fundamentally  different. 

The  actual  size  of  particle  which  has  been  found  to  be  charac- 
teristic of  the  colloid  state  has  been  stated  by  Wo.  Ostwald1  to 
lie  between  1  and  100/u/*.  He  classifies  dispersed  systems  accord- 
ing to  the  following  scheme: 

DISPERSED  SYSTEMS 


Coarse  dispersions 

Colloids 

Molecular  dispersoids 

Greater  than  10(W  in 
size.  Do  not  pass 
through  filter  paper. 
Microscopically  ana- 
lyzable. 

100     to     1.0/x/z    in  size. 
Pass  through  filter  pa- 
per.    Cannot  be  ana- 
lyzed   microscopically. 
Generally  observed  in 
ultramicroscope.       Do 
not  diffuse  or  dialyze. 

Less  than  l.Ojuju  in  size 
Pass  through  filter 
paper.  Cannot  be  ana- 
lyzed microscopically, 
nor  observed  in  ultra- 
microscope.  D  i  ff  u  s  e 
and  dialyze. 

A  colloidal  dispersion  of  a  substance  (the  dispersed  phase)  in  a 
liquid  (the  dispersion  medium)  which  appears  like  a  homogeneous 
solution,  but  has  the  properties  of  a  colloid,  was  called  by  Graham 
a  colloid  sol.  He  gave  the  name  gel  to  the  dispersion  which  was 
obtained  from  a  sol  by  so  changing  the  degree  of  dispersion,  i.e., 
the  size  of  the  dispersed  particles,  that  a  precipitation  or  coagula- 
tion resulted.  For  example,  on  adding  a  salt  solution  to  an 
arsenic  sulphide  sol,  an  immediate  precipitation  of  ordinary  arsenic 
sulphide  took  place,  and  this  substance  he  referred  to  as  the  gel. 
A  similar  addition  to  a  sodium  silicate  sol  resulted  in  the  gelatini- 
zation  of  the  material,  and  this  jelly  was  also  known  as  a  gel. 
There  is  still  some  difference  of  opinion  as  to  whether  the  term 
gel  should  be  applied  to  the  jelly  resulting  from  the  gelatinization 
of  reversible  colloids,  such  as  gelatin,  which  is  brought  about,  for 
example,  by  cooling  a  gelatin  sol.  The  tendency  however  is  to 
apply  the  term  to  both  cases,  and  it  is  so  used  throughout  this 
book. 

Gelatin  water  systems  are  spoken  of  as  reversible  colloid  systems 
because  they  may  be  repeatedly  changed  from  the  sol  to  the  gel 
state,  and  vice  versa,  by  merely  changing  the  temperature  or 


1  Wo.  OSTWALD — FISCHER,  "Theoretical  and  Applied  Colloid  Chemistry," 
N.  Y.  (1917),  20. 


GELATIN  AS  A  LYOPHILIC  COLLOID  163 

other  agency  which  brought  about  the  change.  An  irreversible 
colloid  system  may  not  be  brought  back  to  its  original  condition 
by  any  ordinary  reversal  of  environment. 

A  further  subdivision  of  colloid  systems  is  made  depending 
upon  the  physical  state  of  the  particles  which  constitute  the 
dispersed  phase.  If  these  consist  of  liquid  particles  dispersed 
in  a  liquid,  Wo.  Ostwald1  has  applied  the  term  emulsoid,  while 
if  the  dispersed  phase  consists  of  solid  particles,  likewise  dispersed 
in  a  liquid  medium,  the  term  suspensoid  is  used.  Many  other 
designations  have  been  given  to  these  two  distinctly  different 
types  of  colloids.  Henri2  speaks  of  the  emulsoid  type  as  stabile 
and  the  suspensoid  type  as  instabile,  on  account  of  the  ease  with 
which  the  latter  type  is  precipitated  with  electrolytes.  A.  A. 
Noyes3  calls  them  colloidal  solutions  and  colloidal  suspensions 
respectively.  Perrin4  first  used  the  terms  hydrophilic  and 
hydrophobic,  which  have  been  largely  substituted  by  the  more 
expressive  terms  lyophilic  and  lyophobic,  suggested  by  Freundlich 
and  Neumann.5  The  lyophilic  colloids  are,  according  to  Noyes, 
"viscous,  gelatinizing,  colloidal  mixtures,  not  (easily)  coagulated 
by  salts,"  and  the  lyophobic  colloids  as  "  non-gelatinizing,  but 
easily  coagulable  mixtures."  Martin  Fischer6  carries  the  dis- 
tinction further.  He  considers  that  "a  suspension  colloid 
(hydrophobic  or  lyophobic)  results  whenever  the  colloidally 
dispersed  phase  is  not  a  solvent  for  the  'dispersing  medium';  a 
hydrophilic  or  lyophilic  colloid  whenever  the  dispersed  phase  is 
such  a  solvent  (and  independently  of  the  fact  that  the  subdivided 
phase  is  solid,  liquid  or  gaseous  at  the  temperature  employed)." 
Bancroft7  has  decided  that  the  expressions  hydrophilic  and  hydro- 
phobic  became  meaningless  when  the  original  distinction  between 
the  suspensoid  and  emulsoid  colloids  was  lost,  and  in  charac- 
teristic Bancroft  style  remarks  that "  it  seems  foolish  to  invent  new 
words  when  we  have  two  perfectly  good  ones  with  no  meanings 
attached  to  them,"  and  suggests  "that  hydrophile  be  used  to 
designate  colloidal  solutions  in  water,  and  hydrophobe  for 
colloidal  solutions  in  non-aqueous  solutions." 

1  Wo.  OSTWALD,  op,  cit. 

2  HENRI,  Z.  physik.  Chem.,  61  (1905),  29. 

3  A.  A.  NOYES,  /.  Am.  Chem.  Soc.,  27  (1905),  85. 

4  PERRIN,  /.  Phys.  Chem.,  3  (1905),  50. 

5  FREUNDLICH  and  NEUMANN,  Kolloid-Z.,  3  (1908),  80. 

6  MARTIN  FISCHER,  Science,  N.  S.,  49  (1919),  615. 

7  W.  D.  BANCROFT,  /.  Phys.  Chem.,  19  (1915),  275. 


164  GELATIN  AND  GLUE 

2.   THE  SWELLING,  SOLUTION,  AND  GELATION  OF  GELATIN 

Physical  Equilibria. — When  gelatin  is  allowed  to  remain  in 
cold  water  it  takes  up  many  times  its  original  volume  of  the 
water,  becomes  rubbery  and  jelly-like,  but  rigidly  retains  its 
shape,  and  only  traces  of  the  gelatin  pass  into  solution.  The 
presence  of  electrolytes  in  the  solution  greatly  modifies  the 
amount  of  water  which  will  be  imbibed  by  the  gelatin,  somo, 
increasing,  others  lessening  the  degree  of  swelling.  On  warming 
the  swollen  gelatin  and  water  the  amount  of  gelatin  entering 
solution  increases  but  very  slightly  until  a  particular  temperature 
is  reached  at  which  the  mass  loses  its  rigidity  of  form  and  enters 
into  an  apparently  homogeneous  phase  with  the  solvent.  This 
temperature  is  commonly  known  as  the  melting  point  of  the  jelly. 

Volume,  Pressure,  and  Heat  Effects  of  Swelling. — The  mechan- 
ism of  this  rather  remarkable  property  of  swelling;  the^ 
fundamental  processes  underlying  and  involved  in  the  phenome- 
non; and  the  many  conditions  affecting  the  process,  have  been  the 
subject  of  a  large  number  of  investigations.  As  far  back  as 
1870  Quincke1  showed  that  the  swelling  of  gelatin  was  not  an 
altogether  simple  absorption,  for  he,  observed  that  the  process 
involved  a  volume  contraction.  The  volume  of  the  swollen 
jelly  was  not  as  great  as  the  sum  of  the  volumes  of  the  unswollen 
gelatin  and  the  water  taken  up. 

Hatschek2  has  described  an  experiment  which  strikingly  demonstrates 
this  volume  contraction.  One  gram  of  gelatin  is  placed  in  a  pycnometer, 
the  latter  then  filled  with  water  and  weighed.  It  is  placed  in  water,  left 
until  the  gelatin  has  become  fully  swollen,  then  taken  out  and  again  weighed. 
The  increase  in  weight  represents  directly  the  amount  of  contraction  of  the 
original  system:  e.g.,  the  amount  of  water  that  has  entered  the  pycnometer 
owing  to  the  contraction  of  the  gelatin  plus  imbibed  water.  In  one  experi- 
ment Hatschek  found  a  volume  contraction  of  nearly  2  per  cent  of  the 
original  volume.  If  the  same  effect  were  to  be  obtained  by  mechanical 
compression  of  the  water,  a  pressure  of  about  400  atmospheres  would  be 
required. 

Since  the  compression  of  a  liquid  involves  the  liberation  of 
heat,  it  becomes  obvious  that  the  swelling  of  gelatin  must  be, 
a  priori,  an  exothermic  process.  That  heat  is  indeed  liberated 
was  first  shown  by  Wiedemann  and  Ltideking3  in  1885  who 

1  QUINCKE,  Arch.  Ges.  Physiol.,  3  (1870),  332. 

2  E.  HATSCHEK,  "An  Introduction  to  the  Physics  and  Chemistry  of  Col- 
loids," London  (1913),  55. 

3  WIEDEMANN  and  LUDEKING,  Ann.  Physik.  Chem.,  25  (1885),  145. 


GELATIN  AS  A  LYOPHILIC  COLLOID 


165 


reported  the  following  amount  of  heat  in  gram-calories  liberated 
per  gram  of  substance: 

TABLE  34. — LIBERATION  OF  HEAT  ON  THE  SWELLING  OF  GELS 


Gel 


Gram-calories  per  gram  of  gel 


Gelatin 

Starch 

Gum  arabic 

Gum  tragacanth 


5.7 

6.6 

9.0 

10.3 


Further  evidence  that  the  swelling  of  gels  is  more  than  the 
mere  taking  up  of  water  within  the  pores  or  around  the  molecules, 
as  a  sponge  takes  up  water,  is  shown  very  strikingly  in  an  experi- 
ment by  Reinke.1  He  confined  circular  disks  of  the  foliage  of 
Laminaria,  a  seaweed,  in  a  cylinder,  and  placed  above  this  a 
weighted  piston  containing  numerous  small  perforations  through 
which  the  water  reached  the  gel.  fThe  following  table  shows  the 
pressure  on  the  piston  in  atmospheres  (kg.  per  sq.  cm.)  and  the 
accompanying  increase  in  volume  of  the  gel  at  that  pressure. 
Data  obtained  by  Posnjak2  are  included  in  the  second  portion 
of  the  table. 


TABLE  35. — EFFECT  OF  PRESSURE  ON  THE  SWELLING  OF  GELS 


Pressure  in  atm. 

Percentage  in- 
crease in  volume 

Pressure  in  cm. 
mercury 

Grams  water  per 
gram  gelatin 

41.2 

16 

38.3 

2.56 

31.2 

23 

82.4 

2.06 

21.2 
11.2 

35 

1       89 

156.0 
240.0 

1.46 
1.28 

7.2 

97 

303.0 

1.10 

3.2 

205 

377.0 

0.92 

1.2 

318 

1.0 

330 

1  Described  by  HATSCHEK,  lib.  cit.,  56. 

2  E.  POSNJAK,  Kolloidchem.  Beihefte,  3  (1911),  417. 


166  GELATIN  AND  GLUE 

That  the  gel  increases  in  volume  330  per  cent  at  atmospheric 
pressure  seems  remarkable,  but  much  more  extraordinary  is  the 
observation  that  even  at  the  high  pressure  of  over  41  atmospheres 
the  gel  still  absorbed  water  to  produce  an  increase  in  volume  of 
16  per  cent.  This  makes  it  clear  that  a  swollen  jelly,  unlike  a 
sponge,  may  not  be  made  to  give  up  its  imbibed  water  by 
moderate  pressure  alone. 

Velocity  of  Swelling. — The  time  relations  of  swelling  phe- 
nomena were  early  studied  by  Hofmeister.1  He  found  that  the 
maximum  velocity  of  swelling  was  attained  immediately  on 
immersion  of  the  gel,  and  decreased  regularly  as  swelling  pro- 
ceeded. This  action  is  in  effect  an  application  of  the  law  of  mass 
action  of  Guldberg  and  Waage.2  In  order  to  express  the  facts 
mathematically  Hofmeister  developed  the  following  equation: 


W  = 


in  which  W  is  the  weight  of  water  absorbed  by  unit  weight  of 
gel  in  time  t,  P  is  the  maximum  amount  of  water  which  the  unit 
weight  of  gel  will  imbibe,  c  is  a  constant,  and  d  is  th'e  thickness  in 
millimeters  of  the  plate  at  its  maximal  degree  of  swelling.  Thus 
the  greater  the  value  of  P,  the  greater  the  velocity  of  the  swelling 
at  any  moment. 

An  investigation  conducted  by  Pauli3  led  him  to  the  adoption 
of  a  slightly  different  formula.  He  found  evidence  that  the 
swelling  of  a  given  particle  was  dependent,  not  only  on  the  free 
water  present,  but  was  also  influenced  by  the  water  content  of 
each  neighboring  particle.  He  wrote  the  equation: 

1  M  -  Q 

x^r^T^w^Q.' 

in  which  K  is  9,  constant  which  varies  inversely  with  the  thickness 
of  the  plate,  Q  is  the  quantity  of  water  taken  up  by  unit  weight 
of  gelatin  in  time  t,  Qi  the  quantity  of  water  taken  up  by  unit 
weight  of  gelatin  in  time  ti,  and  M  is  the  maximal  degree  of 
swelling  attained.  This  equation  expresses  the  velocity  of 

1  HOFMEISTER,  Arch,  exptl.  Path.  Pharm.,  27  (1890),  395;  28  (1891),  210. 

2  Cf.  NERNST,  "Theoretical  Chemistry,"  6th  ed.,  London  (1911),  445. 

3  PAULI,  Arch,  exptl.  Path.  Pharm.,  36  (1895),  100;  57  (1897),  219;  71 
(1898),  333. 


GELATIN  AS  A  LYOPHILIC  COLLOID  167 

swelling  as  directly  proportional  at  any  instant  to  the  swelling 
which  it  must  yet  undergo  to  obtain  its  maximum.  Hofmeister's 
formula  considers  the  velocity  of  swelling  at  any  moment  as 
proportional  to  the  square  of  the  swelling  which  it  has  yet  to 
undergo. 

Hofmeister  further  found  that  after  the  attainment  of  the 
maximum  swelling  the  outer  layers  of  the  gelatin  passed  slowly 
into  solution,  and  the  more  readily  the  larger  the  amount  of 
acid  or  alkali  present  in  the  watery  solvent. 

Equilibrium  between  the  Liquid  and  Vapor  Phases. — vonSchroed- 
er1  has  reported  that  gelatin  in  equilibrium  with  saturated  water 
vapor  will  still  take  up  large  amounts  of  water  and  become  much 
more  highly  distended  if  placed  in  liquid  water  at  the  same  tem- 
perature.^ He  found  a  plate  of  gelatin  weighing  0.904  grams  to 
absorb  only  0.37  g.  of  water  upon  remaining  in  an  atmosphere  of 
saturated  water  vapor  for  eight  days,  at  the  end  of  which  time 
the  weight  remained  constant.  But  on  placing  the  plate  in 
water  at  the  same  temperature  5.63  g.  of  water  were  further 
absorbed  in  one  hour.  Conversely,  gelatin  that  has  been  brought 
into  equilibrium  with  liquid  water* was  found  to  give  up  water 
when  transferred  to  an  atmosphere  of  saturated  vapor. 

Wolf  and  Buchner2  repeated  the  investigation  and  took  especial 
care  upon  the  elimination  of  possible  fluctuations  in  temperature. 
They  employed  vessels  silvered  on  their  internal  surfaces,  and 
a  thermostat  of  high  reliability.  It  was  found  possible  by  them 
to  transfer  gelatin  in  equilibrium  with  liquid  water  into  the 
saturated  vapor  phase  without  a  subsequent  loss  in  weight 
occurring.  But  upon  even  the  slightest  fluctuation  in  tempera- 
ture within  the  apparatus,  loss  in  weight  of  the  swollen  gel  took 
pl^ce.  But  it  was  observed  that  an  entirely  similar  loss  in 
weight  occurred  from  open  dishes  of  pure  water  placed  beside 
the  gelatin.  The  explanation  advanced  was  that  a  distillation 
of  the  water  from  the  gejatin  or  dishes  of  water  on  to  the  walls  of 
the  containing  vessel  occurred  due  to  a  temperature  gradient. 

Miss  Lloyd3  has  pointed  out,  however,  that,  in  spite  of  the 
reliability  of  the  results  of  Wolff  and  Buchner,  in  systems  con- 
sisting of  acid  or  alkaline  gelatin  two  separate  equilibria  do 
actually  occur  in  the  fluid  and  gaseous  media,  but  that  the 

1  VON  SCHROEDER,  Z.  physik.  Chem.,  46  (1903),  74;  109. 

2  WOLFF  and  BUCHNER,  Z.  physik.  Chem.,  89  (1915),  271. 

3  D.  J.  LLOYD,  Biochem.  J.,  14  (1920),  156. 


168  GELATIN  AND  GLUE 

existence  of  these  two  points  follows  naturally  from  Donnan's 
membrane  potential  theory,1  and  does  not,  therefore,  form  an 
exception  to  the  second  law  of  thermodynamics.  Miss  Lloyd 
demonstrated  that  both  the  degree  and  the  direction  of  the 
change  in  water  absorption  of  a  gelatin  gel  upon  transferring  from 
a  liquid  to  a  vapor  phase  could  be  controlled  by  the  pH  of  the 
solution.  That  is,  it  was  found  possible  by  a  variation  in  the 
reaction  to  determine  at  will  whether  a  gel  should  lose  weight, 
gain  weight,  or  remain  constant  upon  being  transferred  from  the 
liquid  to  the  saturated  vapor.  At  concentrations  of  N/20  or 
higher  of  hydrochloric  acid  or  sodium  hydroxide  the  gel  was 
found  to  gain  in  weight.  At  a  concentration  of  N/200  it  lost 
weight  upon  being  transferred  to  the  saturated  vapor,  and  the 
loss  was  at  its  maximum  at  this  concentration.  Upon  further 
decreasing  the  normality  of  either  acid  or  base  the  losses  became 
less,  and  at  the  value  of  water  remained  practically  unchanged. 
The  loss  occurring  from  the  alkaline  gelatins  was  much  greater 
(at  the  same  concentrations)  than  that  from  the  acid  gelatins. 

Donnan  has  shown  that  if  a  system  containing  one  ion  that 
will  not  pass  through  a  given  membrane  be  separated  by  such  a 
membrane  from  another  system  all  of  whose  ions  are  permeative, 
the  concentration  of  the  permeative  ions  will  differ,  at  equilib- 
rium, on  the  two  sides  of  the  membrane,  it  being  the  higher 
on  the  side  free  from  the  impermeative  ion.  Miss  Lloyd  writes 
the  gelatin  hydrochloric  acid  equilibrium,  when  gelatin  chloride 
is  separated  from  hydrochloric  acid  by  a  membrane  impermeable 
to  gelatin,  but  permeable  to  the  other  ions : 


G  +  Cl  +  H  +  Cl 


V.V1 


I. 


H  +  C1 

C3      C3 

II. 


where  C2  and  C3  represent  the  concentrations  of  ionized  hydro- 
chloric acid  in  the  jelly  and  liquid  phases  respectively,  and  Ci 
the  concentration  of  the  gelatin  ion.  Assuming  complete  ioniza- 
tion,  C3  must  always  be  greater  than  C2. owing  to  the  electrostatic 
repulsion  between  non-diffusible  G  and  diffusible  H.  If,  now, 
the  acid  solution  of  phase  II  is  replaced  by  saturated  water  vapor 
the  system  becomes  unstable,  and  H  and  Cl  ions  must  be  trans- 
ferred, together  with  liquid  water,  across  the  membrane.  This 
is  sufficient  to  account  for  the  losses  in  weight  of  water  observed. 
1  F,  DONNAN,  Z.  Electrochem.,  17  (1911),  572.  See  also  page  128. 


GELATIN  AS  A  LYOPHILIC  COLLOID  169 

The  Rate  of  Solution.  —  The  temperature  of  the  liquid  also  has 
an  effect  upon  the  amount  of  water  taken  up  in  a  given  period  of 
time.  Reasoning  from  the  principle  enunciated  by  LeChatelier, 
inasmuch  as  the  swelling  process  involves  a  liberation  of  heat, 
the  more  rapidly  the  heat  produced  is  carried  away,  the  more 
rapid  will  be  the  process  of  water  absorption.  In  other  words,  a 
low  temperature  will  favor,  and  a  high  temperature  depress,  the 
rate  of  water  absorption  or  swelling.1  It  is  well  known  that  in 
order  to  get  glue  or  gelatin  into  solution  it  is  very  desirable  that 
the  substance  be  first  soaked  in  cold  water  for  several  hours,  or 
until  it  is  well  swollen,  and  then  warmed.  If  the  glue  be  put 
directly  into  hot  water  the  amount  and  rate  of  swelling  will  be, 
according  t6  the  above  theory,  greatly  decreased,  and  solution 
must  therefore  take  place  from  the  outer  surface  only,  instead 
of  from  the  enormous  surface  produced  by  the  network  of  minute 
capillary  spaces  which  are  assumed  to  exist  in  the  swollen 
substance. 

The  conditions  governing  the  rate,  of  solution  of  crystalloids 
have  been  studied  by  Noyes  and  Whitney.2  They  find  that  when 
no  chemical  reaction  other  than  a  possible  formation  of  "solvates" 
is  involved,  the  rate  of  solution  in  water  at  any  given  moment  is 
proportional  to  the  difference  between  its  concentration  at  that 
moment  and  its  concentration  at  saturation.  They  explain  this 
by  supposing  that  at  the  crystal-solvent  interface  there  exists  a 
film  of  the  saturated  solution,  and  the  thickness  of  this  film  is 
dependent  on  the  rate  of  stirring  of  the  mixture.  Thus  the  rate 
with  which  the  constituents  of  this  saturated  film  diffuse  into 
the  outer  liquid  determines  the  velocity  of  solution,  or  expressed 
mathematically: 


where  a  is  the  concentration  of  a  saturated  solution,  x  the  con- 
centration of  the  solution  at  any  moment  t,  s  the  area  of  the 
surface  of  the  solute,  and  k  a  constant  which  varies  with  the 
rate  of  diffusibility  of  the  dissolved  molecules:  e.g.,  with  the  rate 
of  stirring  and  the  temperature. 

1  First  pointed  out  by  KORNER,  "  Beitrage  zur  wissenschaftlichen  Grund- 
lage  der  Gerberei,"  Freiberg  (1899). 

2  NOTES  and  WHITNEY,  Z.  physik.  Chem.,  23  (1897),  689. 


170  GELATIN  AND  GLUE 

Robertson1  concludes  from  a  study  of  casein  that  the  factor 
which  determines  the  rate  of  solution  of  that  protein  is  the 
velocity  with  which  it  is  wetted  by  the  solvent.  But  a  crystal  is 
wetted  only  on  its  external  surface,  and  this,  inasmuch  as  it 
takes  place  instantly,  becomes  of  practically  negligible  impor- 
tance in  comparison  with  the  rate  of  diffusion  of  the  dissolved 
molecules.  But  proteins  contain  a  multitude  of  capillary  spaces 
which  are  not  wetted  instantaneously,  and  much  time  is  required 
for  the  water  to  reach  every  portion  of  the  internal  surface. 
Furthermore  diffusion  from  these  capillary  spaces  of  dissolved 
molecules  must  be  vastly  slower  than  from  the  external  surface. 
Thus  the  rate  of  solution  of  such  a  material  will  be  proportional 
to  the  rate  of  penetration  of  the  solvent,  and  the  rate  of  diffusion 
of  the  dissolved  molecules  from  the  interior  to  the  exterior  of  the 
mass.  Robertson  and  Miyake2  have  shown  that  solution  of 
casein  in  alkaline  solutions  takes  place  according  to  the  equation : 

x  =  Ktm, 

in  which  x  is  the  amount  dissolved  in  time  t,  and  K  and  m  are 
constants  which  vary  with  the  nature  and  concentration  of  the 
alkali  and  the  mass  of  casein  used.  Differentiating  we  obtain: 

%  =  Kmt-i. 
at 

The  product  Km,  termed  by  Robertson  and  Miyake  the  coefficient 
of  penetration,  expresses  the  constant  proportionality  between 
the  velocity  of  solution  and  an  exponent,  characteristic  of  each 
solvent,  of  the  time  of  exposure  of  the  protein  to  the  solvent. 

It  will  be  obvious  from  the  foregoing  that  if  a  piece  of  dry 
gelatin  or  glue  be  placed  in  hot  water,  swelling  will  be  to  a 
great  extent  inhibited,  and  solution  will  take  place  from  the 
outer  surface  only,  while  if  the  gelatin  be  first  swollen  in  cold 
water,  and  then  placed  in  hot  water,  solution  will  take  place 
from  the  entire  inner  as  well  as  outer  surface,  and  therefore  be 
rapid. 

The   conception  of  a  structure3  of  some  kind  seems  to  be 

*  l  T.  B.  ROBERTSON,  "Physical  Chemistry  of  the  Proteins,"  New  York 
(1918),  286. 

2  ROBERTSON  and  MIYAKE,  «/.  Biol.  Chem.,  25  (1916),  351;  26  (1916),  126. 
Vide  also  ROBERTSON,  lib.  cit.,  280. 
page'136. 


GELATIN  AS  A  LYOPHILIC  COLLOID  171 

essential  to  account  for  the  above  action.  In  the  dry  gelatin 
the  structure  exists  apparently,  but  the  walls  are  collapsed  one 
upon  another  into  a  rigid  and  but  slightly  porous  mass.  Cold 
water  penetrates  these  walls  and  distends  the  cells,  and  upon 
warming,  solution  takes  place  from  interior  as  well  as  exterior. 
Hot  water  added  directly  before  swelling  could  effect  solution 
only  by  the  slow  process  of  dissolving  the  outermost  layers 
before  the  underlying  ones  could  be  affected. 

The  degree  of  swelling  which  any  given  gelatin  may  undergo 
is  dependent  upon  the  previous  history  of  the  sample.  If  the 
gelatin  has  been  made  up  in  concentration  of  10,  20  and  30  per 
cent  and  allowed  to  dry  out  to  a  uniform  concentration  of  say  90 
per  cent,  it  is  found  that  the  most  water  will  be  reabsorbed  by  the 
sample  made  from  the  lowest  concentration,  and  the  least  by 
the  sample  made  from  the  highest  concentration  of  gelatin.  If  the 
samples  made  from  the  10  and  from  the  30  per  cent  solutions 
are  both  allowed  to  absorb  water  until  their  concentration  is  the 
same,  say  30  per  cent,  then  the  two  will  not  act  similarly  when 
allowed  to  remain  longer  in  water,  btit  the  former  will  continue 
to  absorb  water  rapidly,  while  the  latter  will  continue  but  very 
slowly  or  not  at  all.  This  also  is  explainable  by  assuming  a 
Itructure  to  develop  after  the  latest  warming.1  A  gelatin  tends 
so  absorb  water  to  a  dilution  equal  to  that  which  it  had  at  the 
tast  warming,  but  not  very  much  over  this. 

Osmotic  Phenomena  in  Swelling. — It  has  been  definitely 
established  that  gelatin  exerts  a  small  but  appreciable  osmotic 
pressure.2  A  consideration  of  this  fact  leads  to  the  conclusion 
that  osmotic  forces  must  play  a  role  in  the  swelling  of  proteins. 
The  gelatin  surface  acts  as  a  semipermeable  membrane,  admitting 
of  the  passage  of  water  and,  to  a  somewhat  lesser  degree,  of 
electrolytes,  while  depriving  any  colloid  of  this  prerogative. 
Thus  water  and  other  electrolytic  solutions  may  pass  freely 
into  the  gelatin  complex,  but  any  gelatin  which  they  may  dissolve 
will  not  only  fail  to  pass  out,  except  at  the  surface,  but  will  also 
deprive  the  imbibed  solution  of  power  to  again  pass  into  the  sur- 
rounding solvent.  (The  laws  of  osmotic  equilibrium  are  too  well 
known  to  justify  a  discussion  of  them  in  a  book  of  this  nature,  but 
they  may  be  found  in  any  standard  work  on  physical  chemistry.3) 

1  Vide  page  141. 

2  Vide  page  97. 

3  Cf.  texts  on  Physical  Chemistry  by  Nernst,  Walker,  Lewis,  etc. 


172  GELATIN  AND  GLUE 

Procter1  regards  the  situation  somewhat  differently  by  con- 
sidering a  condition  of  equilibrium  to  exist  only  "when  the 
attraction  of  the  water  molecules  for  the  gelatin  is  equal  to 
the  sum  of  the  cohesive  attraction  of  the  gelatin  for  itself  and  the 
internal  attraction  of  the  water  outside."  In  other  words, 
Procter  considers  that  there  are  three  forces  involved:  (1), 
an  attraction  of  water  molecules  for  each  other;  (2),  an  attraction 
of  gelatin  molecules  for  each  other,  which  he  calls  the  cohesion 
of  the  gelatin;  and  (3),  an  attraction  of  water  molecules  for 
gelatin  molecules.  To  these  should  be  added  a  forth  force, 
e.g.,  the  attraction  of  gelatin  molecules  for  water  molecules. 
At  equilibrium  (1)  +  (2)  +±  (3),  and  any  alteration  in  the  system 
gelatin-water  by  which  any  of  these  factors  would  be  changed 
would  of  course  affect  the  equation  in  one  direction  or  the  other. 
For  example,  although  the  swelling  process  is  exothermic  and 
evolves  heat,  yet  the  solution  process,  on  the  other  hand,  is 
endothermic  and  absorbs  heat,  and  is  therefore  favored  by  higher 
temperatures.  That  is,  the  cohesion  of  the  gelatin  molecules 
decreases,  and  the  attraction  of  gelatin  to  water  accordingly 
increases,  with  rise  in  temperature.  Alcohol  however  is  not  a 
solvent  for  gelatin:  that  is,  the  attraction  of  alcohol  to  gelatin 
is  nil,  so  if  gelatin  swollen  with  water  were  put  into  alcohol,  the 
sum  of  the  attraction  of  the  water  and  alcohol  and  the  cohesive- 
ness  of  the  gelatin  would  be  much  greater  than  the  mean  of  the 
attraction  of  the  water  and  of  the  alcohol  for  the  gelatin.  The 
obvious  effect  would  be  therefore: — from  the  failure  of  the  alcohol 
to  pass  into  the  gelatin,  and  the  free  passage  of  water  out  of  the 
same — a  contraction  of  the  swollen  gelatin. 

When  any  gel  is  allowed  to  remain  for  a  number  of  hours  or 
days,  protected  against  infection  with  microorganisms,  and  also 
against  evaporation,  a  separation  into  two  phases  takes  place 
which  phenomenon  was  called  by  Graham  syneresis.  Wo. 
Ostwald2  prefers  to  regard  the  swelling  process  as  the  reverse  of 
syneresis.  He  points  out  that  during  swelling  a  small  amount 
of  the  colloid  dissolves  forming  a  dilute  colloidal  solution, 
while  in  syneresis  a  small  amount  of  the  colloid  is  excreted  into 
a  separate  layer  forming  also  a  dilute  colloidal  solution.  He 

1  PROCTER,  "The  Principles  of  Leather  Manufacture,"  London  &  N.  Y. 
(1903),  82. 

2  OSTWALD  and  FISCHER,  "Theoretical  and  Applied  Colloid  Chemistry," 
N.Y.  (1917),  100. 


GELATIN  AS  A  LYOPHILIC  COLLOID  173 

emphasizes  the  existence  of  structure,  as  reported  by  Blitschli 
and  Quincke,  as  necessary  in  order  that  swelling  and  syneresis 
may  occur,  and  argues  that  swelling  consists,  firstly,  in  an  increase 
in  the  degree  of  dispersion  of  the  gelatin,  and,  secondly,  in  a  solvcir 
tion  of  the  more  highly  dispersed  molecules.  Since  these  two 
effects  tend  to  influence  the  volume  changes  in  opposite  direc- 
tions, the  equilibrium  between  them  determines  the  velocity  and 
degree  of  swelling. 

Smith1  has  performed  experiments  which  lead  him  to  believe 
that  the  swelling  of  gelatin  is  the  result  of  osmotic  pressure 
within  the  jelly,  the  jelly  acting  as  an  ''imperfectly  resisting" 
membrane.  He  finds  that  although  the  osmotic  pressure  at 
the  optimum  concentration  of  univalent  acids  and  bases  is  the 
same,  the  swelling  is,  however,  much  less  in  alkalies  because  of 
the  ''weakened  membrane  effect." 

Chemical  Phenomena  in  Swelling.  —  Jones  and  his  collabo- 
rators2 have  accumulated  an  abundance  of  data  which  point 
to  the  existence  of  solvates  or  hydrates,  oi  both  the  ions  and  the 
molecules  of  various  substances  when  in  solution.  Their  theory 
points  to  a  kind  of  chemical  combination  between  the  elements  of 
water  and  the  ions  or  molecules  in  question.  There  seems  to  be 
an  equilibrium  established  between  the  solvated  and  non- 
solvated  molecules,  and  this  equilibrium  is  altered  by:  (1),  the 
temperature  of  the  solution,  and,  (2),  the  presence  of  other 
substances  which  compete  for  the  water.  Where  proteins  are 
concerned  there  are  several  ways  in  which  the  elements  of 
water  may  combine  to  form  the  solvated  molecules.  The  water 
may  combine  with  the  terminal  —  NH2  or  —  COOH  groups,  or 
with  the  internal  —  NHOC  —  groups,  in  the  latter  case  resulting 
in  a  depolymerization  of  the  molecule.  Robertson  writes  the 
reactions  as  follows:3 

NH.OC.R.NH  xNH.OC.R.NEU 


R  +  H20  =  R< 

XCOOH 

NH.OC.R.NH2  /NH.OC.R.NHgOH 

R  +  H20  =  R( 


\ 


COOH  XCOOH 


1  C.  R.  SMITH,  J.  Am.  Chem.,  Soc.,  43  (1921),  1350. 

2  H.  C.  JONES  and  K.  OTA,  Am:  Chem.  J.,  22  (1899),  5;  JONES  and  UHLER, 
ibid.,  34  (1905),  291;  JONES,  Z.  physik.  Chem.,  74  (1910),  325. 

3  ROBERTSON,  lib.  cit.,  127. 


174  GELATIN  AND  GLUE 

/NH.OC.R.NHgOH  XNH3OOC.R.NH3OH 

+  H20  = 


XCOOH  COOH 


/NHaOOC.R.NH,OH  7NH3OH         yNH3OH 

/  +  H20  =  R/  +    R/ 

XCOOH  XCOOH  XCOOH 


As  evidence  in  favor  of  the  solvate  theory  Pauli1  points  out 
that  when  a  protein  goes  into  solution  heat  is  absorbed,  while  on 
swelling  heat  is  liberated.  He  considers  the  heat  evolved  on 
swelling  to  be  the  result  of  the  chemical  combination  of  the 
water  and  gelatin  molecules — the  solvation — and  the  heat 
absorbed  on  solution  as  a  purely  physical  effect. 

Probably  more  work  has  been  done  upon  the  effect  of  acids  and 
alkalies  on  the  swelling  of  proteins,  than  on  any  other  influence 
attending  this  phenomenon.  Gelatin  possesses  the  very  peculiar 
property  of  being  able  to  absorb  either  acids  or  alkalies  from 
dilute  solutions  (about  tenth  normal)  to  such  a  degree  that  the 
liquid  bathing  the  gelatin  may  be  quite  neutral,  as  far  as  the 
litmus  test  can  reveal.  The  resulting  gelatin  has  moreover 
acquired  the  ability  to  take  up  a  very  much  larger  amount  of 
water  than  it  could  do  before  such  treatment.  When  this 
effect  is  subjected  to  quantitative  methods,2  it  is  found  that  very 
dilute  solutions  of  acids  (less  than  N/256)  tend  rather  to  decrease 
the  swelling  capacity  of  gelatin,  but  from  that  concentration  up  to 
about  normal  the  degree  of  swelling  increases,  at  first  rapidly,  but 
gradually  falling  off  as  the  latter  value  is  approached.  At  still 
higher  concentrations  the  degree  of  swelling  again  becomes 
less.  In  the  presence  of  alkalies  the  swelling  proceeds  regularly 
from  the  neutral  point  until  a  concentration  is  reached  at  which 
the  gelatin  dissolves,  i.e.,  about  normal  at  10°C.  Salts  in  general 
exert  a  depressive  effect  on  swelling,  but  this  is  determined  by  the 
hydrogen  ion  concentration  as  will  be  pointed  out  later. 

The  most  generally  accepted  explanation  of  the  influence  of 
electrolytes  on  swelling  phenomena  is  to  be  found  in  the  com- 
bined effect  of  osmotic  and  chemical  forces.  Hydrochloric 
acid,  for  example,  has  the  ability  to  pass  freely  into  and  out  of  a 
gelatin  gel.  So,  if  gelatin  is  immersed  in  a  dilute  solution  of  this 

1  PAULI,  Arch.  ges.  Physiol,  57  (1897),  219;  71  (1898),  333. 

2  Cf.  especially  the  curves  by  Wo.  OSTWALD,  Pfluger's  Arch.  Physiol., 
108  (1905),  563;  and  by  J.  LOEB,  J.  Gen.  Physiol.,  3  (1920),  253;  254;  256. 


GELATIN  AS  A  LYOPHILIC  COLLOID  175 

acid,  both  the  ions  of  the  acid  and  the  water  will  diffuse  into  the 
gelatin,  and  if  no  other  forces  were  operative  it  would  be  expected 
that  the  concentration  of  the  acid  in  the  water  would  be  the  same 
both  inside  and  outside  of  the  protein.  If  a  gelatin  thus  swollen 
with  acid  were  to  be  placed  in  pure  water,  the  acid  would  diffuse 
out  into  the  water  until  the  acid  concentration  were  again  iden- 
tical in  both  phases,  and  upon  numerous  repetitions  of  this 
process,  or,  which  would  amount  to  the  same  thing,  on  placing 
the  gelatin  in  running  pure  water,  all  of  the  acid  would  even- 
tually have  diffused  out.  This  would  also  apply  equally  to  alkalies. 
But  it  has  been  shown  by  numerous  investigators1  that  gelatin 
is  an  amphoteric  colloid,  that  is,  it  is  capable  of  combination  with 
either  anions  or  cations  depending  upon  the  hydrogen  ion  concen- 
tration of  the  meditim  in  which  it  is  immersed.  For  example,  in 
hydrochloric  acid  solution  gelatin  chloride  will  be  formed  and  the 
gelatin  is  electropositive,  migrating,  in  an  electric  field,  to  the 
cathode.  In  a  solution  of  sodium  hydroxide,  sodium  gelatinate 
will  be  produced,  in  which  the  gelatin  is  electronegative  and,  in  an 
electric  field,  migrates  to  the  anode.  At, the  isoelectric  point,  that 
is  at  the  condition  of  electroneutrality  at  which  the  gelatin  will 
not  migrate  to  either  electrode,  the  gelatin  exists  as  a  perfectly 
neutral  molecule  consisting  of  free  protein,  or,  in  some  excep- 
tional cases,  of  base-protein-acid.  It  has  been  shown  by  Loeb,2 
Michaelis3  and  others  that  the  isoelectric  point  for  gelatin  lies  at  a 
hydrogen  ion  concentration  of  CH  =  2.10  X  10~5,  or  in  Soren- 
sen's  logarithmic  symbol  pH  =  4.7.  This  value,  it  will  be 
observed,  lies  upon  the  acid  side  of  the  neutral  point  of  water 
(CH  =  1  X  iO~7  or  pH  =  7.0).  Loeb  has  furthermore  shown 
that  many  of  the  properties  of  gelatin,  as  the  conductivity, 
osmotic  pressure,  swelling,  alcohol  number,  and  viscosity,  have 
their  minimum  value  at  the  isoelectric  point  and  increase  on 
either  side  of  that  point  with  varying  degrees  of  rapidity  accord- 
ing to  the  charge  on  the  ion,  etc.4  Simply  expressed,  this 
means  that  isoelectric  gelatin — slightly  acid  to  litmus — will 
swell  the  least  of  any,  and  that  the  addition  of  either  acid  or 
alkali,  forming  gelatin  salt  or  metal  gelatinate  respectively,  will 
result  in  an  increase  in  the  swelling. 

\Vide  Chap.  V. 

2  J.  LOEB.,  J.  Gen.  PhysioL,  1  (1918-19),  39;  237;  363;  483;  559. 
8  MICHAELIS,  "Die  Wasserstoffionenkonzentration,"  Berlin  (1914). 
4  Vide  Chap.  V. 


176  GELATIN  AND  GLUE 

It  has  already  been  stated  that  gelatin  immersed  in  dilute 
(tenth  normal)  acid  will  absorb  the  acid  until  the  solution  sur- 
rounding it  is  nearly  neutral.  This  could  not  easily  be  accounted 
for  by  osmotic  forces  alone.  It  might  be  argued  that  the  ions  of 
the  acid  were  condensed  upon  the  surface  of  the  gelatin,  but  the 
evidence  above  referred  to  leaves  little  doubt  that  actual  chem- 
ical combination  has  taken  place.  It  is  true  that  practically  all  of 
the  acid  may  be  removed  by  prolonged  dialysis  in  running  water, 
but  this  could  easily  be  accounted  for  by  assuming  an  hydrolytic 
dissociation  of  the  gelatin  salt.  Any  salt  consisting  of  a  com- 
bination of  strong  and  weak  cation  and  anion  exhibits  the  prop- 
erty of  dissociation  into  its  constituent  acid  and  base,  and  a 
gelatin  salt  may  react  in  a  similar  manner. 

Procter's  Theory  of  Swelling. — The  quantitative  relations  of  the 
acid-gelatin  equilibrium  have  been  investigated  by  Procter  and 
his  collaborators.1  They  find  that  the  amount  of  acid  " bound" 
by  gelatin  at  the  attainment  of  maximal  swelling  is  0.7  to  0.8  X 
10~3  equivalents  per  gram,  when  the  initial  concentration  of 
the  acid  is  between  0.01  and  0.25N.  They  find  that  this  value 
is  practically  the  same  for  all  strong  acids,  but  becomes  smaller 
as  weaker  acids  are  used;  and  that  the  concentration  of  the  acid 
in  the  surrounding  solution  has  but  little  influence  upon  the 
amount  of  acid  chemically  combined.  The  degree  of  swelling 
however  increases  with  increasing  concentration  up  to  a  certain 
point,  and  thereafter  decreases.  The  explanation  advanced  by 
Procter  assumes  the  existence  of  an  osmotic  pressure  within  the 
gelatin  produced  by  the  imprisonment  of  chloride  ions.  He 
considers  that  the  gelatin  will  continue  to  swell  until  the  chloride 
ions  within  the  gelatin  are  in  osmotic  equilibrium  with  those 
in  the  outer  solution.  Increases  in  the  concentration  of  the 
chloride  ions  in  the  outer  solution  will  eventually  however 
exert  a  repressive  action  upon  the  ionization  and  hydrolytic 
dissociation  of  the  gelatin  chloride  which  will  more  than  offset  the 
tendency  to  swell  due  to  a  larger  amount  of  the  gelatin  salt  being 
formed,  and  from  that  point  swelling  will  decrease,  rather  than 
increase,  with  further  additions  of  acid.  In  support  of  this 
theory  Procter  recalls  that  additions  of  sodium  chloride  produce 
decreases  in  the  swelling  in  a  similar  way  to  hydrochloric  acid, 

1  PROCTER,  lib.  cit.,  86;  Collegium,  (1915);  Trans.  Chem.  Soc.,  London,  105 
(1914),  313;  PROCTER  and  BURTON,  J.  Soc.  Chem.  Ind.,  35  (1916),  404; 
PROCTER  and  WILSON,  J.  Chem.  Soc.,  109  (1916),  307. 


GELATIN  AS  A  LYOPHILIC  COLLOID  177 

and  argues  that  the  effect  is  in  accordance  to  the  well  known 
mass  law,  the  excess  of  chloride  ions  of  the  salt  having  the  same 
influence  as  those  of  the  acid  in  repressing  ionization  and  hydro- 
lytic  dissociation.  Procter  affirms  that  the  effect  of  the  salt 
cannot  be  a  dehydrating  action,  "  since  concentrated  sodium 
chloride  solutions  have  no  dehydrating,  but  rather  a  swelling 
effect  on  gelatin  in  the  absence  of  acid." 

Robertson1  however  considers  the  assumption  that  the  external 
acid  exerts  an  osmotic  pressure  as  "unnecessary  and  inconsistent 
with  the  fact  that  the  acid  freely  penetrates  the  'gelatin  and 
combines  with  it."  He  regards  the  action  as  due  to  a  competition 
between  the  inorganic  salt  and  the  gelatin  salt  for  water.  Since 
gelatin  is  readily  permeable  to  the  ions  of  inorganic  salts  it  is 
difficult  to  understand  a  development  of  osmotic  pressure  in  the 
system  as  due  to  inorganic  ions. 

Procter  has  advanced  the  hypothesis  that  the  anicns  obtained 
by  ionization  of  a  gelatin  salt  are  held  within  the  mass  by  electro- 
static forces,  since  they  are  not  able  to»pass  beyond  the  sphere 
of  attraction  of  the  colloid  cations.  The  latter  being  of  a  colloid 
nature  are  necessarily  held  within  the  gelatin  structure.  The 
only  way,  therefore,  in  which  the  osmotic  pressure  of  the  anions 
may  become  operative  is  not  by  their  own  movement  but  by  the 
passage  of  water  into  the  gelatin.  Procter  points  out  three 
objections  to  this  view.  If  his  theory  were  correct  the  force 
requiring  movement  of  water  would  be  the  electrostatic  tension 
preventing  the  escape  of  anions  into  the  outer  solution.  This 
would  result  in  a  difference  of  potential  between  the  internal 
jelly  and  the  solutions,  but  Ehrenberg2  has  been  unable  to  detect 
any  such  difference  in  potential.  Robertson  however  regards 
the  colloid  particles  themselves,  nather  than  the  inorganic  ions, 
as  responsible  for  the  osmotic  pressure,  for,  since  they  may  not 
pass  the  boundary  of  the  gelatin,  they  necessarily  compel  the 
compensating  migration  of  water.  "The  increased  swelling 
capacity  of  gelatin  in  solutions  of  acids  or  alkalies  is  merely  the 
expression  of  the  fact  that  the  ionization  of  the  protein  salt 
leads  to  an  increase  in  the  number  of  colloid  particles  per  unit 
volume  of  the  jelly  and  possibly  also  in  part  to  the  fact  that 


1  ROBERTSON,  lib.  cit.,  296. 

2  EHRENBERG,  Biochem.  Z.,  53  (1913),  356.     Loeb's  experiments  on  P.D. 
indicate  that  Ehrenberg's  results  are  incorrect. 

12 


178  GELATIN  AND  GLUE 

protein  ions  have  a  greater  affinity  for  water  than  undissociated 
protein  molecules." 

This  much  may  be  considered  as  established  beyond  any  reason 
of  doubt:  the  addition  of  sodium  chloride  to  an  acid  solution 
increases  the  hydrogen-ion  concentration  of  the  solution,  while 
the  addition  of  the  same  salt  to  an  alkaline  solution  increases  the 
hydroxyl-ion  concentration.1  The  author2  has  found  that  the 
action  of  sodium  hydroxide  is,  first,  to  promote  strongly  the 
hydration  of  the  gelatin,  which  takes  place  in  proportion  to  the 
amount  of  sodium  gelatinate  produced,  and,  secondly,  to  dis- 
solve the  gelatin.  The  latter  effect  becomes  noticeable  in  con- 
centrations of  N/4  sodium  hydroxide,  and  at  N/l  the  latter 
effect  predominates  over  the  former  and  solution  takes  place. 
There  appears  to  be  no  reduction  in  the  swelling  except  that  due 
to  solution.  Sodium  chloride  acts  in  a  similar  way  as  regards 
swelling,  but  does  not  result  in  solution.  From  an  application 
of  the  mass  law  we  would  expect  that  an  addition  of  sodium 
chloride  to  a  solution  of  sodium  hydroxide  would  increase  the 
sodium  ions  and  depress  the  hydroxyl  ions  present.  That  the 
reverse  actually  takes  place  is  due  to  the  hydration  of  the  sodium 
and  chloride  ions  added.  As  the  liquefaction  of  gelatin  by 
sodium  hydroxide  is  due  to  the  hydroxyl  ions  in  all  probability, 
it  is  evident  that  the  amount  of  hydroxyl  ions  by  which  the 
solution  is  enriched  upon  the  addition  of  sodium  chloride  is  still 
too  small  to  produce  that  effect. 

A  second  objection  to  Procter's  hypothesis  lies  in  the  fact 
that,  according  to  his  theory,  no  equilibrium  would  be  established, 
but  swelling  would  proceed  indefinitely,  which  is  not  the  case. 
Procter  considers  that  the  force  which  opposes  an  indefinite 
increase  in  swelling,  and  limits  the  latter  to  a  well  defined  maxi- 
mum is  the  tension  of  the  elastic  colloid  network.  By  applying 
Hooke's  law  Procter  derives  the  following  relation  defining  the 
equilibrium  : 


in  which  e  is  the  tension  of  the  colloid  network,  C  the  modulus  of 

1  ARRHENIUS,  Z.  physik.  Chem.,  31  (1899),  197;  POMA,  ibid.,  88   (1914), 
671;  HARMED,  /.  Am.  Chem.  Soc.,  37  (1915),  2460;  FALES  and   NELSON, 
ibid.,  37  (1915),  2769;  THOMAS  and  BALDWIN,  ibid.,  41  (1919),  1981;  WILSON, 
ibid.,  42  (1920),  715. 

2  R.  H.  BOGUE,  /.  Ind.  Eng.  Chem.,  14  (1922),  32. 


GELATIN  AS  A  LYOPHILIC  COLLOID  179 

elasticity,  V  the  maximum  volume  attained  by  one  gram  of 
gelatin,  and  g  the  specific  gravity  of  the  gelatin.  He  finds  the 
modulus  of  elasticity  (C)  decreases  with  rise  in  temperature, 
from  0.00125  at  7°,  to  0.00021  at  18°.  The  value  of  e  varies  as  the 
degree  of  swelling,  at  first  increasing  and  later  decreasing  with 
continued  additions  of  acid. 

Mathematical  Confirmation  of  Procter's  Theory. — A  more  ex- 
tensive mathematical  interpretation  of  these  relations  is  given 
by  J.  A.  and  W.  H.  Wilson.1  Starting  with  a  purely  hypothet- 
ical colloid  jelly,  G,  they  have  developed  curves  which  conform 
in  a  striking  manner  to  jiata  which  were  obtained  by  Procter 
experimentally.  The  hypothetical  colloid  G  is  assumed  to  be 
completely  permeable  to  water  and  to  all  dissolved  electrolytes; 
to  be  elastic;  to  follow  Hooke's  law;  and  to  combine  chemically 
with  the  positive  but  not  the  negative  ion  of  a  binary  electrolyte 
MN,  in  accordance  with  the  equation: 

[G]  X  [M+]  =  K[GM+].  (1) 

On  immersing  G  in  an  aqueous  solution  of  M N  the  solution 
penetrates  G  which  thereupon  combines  with  some  of  the  ions 
M+,  removing  them  from  solution.  The  solution  within  the 
jelly  will  consequently  have  a  greater  concentration  of  N~  than 
of  M+,  while  in  the  outside  solution  the  concentrations  of  the  ions 
of  the  electrolyte  must  be  equal. 

In  the  external  solution  then,  let 

x  =  [M+]  =  [N~], 

y  =  IM+], 

and 

z  =  [GM+]; 

whence  § 

[N-]  =  y  +  z. 

Assuming  the  transfer  of  an  infinitesimally  small  amount,  dn 
mols,  of  M+  and  N~  from  the  external  solution  to  the  jelly  phase, 
the  relation  between  the  concentrations  of  diffusible  ions  of  the 
two  phases  at  equilibrium  was  derived,  yielding  the  equation: 

dn  RT  log  x/y  +  dn  RT  log  x/(y  +  z)  =0; 
whence 

x2  =  y(y  +  z).  (2) 

Since  the  sum  of  two  numbers  that  are  unequal  is  greater  than 
1  J.  A.  and  W.  H.  WILSON,  J.  Am.  Chem.  Soc.,  40  (1918),  886. 


180  GELATIN  AND  GLUE 

the  sum  of  two  others  that  are  equal  but  which  when  multiplied 
give  products  identical  with  the  unequal  pair,  it  follows  that: 

2y  +  z  >  2x, 

for  the  total  concentration  of  ions  2y  +  z  of  the  jelly  is  greater 
than  the  ionic  concentration  2x  of  the  external  solution.  Now 
by  letting  this  excess  in  the  value  of  2y  +  z  over  that  of  2x  be 
represented  by  e,  the  general  equation: 

2x  +  e  =  2y  +  z,  (3) 

is  obtained,  which  is  mathematical  proof  of  the  preponderance 
of  the  concentration  of  diffusible  ions  in  the  jelly  phase  over  that 
of  the  external  solution. 

Since  [N~]  is  greater  in  the  jelly  than  in  the  surrounding  solu- 
tion, these  ions  will  tend  to  diffuse  out  from  the  jelly,  but  electro- 
static forces  make  this  impossible  except  they  be  accompanied 
by  their  colloid  cations.  The  cohesive  forces  of  the  elastic  jelly 
tend  to  resist  this  outward  pull  by  the  value  e,  and  according  to 
Hooke's  law: 

e  =  CV,  (4) 

where  C  is  a  constant  and  V  the  increase  in  volume  in  cubic 
centimeters  of  one  millimole  of  the  colloid. 
Taking  unit  quantity  of  G: 

[G\  +  [GM+]  =  1/(F  +  a), 
or 

[G]  =  l/(V  +  a)  -Z,  (5) 

where  a  is  the  free  space  within  the  jelly  before  swelling  through 
which  the  ions  may  pass.  This  will  be  nearly,  but  not  quite,  as 
great  as  the  initial  volume  of  the  colloid.  Where  a  =  0, 

1/(F  -  Z)y  =  Kz.  (6) 

From  (2)  and  (3) 

Z  =  e  +  2  Vq/, 
or 

Z  =  CV  +  2VCVy-  (7) 

And  from  (6)  and  (7)  : 

V(K  +  y)(CV  +  2VCVy)  ~  y  =  0.  (8) 


The  only  variables  are  V  and  y,  and  if  the  constants  K  and  C 
are  known  the  values  of  either  variable  may  be  plotted  in  terms 
of  the  other.  Knowing  y  and  F,  the  value  of  Z  may  be  calcu- 


GELATIN  AS  A  LYOPHILIC  COLLOID 


181 


lated  from  (7),  and  knowing  y  and  Z,  the  value  of  x  may  be 
calculated  from  (2),  and  e  from  (4). 

The  values  of  K  and  C  have  been  calculated  by  Procter  and 
Wilson.1  K  was  obtained  by  running  successive  portions  of 
standard  hydrochloric  acid  into  a  gelatin  solution  of  known 
volume,  and  determining  the '  hydrogen  ion  concentration  by 
means  of  the  hydrogen  electrode  after  each  addition.  C  was 
calculated  from  values  obtained  for  e  and  V.  Their  results  gave 


The  lines ,  ^present  the  theoretical  0 
cur ves  for  K=.  000/5  and  C=.  0003        A 


The  marks  indicate  experimental  determinations 


.oe 


.18 


.20 


.04          .06          .08  .10  .IE  .14          .16 

Moles  per  Lifer  of  Hydrogen  in  External  Solution 

FIG.  19. — longenic  equilibria  in  gelatin  systems.  I.     (By  permission  of  the  Jour- 
nal of  the  American  Leather  Chemists'  Association.) 

a  value  to  K  of  1.5  X  10~4  and  §to  C  of  3  X  10~4  at  18°C. 
Upon  substituting  these  values  in  the  equations  the  curves  shown 
in  Figs.  19  and  20  are  obtained.2  The  ordinates  in  Fig.  19 
indicate  the  molsper  liter  of  the  ions  enumerated,  and  the  abscissa 
the  mols  per  liter  of  hydrogen  ion  in  the  external  solution.  In 
Fig.  20  the  ordinates  are  the  cubic  centimeters  of  solution 
absorbed  by  one  millimol  of  gelatin.  The  lines  in  all  cases 
represent  the  curves  derived  theoretically  from  the  foregoing 
mathematical  formulae,  while  the  marks  indicate  the  values 
obtained  by  Procter3  some  years  earlier.  It  will  be  observed 

1  H.  R.  PROCTER  and  J.  A.  WILSON,  J.  Chem.  Soc.,  109  (1916),  307. 

2  J.  A.  WILSON,  J.  Am.  Leather  Chem.  Assn.,  13  (1918),  184-5. 

3  H.  R.  PROCTER,  J.  Chem.  Soc.,  105  (1914),  317. 


182 


GELATIN  AND  GLUE 


that  the  theoretical  and  experimental  findings  coincide  in  a 
striking  manner.  In  Fig.  20  Wilson  assumes  the  value  of  768 
as  the  molecular  weight  of  gelatin,  having  previously  found  this 
value  in  connection  with  experiments  upon  the  combining  equi- 
valent of  gelatin,  assuming  the  latter  to  act  as  a  monacid  base  in 
dilute  acid  solutions.1  Procter's  values,  which  were  based  upon 
his  figure  of  839  for  the  molecular  weight  of  gelatin,2  assuming 
the  substance  to  act  as  a  diacid  base  in  dilute  acid  solutions,  were 
recalculated  to  the  former  value.  The  perfection  of  the  latter 


The  line  represents  the  theoretical  curve  for 
H = .  00015  and  C= .  0003 


Circles  indicate  experimental  determinations 


.16 


18 


20 


.04.          .06          .08  .10  .12  .14 

Concentration  of  Hydrion  in  External  Solution 

FIG.  20.— longenic  equilibria  in  gelatin  systems.  II.      (By  permission  of  the  Jour- 
nal of  the  American  Leather  Chemists'  Association.) 

curve  may  be  urged  as  proof  that  the  equivalent  weight  of  gelatin 
is  at  least  of  the  order  of  magnitude  of  768. 

Influence  of  Salts  upon  Swelling  and  Solution. — The  specific 
influence  of  salts  upon  the  swelling  and  solution  of  gelatin  has 
been  investigated  by  a  number  of  workers.  Loeb3  has  found  the 
valence  of  the  cation  or  anion  with  which  the  gelatin  is  brought 
into  combination  to  exert  a  profound  influence.  This  will  be  con- 
sidered in  detail  in  Chapter  V. 

Fischer's  Theory  of  Salt  Action. — Fischer  and  his  collaborators4 
have  studied  not  only  the  effects  of  simple  neutral  salts,  but  also 
the  effects  of  " buffer"  mixtures  (phosphates,  citrates,  and  car- 
bonates) upon  swelling  and  solution  of  gelatin  and  fibrin.  In 
the  phosphate  series  they  added  uniformly  varying  amounts  of 

1  J.  A.  WILSON,  J.  Am.  Leather  Chem.  Assn.,  12  (1914),  317. 

2  H.  R.  PROCTER,  loc.  cit.,  320. 

3  J.  LOEB,  op.  cit. 

4  FISCHER  and  HOOKER,  J.  Am.  Chem.  Soc.,  40  (1918),  272;  FISCHER  and 
COFFMAN,  ibid.,4Q  (1918);  303;  FISCHER,  HOOKER,  BENZINGER,  and  COFFMAN, 
Science,  N.  S.,  46  (1917),  189. 


GELATIN  AS  A  LYOPHILIC  COLLOID  183 

phosphoric  acid,  mono-,  di-,  and  trisodium  phosphate  and  sodium 
hydroxide  to  a  constant  amount  of  gelatin,  and  noted  the  degree 
of  swelling  and  the  effect  of  the  added  substances  upon  the  solu- 
tion of  the  gelatin.  In  the  citrate  series  they  used  citric  acid, 
mono-,  di-,  and  trisodium  citrate,  and  sodium  hydroxide. 

The  data  obtained  by  these  investigators  show  that  the  curves 
for  swelling  and  for  solution  run  parallel,  the  greatest  swelling 
and  the  earliest  signs  of  liquefaction  being  brought  about  in  the 
pure  acid  or  the  pure  base,  while  at  the  point  of  minimum  swelling 
the  gelatin  is  most  insoluble.  "These  investigators  conclude, 
however,  that  although  the  same  causes  may  bring  about  the  two 
phenomena,  e.g.,  swelling  and  solution,  they  are  nevertheless 
totally  different  processes,  and  liquefaction  is  not,  as  has  com- 
monly been  stated,  the  extreme  of  what  in  lesser  degree  is  called 
swelling.  "Hydration,"  say  Fischer  and  Coffman,  "is  to  be 
regarded  as  a  change  through  which  the  protein  enters  into 
physicochemical  combination  with  its  solvent  (water);  'solution,' 
as  one  which  can  be  most  easily  understood  at  the  present  time  as 
the  expression  of  an  increase  of  the  degree  of  dispersion  of  the 
colloid."  High  temperatures,  acids,  and  alkalies  cause  the  colloid 
particles  to  become  smaller,  and  when  in  this  condition  the  gelatin 
is  liquid  and  clear,  while  under  the  reverse  conditions  the  gels 
become  solid  and  opalescent.  Fischer  considers  that  the 
warming  of  a  gelatin  water  system  displaces  it  from  the  side  of  a 
solution  of  water  in  gelatin  towards  that  of  gelatin  in  water. 
In  the  latter  the  particles  are  smaller,  more  nearly  in  "true" 
solution,  and  therefore  the  system  is  also  clearer.  Neutral  gela- 
tin takes  up  some  water  but  the  addition  of  acid  or  alkali  leads 
to  the  formation  of  gelatin  salts  and  gelatinates  which  not 
have  a  greater  capacity  for  absorbing  water  but  also  a  greater^  / 
solubility  in  water.1  If  the  alkali  or  acid  is  added  to  a  neutral  / 
gelatin  water-system  near  its  gelation  point  the  mixtures  clearj 
and  become  liquid  because  the  solubility  of  the  gelatinate  or 
gelatin  salt  in  the  water  dominates  the  system.  In  this  region 
signs  of  "going  into  solution"  become  manifest.  Fischer 
places  especial  stress  upon  the  relations  which  these  observations 
bear  to  physiological  and  pathological  processes,  as  are  observed 
in  edema,  excessive  turgor,  and  plasmoptysis,  and  other  "soften- 
ing" conditions  in  the  tissue. 

M1  M!RTIN  H.  FISCHER  and  MARION  HOOKER,  Science,  48   (1918),   143; 
ARTIN  H.  FI^GHER,  Science,  49  (1919),  615. 


184  GELATIN  AND  GLUE 

The  exact  relations  which  obtain  between  the  swelling,  vis- 
cosity, and  hydrogen  ion  concentration  have  been  investigated 
in  the  author's  laboratory.1  The  results  obtained  agree  well 
with  those  of  Fischer,  but  the  additional  information  upon  the 
pH  values  is  even  more  conclusive  in  its  deductions.  It  was 
found  that  the  swelling  and  viscosity,  which  are  at  their  minimum 
at  a  pH  of  4.7,  increase  regularly  with  a  rise  in  pH  to  about  8.5, 
but  that  above  that  value  they  decline  slightly  due  to  an  increas- 
ing solubility.  That  is,  the  hydrogen-ion  concentration  deter- 
mines the  solvation,  and  the  solvation  in  turn  determines 
viscosity,  swelling,  and  jelly  consistency.  When  the  solvation, 
which  may  be  defined  as  the  volume  occupied  by  unit  weight  of 
dispersed  phase,  is  very  small,  these  other  properties  will  likewise 
be  small;  when  the  solvation  is  very  high  then  the  tendency  to  go 
into  solution  is  increased,  and  the  viscosity  and  jelly  consistency 
are  again  low.  At  intermediate  degrees  of  solvation  the  above 
properties  attain  the  maximum  value. 

Loeb's  Theory  of  Salt  Action. — The  relationship  between  hydro- 
gen ion  concentration  and  solvation,  as  described,  is  not  at  all 
antagonistic  to  Loeb's  theories  of  gelatin  ionization  and  salt 
formation,  as  some  writers  have  assumed.  Without  the  exact 
measurements  dependent  upon  the  laws  of  classical  chemistry, 
the  colloid  chemistry  of  the  proteins  becomes  a  hopeless  "slough 
of  despond."  Everything  is  speculative;  but  little  is  based  upon 
quantitative  evidence.  By  the  masterly  efforts  of  S0rensen, 
Michaelis,  and  Loeb  we  are  finding  that  these  great  aggregates 
of  molecules  called  proteins  and  colloids  are  susceptible  of  much 
the  same  type  of  metamorphos  as  the  better  understood  inorganic 
substances.  But  the  conception  of  colloid  chemistry  as  the 
chemistry  of  special  dimensions  need  not  be  laid  aside.  Special 
conditions,  such  as  hydrogen  ion  concentration,  that  control 
ionization,  conductivity,  and  osmotic  pressure,  also  control  solva- 
tion and  degree  of  dispersion. 

Donnan2  in  1911  proposed  a  theory  of  membrane  equilibrium 
to  account  for  the  differences  in  osmotic  pressure,  conductivity, 
etc.,  observed  between  two  electrolytic  solutions,  separated  by  a 
membrane,  both  ions  of  one  of  these  solutions  being  permeable 
in  the  membrane,  and  one  of  the  ions  of  the  other  solution  being 

1  R.  H.  BOGUE,  /.  Ind.  Eng.  Chem.,  14  (1922)  32. 

2  F.  G.  DONNAN,  Z.  Electrochem.,  17  (1911),  572;  DONNAN  and  HARRIS, 
/.  Chem.  Soc.,  99  (1911),  1554;  DONNAN  and  GARNER,  ibid.,  115  (1919),  1313. 


GELATIN  AS  A  LYOPHILIC  COLLOID  185 

impermeable.  Procter1  made  use  of  this  theory  to  account  for 
the  swelling  of  gelatin  in  the  presence  of  inorganic  ions.  Taking 
gelatin  chloride  as  an  example,  placed  in  a  solution  containing 
the  ions  of  a  chloride,  he  denned  the  swelling  as  determined  by 
the  difference  between  the  concentration  of  the  free  ions  in  the 
interior  of  the  jelly  and  the  concentration  of  the  free  ions  in  the 
external  solution.  He  did  not  attempt  to  explain  the  causes  for 
the  differences  in  ion  concentration  observed,  but  Loeb2  has 
shown  that  such  differences  are  defined  by  the  hydrogen  ion. 
The  potential  difference  may  also"  be  calculated  very  accurately 
from  the  difference  of  pH  inside  minus  pH  outside  of  the  jelly 
on  the  basis  of  Nerst's  formula 


which  at  room  temperature  and  for  n  =  1  becomes 

0.058  log  £!. 
C2 

In  the  experiment  the  gelatin  in  1  per  cent  solution  in  sodium 
nitrate  of  pH  3.5  was  placed  in  collodion  bags  and  immersed  in 
water  containing  similar  quantities  of  sodium  nitrate,  and 
hydrochloric  acid  of  pH  3.0.  If  Ci  represents  the  concentration 
of  free  hydrochloric  acid  in  the  gelatin  solution,  and  C*  the 
concentration  of  the  same  in  the  outside  solution,  the  value  log- 

C 

fT  becomes  equal  to  (pH  inside  —  pH  outside)  .     The  difference 

02 

in  pH  multiplied  by  58  or  59  (corrected  for  temperature)  would 
therefore  express  the  potential  difference  between  the  inside  and 
outside  solutions  in  millivolts,  if  Nerst's  formula  is  applicable. 
By  actual  measurement  the  calculated  and  observed  values  for 
potential  difference  are  practically  identical.  The  two  solutions 
are  in  ionic  equilibrium  when  the  one  per  cent  gelatin  chloride 
solution  is  at  pH  3.5  and  the  external  aqueous  hydrochloric  acid 
solution  is  at  pH  3.0. 

Loeb  also  made  the  important  observation  that  the  potential 
difference  between  the  two  solutions  diminished  as  the  concen- 
tration of  salt  present  increased.  from  0  to  M/32.  At  the  latter 
concentration  it  had  nearly  reached  zero.  He  therefore  argues 
that  "the  depressing  influence  of  salts  upon  the  swelling  of  gelatin 

1  H.  R.  PROCTER,  /.  Chem.  Soc.,  105  (1914),  313;  PROCTER  and  WILSON, 
ibid.,  109  (1916),  307. 

2  J.  LOEB,  J.  Gen.  PhysioL,  3  (1921),  557;  667;  691.     Vide  also  Chap.  V. 


186  GELATIN  AND  GLUE 

is  due  to  a  diminution  of  the  difference  of  pH  inside  and  outside 
of  the  gel,  and  that  the  curves  expressing  the  influence  of  neutral 
salts  on  the  value  of  pH  inside  minus  pH  outside  the  gel,  and  on 
the  swelling  run  approximately  parallel." 


3.  THE  VISCOSITY  OF  GELATIN 

General  Considerations. — When  crystalloids  go  into  solu- 
tion the  viscosity  of  the  solution  is  usually  slightly  greater  than 
that  of  the  pure  solvent,  and  the  viscosity  of  such  solutions 
normally  increases  with  the  concentration,  but  at  a  somewhat 
greater  rate,  so  that  the  curve  illustrative  of  this  principle  is 
convex  towards  the  concentration  axis.  There  are  some  excep- 
tions to  this  general  expression  wherein  a  decrease  in  viscosity  is 
observed,  as  the  solution  of  naphthalene  in  alcohol,  or  of  lithium 
chloride  and  potassium  chloride  in  water,  which  are  generally 
explained  as  due  to  a  depolymerization  of  the  solvent  by  virtue  of 
the  dielectric  effect  of  the  solute. 

In  the  case  of  suspensoids  in  dilute  solution  very  slight  differ- 
ences only  are  observed  between  their  viscosity  and  the  viscosity 
of  the  pure  dispersion  medium.  In  concentrated  solutions  of 
suspensoids  however  there  may  be  so  much  solid  material  in 
proportion  to  the  liquid  present  that  a  paste  may  result,  which 
is,  of  course,  very  viscid. 

Both  of  the  above  types  of  dispersion  differ  quite  materially 
from  that  which  is  encountered  in  the  class  of  colloids  of.  the 
emulsoid  type.  In  the  latter  case,  we  have,  according  to  the 
viewpoint  of  Wo.  Ostwald,  an  apparently  homogeneous  mixture 
of  two  immiscible  liquids,  the  one  being  suspended  in  very  fine 
droplets  throughout  the  other,  e.g.,  the  one  discontinuous,  and 
known  as  the  dispersed  phase,  the  other  continuous,  and  known 
as  the  dispersion  medium.  In  this  type  of  colloid  solution  the 
viscosity  constitutes  one  of  its  most  striking  properties.  Even 
in  very  dilute  solutions  e.g.,  0.1  per  cent,  the  viscosity  is  materi- 
ally higher  than  that  of  the  dispersion  medium,  and  upon  slightly 
increasing  the  concentration  the  viscosity  increases  enormously. 
This  property,  alluded  to  by  Ostwald  and  others  as  the  internal 
friction  of  the  emulsoid,  is  also  very  variable  with  slight  altera- 
tions in  the  conditions  to  which  it  is  subjected.  These  are 
described  below. 


GELATIN  AS  A  LYOPHILIC  COLLOID  187 

Conditions  Affecting  Viscosity. — In  the  case  of  molecularly 
dispersed  solutions  the  viscosity  is  completely  denned  by  the 
concentration  and  temperature.  We  are  dealing  with  only 
three  variables,  and  we  may  plot  precise  viscosity-temperature 
curves  at  constant  concentration,  or  viscosity-concentration 
curves  at  constant  temperature.  In  the  suspensoids,  where  the 
conditions  involve  a  suspension  of  solid  particles  in  a  liquid 
probably  only  one  further  variable  enters,  i.e.,  the  size  of  the 
suspended  particle.  The  greatest  viscosity  seems  to  attend  a 
medium  degree  of  dispersion. 

But  many  other  factors  are  found  to  influence  the  viscosity  of 
emulsoids.  In  addition  to  concentration,  temperature,  and  degree 
of  dispersion,  Wo.  Ostwald1  has  listed  as  of  importance:  solvate 
formation,  electric  charge  or  ionization,  the  previous  thermal 
treatment,  the  previous  mechanical  treatment,  inoculation  with 
other  colloids,  time,  and  addition  of  foreign  substances.  Such  an 
array  of  variables  makes  it  seem  a  most  difficult  if  not  impossible 
accomplishment  to  determine  a  precise  curve  for  viscosity  with  any 
other  property,  as  the  effect  of  all  other  variables  should  either  be 
completely  eliminated  or  reduced  to  a  negligible  influence.  The 
latter  condition  may  however  be  approached,  and  by  systemati- 
cally controlling  some  of  the  variables,  curves  for  the  others  may 
be  obtained  which  have  pro^fved  of  the  greatest  service,  both  in 
assisting  to  a  more  appreciative  understanding  of  the  principles 
underlying  changes  in  state  of  colloid  systems,  and  in  processes 
of  control  and  analysis  in  certain  instances.  The  above  men- 
tioned factors  will  be  considered  in  detail  in  the  following 
paragraphs.2 

Concentration. — If  the  temperature  of  a  gelatin  sol  is  held 
constant,  and  the  variation  in  viscosity  due  to  alteration  in  the 
concentration  is  measured,  it  will  be  found  that  the  curve  is 
linear  at  very  low  concentrations,  but  becomes  curvilinear  at 
higher  concentrations.  That  is,  when  the  gelatin  is  present  in 
less  than  1  or  2  per  cent  concentrati'on  the  increase  in  visco- 
sity is  a  linear  function  of  the  concentration.  As  soon,  however, 

1  Wo.  OSTWALD,  Trans.  Faraday  Soc.,  9  (1913),  34. 

2  It  should  be  emphasized  that  the  conclusions  which  follow  were  based 
largely  upon  experiments  in  which  the  pH  was  not  considered.     The  work 
of  Loeb,  Bogue  and  others,  described  in  the  following  chapter,  and  in  the 
section  on  Structure  in  Chap.  Ill,  makes  possible  a  considerable  simplification 
of  the  relations. 


188 


GELATIN  AND  GLUE 


as  the  volume  occupied  by  the  dispersed  phase  (which  includes 
the  volume  of  water  which  is  associated  with  the  gelatin  mole- 
cules in  hydrated  condition)  reaches  about  50  per  cent  of  the 
total  volume  of  the  system  the  viscosity  curve  will  no  longer 
remain  linear,  but  rises  ever  more  and  more  rapidly  with  each 
increase  in  concentration.  This  is  explained  by  Hatschek's 
36 


10     II     12 


<  89 

Per  Cent  Gelatin  Concentration 
FIG.  21. — Relation  of  viscosity  to  concentration  in  gelatin  solutions. 

theory1  which  assumes  that  the  viscosity  curve  will  remain 
linear  as  long  as  the  particles  of  dispersed  phase  do  not  touch 
each  other,  but  becomes  curvilinear  when  they  occupy  so  great  a 
volume  that  they  are  in  close  contact.  The  relations  between 
viscosity  and  concentration  of  gelatin  sols  have  been  rigidly 
studied  by  the  author2  and  the  curves  for  a  gelatin  of  three  differ- 

1  Vide  page  200. 

2  R.  H.  BOGUE,  /.  Am.  Ghent.  Soc.,  43  (1921),  1764. 


GELATIN  AS  A  LYOPHILIC  COLLOID 


189 


ent  hydrogen  ion  concentrations,  at  35°C.,  are  shown  in  Fig.  21. 

Temperature. — P.    von    Schroeder1    has    shown    that    while 

water  increases  in  viscosity  about  18  per  cent  on  being  reduced 

200 


180 
160 

140 
120 


8ico 


80 
60 
40 
20 
0 


Mac  Michael  Vi'sco$ime1-er  Used 


7 


iso    i4o    i3o     ieo     no      ioo     90     so 

Temperature,  cleg. Fahr 
FIG.  22. — The  effect  of  temperature  upon  the  viscosity  of  hide  glues. 

in  temperature  from  31  to  21°C.,  a  3  per  cent  solution  of  gelatin 
through  the  same  range  showed  an  increase  of  nearly  1,000  per 
cent.  At  higher  temperatures  however  a  reduction  of  10° 
would  reveal  a  very  much  smaller  increase.  The  same  type  of 
curve  is  obtained  with  glues  as  with  gelatin,  except  that  as  the 
grade  of  the  glue  decreases,  e.g.,  as  its  content  of  gelatin  (hydrata- 
ble  material)  diminishes,  the  temperature  at  which  a  rapid  rise 
begins  is  lowered.  This  will  be  clear  from  an  inspection  of  Figs. 
22  and  23,  which  show  the  viscosity  in  MacMichael  degrees 
of  standard  hide  and  bone  glues. 

The    influence    of   added  substances  upon  the  temperature- 
viscosity  curve  has  been  studied  by  the  author.2     Glues  which 

1  P.  VON  SCHROEDER,  Z.  physik.  Chem.,  46  (1903),  75. 

2  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  5. 


190 


GELATIN  AND  GLUE 


had  been  treated  with  alums  were  found  to  react  to  increase  in 
temperatures  by  a  very  profound  drop  in  viscosity,  which  con- 
tinued regularly.  Glues  that  had  been  treated  with  formalde- 
hyde responded  in  a  different  way.  Between  30  and  40°  the 
viscosity  drops  as  the  temperature  rises,  which  is  the  normal 
behavior  of  untreated  glues.  Between  40  and  45  or  50°,  how- 
200, 


150 


90      80 


130       120      110       100 
Temperature,  Fahrenheif 
FIG.  23. — The  effect  of  temperature  upon  the  viscosity  of  bone  glues. 

ever,  the  viscosity  remains  nearly  constant,  and  above  50°  it 
rises  sharply  to  the  setting  point.  These  data  are  illustrated 
in  Figs.  24  and  25. 

Degree  of  Dispersion. — Of  the  many  factors  which  influence 
the  viscosity  of  emulsoids  the  degree  of  dispersion  is  probably  of 
greater  importance  than  any  other  with  the  exception  of  con- 
centration and  temperature.  It  is  in  fact  highly  probably  that 
the  influence  of  many  if  not  all  of  the  other  factors  affecting 
viscosity  is  brought  about  through  changes  in  the  size  of  the 
particles  of  the  dispersed  phase.  Thus  alterations  in  the  con- 
centration or  the  temperature  of  the  solution,  as  also  the  addition 


GELATIN  AS  A  LYOPHILIC  COLLOID 


191 


of  salts,  acids,  or  alkalies,  very  probably  alters  the  degree  of 
ionization  of  the  gelatin  or  the  gelatin  salt  in  solution.  Any 
changes  in  the  equilibrium  of  ionization  will  be  reflected  in  like 
changes  in  the  relation  of  positve  to  negative  surface  tension 
between  the  disperse  phase  and  dispersion  medium,  and  altera- 
tions in  this  surface  tension  ratio  must  result  in  changes  in  the 
126 


118 


40         50        60       TO        80 
Temperature,  deg.  Cent. 
FIG.  24. — The  effect  of  temperature  upon  the  viscosity  of  alum-treated  glues. 

size  of  the  particles.1  So  it  seems  highly  probable  that  any 
change  in  viscosity  from  whatever  cause  is  produced,  in  the  last 
analysis,  by  alterations  in  the  degree  of  dispersion  and  solvation. 
The  direction  which  the  viscosity  curve  will  take  upon  changes 
in  degree  of  dispersion  has  been  investigated  by  many  workers, 
and  the  results  obtained  point  to  a  medium  dispersion  as  optimum 
for  a  maximum  viscosity.  For  example  Martici2  found  the 

1Wo.  OSTWALD — FISCHER,  "Handbook  of  Colloid  Chemistry,"  1st  ed., 
(1915),  181.  J.  VAN  BEMMELEN,  "Die  Absorption,  "Gesammelte  Abhandl., 
Dresden  (1910-22). 

2  MARTICI,  Arch,  fisiol.,  4  (1907),  133. 


192 


GELATIN  AND  GLUE 


viscosity  of  oil  emulsions  in  soap  solution  increased  as  the  oil 
droplets  became  smaller.  Buglia1  found  that  milk  which  had 
been  homogenized,  by  directing  a  stream  of  it  swiftly  against 


^ represent  the  cubic 

centimeters  of  JO  per 
formaldehyde 


40        45        50        55 
Temperature,  CerrHgrad  e 


65 


FIG.  25. — The  effect  of  temperature  upon  the  viscosity  of  formaldehyde-treated 

glues. 

an  agate  plate,  showed  a  noticeable  increase  in  viscosity.  In 
this  case  the  particles  of  fat  had  become  smaller.  On  the  other 
hand  Biltz  and  von  Vegesack2  found  the  maximum  viscosity  of 
colloid  solutions  of  night-blue  corresponded  with  the  highest 
molecular  weight,  e.g.,  with  the  largest  size  of  particles. 

The  author3  has  succeeded  in  demonstrating  definite  maxima  in 
viscosity  throughout  a  constantly  changing  dispersity  in  experi- 
ments upon  gelatin  and  glue.  In  studying  the  effects  of  added 
substances  upon  viscosity  it  was  observed  that  the  substances 
added  might  be  divided  into  four  groups  with  respect  to  their 
action  on  viscosity,  e.g.,  (1),  those  which  raise  the  viscosity 
constantly;  (2)  those  which  lower  the  viscosity  constantly;  (3), 
those  which  raise  the  viscosity  to  a  maximum  beyond  which  they 

1  BUGLIA,  Kolloid-Z.,  2  (1908),  353. 

2  BILTZ  and  VON  VEGESACK,  Z.  physik.  Chem.,  73  (1910),  500. 
3BoGUE,  Chem  Met.  Eng.,  23  (1920),  5;  61. 


GELATIN  AS  A  LYOPHILIC  COLLOID  193 

produce  a  drop;  and  (4),  those  which  have  no  appreciable  effect. 
The  third  group  is  the  most  interesting  and  instructive.  We 
find  dilute  sodium  hydroxide  and  acetic  acid  both  to  fall  in  this 
group.  As  small  amounts  of  these  electrolytes  are  added  to  the 
glue  solution,  there  is  observed  a  very  definite  and  constant 
increase  in  viscosity  up  to  a  certain  point,  and  a  slight  further 
addition  results  in  a  sharp  drop  in  viscosity  to  a  solution  of 
nearly  watery  consistency.  While  this  has  been  going  on  the 
solution  has  become  more  and  more  turbid,  but  apparently 
homogeneous  up  to  the  point  of  maximum  viscosity.  But  very 
shortly  after  the  break  in  viscosity  occurs,  the  emulsion  also 
visibly  breaks,  and  a  flocculent  precipitate  is  observed.  The 
constantly  increasing  turbidity  of  the  solution  with  its  ultimate 
break  seems  to  indicate  a  continually  increasing  size  of  particle 
in  disperse  phase,  but  the  existence  of  a  distinct  peak  in  the 
viscosity  curve  can  mean  only  that  during  the  first  part  of  the 
experiment  the  viscosity  increased  with  decreasing  degree  of 
dispersion,  while  during  the  latter  part  the  viscosity  decreased 
with  decreasing  degree  of  dispersion.  In  other  words  the  viscos- 
ity shows  a  distinct  maximum  at  a  medium  dispersity  of  gelatin 
particles.  Alexander,1  however,  prefers  to  regard  this  flocculent 
precipitation  as  resulting  from  "an  increased  dispersion  involving 
or  followed  by  the  formation  of  a  small  quantity  of  an  insoluble 
chemical  or  adsorption  compound."  Exactly  what  this  "insolu- 
ble chemical  or  adsorption  compound"  is,  or  why  it  should  be 
so  produced,  or  his  reasons  for  thinking  that  it  exists,  Alexander 
does  not  make  clear. 

Solvate  Formation. — It  has  already  been  shown  on  page  164 
that  the  swelling  of  gelatin  involves  a  physical  or  a  chemical 
combination  of  the  substance  with  the  elements  of  water.  Ex- 
periments performed  in  the  author's  laboratory2  have  shown 
furthermore  that  the  viscosity  and  degree  of  solvation  are  very 
closely  related.  Five  gelatins  of  varying  hydrogen-ion  concen- 
tration were  subjected  to  very  careful  viscosity  determination  at 
35°C.  by  the  use  of  an  Ostwald  viscosimeter.  The  viscosity 
curves  varied  greatly  as  has  been  shown  on  page  188.  On  calcu- 
lating the  volume  occupied  by  unit  weight  of  dispersed  phase  by 
Hatschek's  formula3  it  is  found  that  this  volume  is  much  greater 

1  J.  ALEXANDER,  /.  Am.  Chem.  Soc.,  43  (1921),  434 

2  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  43  (1921),  1764. 

3  Vide  page  203. 

13 


194  GELATIN  AND  GLUE 

at  a  pH  value  of  3.5,  and  lower  at  a  pH  value  of  4.7  than  at  any 
other,  and  the  coefficient  of  viscosity  is  found  to  vary  in  an 
entirely  similar  manner  as  is  shown  in  the  following  table: 

TABLE  36. — RELATION  OF  VISCOSITY  TO  SOLVATION  6  PER  CENT  SOL 


pH 

Coefficient  of  viscosity 

Volume  per  unit  weight 

3.5 
4.7 

5.8 

6.82 
5.25 
5.97 

10.27 

8.76 
9.57 

Since  the  volume  of  dispersed  phase  per  unit  weight  is  a  direct 
measure  of  solvation,  the  above  data  show  that  the  viscosity  and 
solvation  are  parallel  functions. 

Further  experiments1  have  however  shown  that  the  above 
relations  do  not  hold  strictly  true  under  conditions  near  the 
gelation  point.  On  regularly  increasing  the  hydroxide  ion  con- 
centration of  a  gelatin  sol  it  was  found  that  the  degree  of  swelling 
and  the  viscosity  regularly  increased  until  the  pH  value  had 
reached  about  8.5,  and  thereafter  decreased  slightly.  At  the 
same  time  it  was  observed  that  the  jelly  consistency  remained 
solid  until  the  pH  value  had  reached  8.5,  but  at  that  value  it 
became  soft,  and  at  higher  values  remained  liquid.  At  a  pH 
value  of  4.7  the  jelly  was  again  soft.  That  is,  it  reaches  its  maxi- 
mum consistency  at  a  medium  degree  of  solvation,  for  at  a  pH 
of  4.7  the  solvation  attains  its  minimum  value.  The  viscosity 
increases  regularly  with  the  solvation,  therefore,  up  to  a  point 
where  the  increased  tendency  of  the  gelatin  to  liquify,  or  remain 
fluid  due  to  increasing  solubility,  becomes  greater  than  the 
increasing  tendency  to  become  more  highly  solvated. 

Similar  parallelism  between  viscosity  and  solvation  has  been 
observed  by  Pauli,2  Ostwald,3  Fischer,4  Holmes,5  and  others. 
Pauli  and  Ostwald  have  pointed  out  that  such  close  parallelism 
exists  between  the  relative  properties  of  dilute  and  concentrated 
solutions  of  gelatin  that  the  viscosity  of  liquid  sols  may  be 
employed  for  the  study  of  imbibition  phenomena. 

1  R.  H.  BOGUE,  /.  Ind.  Eng.  Chem.,  14  (1922),  32. 

2  Wo.  PAULI,  "Kolloidchemie  der  Eiweisskorper,"  Dresden,  1920. 

3  Wo.  OSTWALD,  Trans.  Faraday  Soc.,  loc.  cit. 

4  M.  FISCHER,  J.  Am.  Chem.  Soc.,  40  (1918),  303. 

5  H.  HOLMES,  ibid.,  42  (1920),  2049. 


GELATIN  AS  A  LYOPHILIC  COLLOID  195 

Electric  Charge,  or  lonization. — Wo.  Pauli1  considers  that 
changes  in  hydration  are  most  frequently  the  basis  for  increase 
or  decrease  in  the  viscosity  of  protein  solutions,  and  that  altera- 
tions in  the  electric  charges  are  in  turn  most  frequently  the  basis 
for  variations  in  the  hydration. 

When  ordinary  gelatin  is  placed  in  an  electric  field  the  gelatin 
is  usually  found  to  be  ionized,  and  to  migrate  to  the  anode.  If  a 
small  amount  of  acid  is  added  this  migration  becomes  less  and 
less  and  eventually  ceases  altogether.  This  point  is  known  as  the 
isoelectric  point,  and  is  for  gelatin  defined  by  a  hydrogen-ion 
concentration  of  2  X  10~5  or  pH  =  4.7.2  It  has  been  shown  by 
Loeb3  that  the  viscosity  of  a  gelatin  solution  reaches  its  minimum 
at  this  point,  and  that  it  rises  on  either  side  thereof,  but  with 
different  rates  according  to  the  particular  anion  or  cation  with 
which  it  is  brought  into  combination.  This  point  will  receive 
extensive  treatment  in  a  later  section.^  It  is  desired  to  emphasize 
at  this  time  however  that  in  all  probability  ionized  particles 
of  gelatin  impart  to  a  solution  a  greater  viscosity  than  do  the 
natural  unchanged  molecules.  That  this  effect  is  very  closely 
connected  with  hydration  is  shown  by  the  fact  that  at  the 
isoelectric  point  the  swelling  and  the  viscosity  are  also  at  their 
minimum  values.  This  seems  to  indicate  that  the  non-ionized 
particles  possess  the  lowest  power  of  combination  with  water, 
that  this  non-hydration  results  in  a  low  viscosity,  and  that 
viscosity  may  accordingly  be  used  as  a  measure  of  still  another 
property,  i.e.,  the  ionization  of  the  protein.  When  it  is  remem- 
bered that  every  addition  of  any  electrolyte,  or  the  presence  of 
such  an  impurity  in  the  gelatin  or  the  water,  will  probably 
result  in  changes  in  the  hydrogen-ion  concentration,  and  hence 
in  the  ionization  of  the  gelatin,  it  will  readily  be  appreciated  that 
changes  in  viscosity  by  virtue  of  such  changes  in  ionization  and 
hydration  may  be  considerable  and,  in  any  study  of  the  proteins, 
of  the  very  highest  importance. 

The  Previous  Thermal  Treatment. — One  of  the  curious  anoma- 
lies of  a  gelatin  solution  is  shown  by  the  fact  that  if  it  is 
warmed  and  cooled  several  times  it  will  reveal  a  lower  viscosity 

1  Wo.  PAULI,  Trans.  Faraday  Soc.,  9  (1913),  54. 

2  MICHAELIS,     "Die     Wasserstoffionenkonzentration,"     Berlin     (1914). 
Vide  also  appendix  page  581. 

3  J.  LOEB,  J.  Gen.  Phijsiol.,  1  U918-19),  39;  237;  363;  483;  559. 
Vide  Chap.  V. 


196  GELATIN  AND  GLUE 

at  a  given  temperature  than  normally.  Also  if  the  viscosity  at, 
for  example,  32°C.  is  desired,  a  difference  will  be  observed  if  the 
gelatin  solution  is  brought  gradually  up  to  32°  and  then  measured, 
or  if  it  is  first  heated  to  60  or  70°,  and  then  cooled  to  32°  and 
measured.  That  such  decrease  in  viscosity  is  not  the  result  of 
hydrolysis  of  the  gelatin  to  simpler  substances  is  shown  by  the 
fact  that  on  standing  for  several  hours  or  days  the  original  values 
may  again  be  duplicated.  Prolonged  heating  results  in  a 
permanent  decrease  in  viscosity  due  to  such  hydrolysis,  and  in 
the  latter  case  the  former  values  may  not  again  be  obtained. 
The  reason  for  reversible  changes  in  viscosity  is  probably  to  be 
found  in  an  alteration  of  the  structure  of  the  gelatin  sol.  This 
"fra*  been  discussed  in  Chap.  III. 

The  Previous  Mechanical  Treatment. — A  decided  reduction  in 
viscosity  is  also  noted  as  a  result  of  vigorous  agitation,  stirring, 
or  shaking,  or  even  the  passage  of  the  solution  several  times 
through  the  capillary  of  a  viscosimeter  tube.  This  phenomenon, 
as  that  of  the  reduction  of  viscosity  by  preliminary  heating, 
seems  also  to  indicate  a  structure  of  some  kind  in  the  gelatin  sol. 
v — ^feis  subject  is  discussed  at  greater  length  in  Chap.  III. 

f ^  Inoculation    with    Other    Colloids. — The    addition    of    small 

V  quantities  of  a  viscous  colloid  have  been  found  to  raise  the 

\        viscosity,  especially  after  standing  for  some  time,  to  an  incom- 
\     parably  higher  degree  than  could  be  attributed  to  the  increase 
\  in  concentration  due  to  such  addition.     A  small  piece  of  aged 
J  or  gelatinized  gelatin,  for  example,  was  found  by  Garrett1  to 
k^ereatly  accelerate  the  rate  of  increase  of  viscosity  with  time. 
V   Time. — When  the  temperature  is  not  greatly  above  the  con- 
gealing   point    of    gelatin,  very  marked    changes    in   viscosity 
with  time  are  observed,  the  direction  showing  an  increase.     As 
the  temperature  becomes  further  and  further  removed  above 
the  point  of  gelation,  however,  the  changes  in  viscosity  with 
time  become  very  small,   pass  through  a  zero  point,  and  at 
higher  temperatures  show  a  decrease  with  time.2     Experiments 
by  von  Schroeder3  show  an  increase  of  750  per  cent  in  the  vis- 
cosity of  a  gelatin  solution  on  standing  for  60  minutes  at  21°C., 
while  at  24.8°  the  increase  was  1.5  per  cent,  and,  at  31°,  less 
than  0.1  per  cent.     The  following  table  presents  his  data: 

1  GARRETT,  Phil  Mag.  (6),  6  (1903),  374. 

2  See  page  150. 

3  P.  VON  SCHROEDER,  Z.  physik.  Chem.,  46  (1903),  75. 


GELATIN  AS  A  LYOPHILIC  COLLOID 
TABLE  37. — INCREASE  IN  VISCOSITY  WITH  TIME 


197 


Time  in  minutes 

At  21.0° 

At  24.  8° 

At  31.0° 

5 

1.83 

1.65 

1.41 

10 

2.10 

1.69 

1.41 

15 

2.45 

1.74 

1.42 

30 

4.13 

1.80 

1.42 

60 

.13.76 

1.90 

1.42 

These  results  are  shown  graphically  in  Fig.  26. 
The  effect  of  the  addition  of  some  salts  upon  the  viscosity-time 
curve  has  been  studied  by  Gokun.1     He  found  that  the  additions 


10          20  3.0  4-0  50 

Time,  minutes        > 

FIG.  26. — Increase  in  viscosity  of  emulsoids  with  time.     (According  to  the  experi- 
ments of  P.  von  Schroeder,  S.  J.  Levites  and  W.  Biltz.) 

of  small  amounts  of  ammonium  nitrate  accelerated  the  increase 
of  viscosity  with  time;  amounts  between  0.32  and  1.4N  resulted 
in  no  increase  in  viscosity;  and  still  larger  amounts  of  the  salt 
produced  a  decrease  in  viscosity  with  time,  as  takes  place  in 
most  suspensoids.  Fischer  explains  these  apparently  antagon- 
istic salt  effects  by  assuming,  first,  an  emulsification  of  the 
hydrated  salt  (solution)  in  the  gelatin,  and,  second,  that  this  is 
succeeded  by  emulsification  of  the  gelatin  in  the  hydrated  salt 
(solution) . 
The  author2  has  found  that  the  initial  very  high  viscosity  of 


1  GOKUN,  Kolloid-Z.,  3  (1908),  84. 

2  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  5. 


198 


GELATIN  AND  GLUE 


glues  which  have  been  treated  with  alums  declines,  rapidly  with 
time  until  it  approaches  the  initial  value  of  the  untreated  glue. 
In  the  case  of  glues  which  had  been  treated  with  formaldehyde, 
however,  the  opposite  effect  was  observed:  the  viscosity  rose 
very  rapidly  with  time.  These  effects  are  illustrated  in  Figs. 
27  and  28. 

Addition  of  Foreign  Substances. — On  account  of  the  diversity 
of  the  possible  substances  which  may  be  added  to  gelatin, 
both  electrolytes  and  non-electrolytes,  and  the  almost  inde- 


5    10    15  EO   25  30  35  40  45   50  55  60  65  70 

Time  in  Minutes 
Fio.  27. — The  effect  of  time  upon  the  viscosity  of  alum-treated  glues. 

finite  degrees  in  which  such  substances  may  affect  the  ioniza- 
tion,  the  hydration,  the  degree  of  dispersion,  etc.,  of  the  product, 
it  seems  at  first  sight  an  almost  hopeless  proposition  to  arrive  at 
any  satisfactory  basis  for  classifying  such  substances.  Fortu- 
nately this  phase  of  the  gelatin  problem  has  received  considerable 
attention  at  the  hands  of  capable  investigators.  Previous  to  1916 
however  practically  no  success  had  been  attained  in  the  formula- 
tion of  a  scientific  basis  of  classification:  all  such  schemes  were 


GELATIN  AS  A  LYOPHILIC  COLLOID 


199 


of  an  empirical  nature,  as  the  well  known  Hofmeister1  and  Pauli2 
series.  Loeb3  has  come  to  the  conclusion  that  it  is  not  only 
misleading  but  quite  incorrect  to  express  the  effects  of  the  several 
ions  on  viscosity  and  other  properties  in  terms  of  an  empirical 
order.  Loeb  worked,  it  should  be  noted,  with  quantities  of 

84 


0   10  20  30  4-0   50  60  70  80  90 

Time ,  minutes 
FIG.  28. — The  effect  of  time  upon  the  viscosity  of  formaldehyde-treated  glues. 

acids  and  bases  which  produced  equal  hydrogen-ion  concentra- 
tions in  the  solutions,  while  Pauli  and  other  previous  investigators 
in  this  field  had  compared  the  effects  of  equal  quantities  of 
acids  or  bases.  The  diversity  of  their  findings  may  very  likely 
be  ascribed  to  this  fundamental  difference  in  their  method  of 
experimentation.  Fischer,  however,  in  working  with  anhydrous 
soaps  and  solvents  like  alcohol,  benzene,  etc.,  has  obtained  the 
whole  Hofmeister-Pauli  series. 

1  HOFMEISTER,  Arch,  exptl.  Path.  Pharm.,  24  (1888),  247.     Vide  page  243. 

2  PAULI,  Beitr.  physiol.  path.  Chem.,  3  (1903),  225;  Fortschr.  naturwiss. 
Forschung,  4  (1912),  237. 

3  J.  LOEB,  J.  Gen.  Physiol.,  3  (1920),  85. 


200 


GELATIN  AND  GLUE 


A  detailed  treatment  of  this  subject  will  be  deferred  to  the 
following  chapter,  where  all  phases  of  the  ionic  and  amphoteric 
character  of  gelatin  will  be  considered.  In  Fig.  29  will  be  found 
curves  reproduced  from  data  obtained  in  the  author's  laboratory1 
which  illustrate  the  great  differences  in  viscosity  produced  by 
varying  additions  of  electrolytes.  It  has  already  been  pointed 
out  that  the  differences  observed  are  in  all  probability  due  to 

14 
70 
66 
62 


lO 

358 

v/) 


54 
50 
4G 


---T-T  ^&&&gfcZ/&te 

Sulphuric  Acid  (c.c.  ^T^      > 


^ 


$&& 


\ 


28 


0         4          &          IZ         16         EO       24 

Per  Cent  of  Substance  Added 
FIG.  29. — The  effect  of  added  substances  upon  viscosity. 

alterations   produced  in  ionization,   hydration,    and   degree   of 
dispersion. 

The  Theories  of  Viscosity.  Hatschek's  Theory. — A  mathe- 
matical treatment  of  the  principles  of  vicosity  is  one  phase  of 
the  colloid  situation  which  has  not  received  a  very  extended 
investigation.  In  1906  Einstein2  published  the  first  paper  which 
considered  the  general  expression  of  a  law  covering  the  viscosity 
of  colloid  solutions.  Einstein  by  a  thermodynamic  determina- 
tion of  Avagadro's  constant3  obtained  the  expression  for  viscosity : 

1  R.  H.  BOGUE,  loc.  cit. 

2  A.  EINSTEIN,  Ann.  Physik.,  39  (1906),  289. 

3  Avagadro's  constant  is  the  number  of  molecules  in  one  c.c.  of  gas  or 
liquid  at  0°C.  and  760  mm.  pressure.     See  Lewis  "System  of  Physical  Chem- 
istry, 2nd  ed.,  London  (1918),  42. 


GELATIN  AS  A  LYOPHILIC  COLLOID  201 


(i) 

where  r/  is  the  viscosity  coefficient  of  the  liquid,  or  continuous 
phase,  rj  the  viscosity  of  the  system,  and  /  the  ratio  of  total 
volume  of  particles,  or  disperse  phase,  to  total  volume  of  the 
system.     Hatschek1  arrived  at  a  nearly  identical  formula  by  a^ 
somewhat   different  line   of   reasoning.     He   argued   that  if   a( 
liquid  is  contained  between  two  parallel  plates,  the  lower  of/ 
which  is  stationary  while  the  upper  is  moved  at  constant  velocity,  | 
the  liquid  at  the  upper  pole  of  each  particle  in  suspension  will  \ 
move  at  a  somewhat  greater  velocity  than  at  the  lower  pole, 
"  which  is  equivalent  to  a  translatory  movement  of  the  particles 
with  a  velocity  equal  to  half  the  difference  of  the  two  velocities 
prevailing  at  the  two  poles."     On  carrying  through  this  calcula- 
tion Hatschek  obtains  the  formula  :     ^ 

V  =  17(1  +4.5/).  (2) 

It  must  be  emphasized  that  the  above  formulas  do  not  contain 
any  functions  of  either  the  radius  of  the  particles  in  suspension, 
or  the  distance  by  which  such  particles  are  separated,  which 
reduces  the  expression  to  a  statement  that  the  viscosity  of  the 
system  is  independent  of  the  size  of  the  particles,  and  is  a  linear 
function  of  the  volume  of  the  disperse  phase  only.  The  formula 
assumes  that  the  particles  are  spherical,  of  a  smooth  surface,  and 
that  they  do  not  carry  any  layer  of  adsorbed  solvent.  Also, 
inasmuch  as  Stokes'  formula  is  employed  in  the  calculation,  the 
formula  of  Hatschek  holds  good  only  between  those  limits  defined 
by  Stokes'  law.2 

Bancelin,3  using  a  factor  of  2.9f,  found  the  expression  to  hold 
for  gamboge  and  mastic,  and  Harrison4  also  has  found  good 
agreement  in  starch  solutions  up  to  30  per  cent  of  disperse  phase, 
but  found  the  constant  4.75  must  be  employed.  In  an  investiga- 
tion upon  sulphur,  however,  Ode*n5  was  not  able  to  confirm  the 
equation  of  Hatschek,  for  he  found  that  when  the  concentration 

1  E.  HATSCHEK,  Kolloid-Z.,  7  (1910),  301;  8  (1911),  34;  Trans.  Faraday 
Soc.,  9  (1913),  80. 

2  Stokes'  law  is  expressed  by  the  equation  C  =  6  -jrrjr  in  which  C  is  a  con- 
stant dependent  on  the  frictional  resistance  of  the  molecule,  77  the  viscosity 
of  the  solvent,  and  r  the  radius  of  the  molecule  of  the  solute.     See  LEWIS, 
loc.  cit.,  24. 

3  BANCELIN,  Kolloid-Z.,  9  (1911),  154. 

4  HARRISON,  J.  Soc.  Dyers  and  Colonists,  27  (1911). 

5  S.  ODEN,  Z.  physik.  Chem.,  80  (1912),  709. 


202  GELATIN  AND  GLUE 

of  his  sulphur  solution  was  raised  above  a  certain  value  the  vis- 
cosity increased  more  rapidly  than  in  linear  ratio,  and  that  the 
viscosity  of  sols  containing  small  particles  is  higher,  for  equal 
weights,  than  the  viscosity  of  sols  of  larger  particles.  Hatschek 
in  commenting  upon  these  discrepancies  recalls  that  all  that  is 
definitely  known  about  the  disperse  phase  is  its  weight,  whereas 


1 

1 

1 

I 

1 

1 

1 

1 

1 

1 

V 

• 

^^1 

B 

FIG.  30. — Hatschek's  conception  of  the  structure  of  emulsoid  systems  at  rest  (A) 
and  in  process  of  shear  (B). 

the  formula  requires  an  expression  of  volume.  Assumptions  are 
commonly  made  to  the  effect  that  the  particles  are  of  the  same 
size  throughout;  of  the  same  geometrical  shape;  of  the  same 
density  irrespective  of  size,  etc.,  but  it  is  known  that  in  sub- 
stances of  microscopic  dimensions  such  assumptions  are  not 
justifiable.  It  is  furthermore  probable  that  a  rotatory  as  well  as 


GELATIN  AS  A  LYOPHILIC  COLLOID  203 

translatory  motion  is  imparted  to  the  particles  in  suspension, 
which  would  disturb  the  energy  relations  of  the  system.  Finally 
the  capillary  viscosimeter  may  not  be  properly  suited  to  vis- 
cosity measurements  of  colloid  solutions. 

In  order  to  account  for  the  high  viscosities  attained  by  emul- 
soids  Hatschek  further  assumes  that  the  disperse  phase  occupies 
a  large  proportion,  often  times  much  more  than  half,  of  the  total 
volume.  This  assumption  necessitates  the  development  of 
dedecahedral  shaped  particles,  each  face  of  which  is  adjacent  to  a 
corresponding  face  of  another  particle,  but  separated  therefrom 
by  a  film  of  the  dispersion  medium.  This  condition  is  shown  in 

A,  Fig.  30.     When  such  a  system  is  subjected  to  shear,  the 
polyhedra  must  slide  over  one  another  until  a  position  of  least 
resistance  is  attained,  and  in  this  latter  position  the  shearing  takes 
place  only  in  the  horizontal  films  of  continuous  phase  as  shown  in 

B,  Fig.  30.     Under  such  circumstances  neither  the  viscosity  of 
the  disperse  phase  nor  the  interfacial  tension  enter  into  the  calcu- 
lation, and  Hatschek  produces  the  following  equation,  in  which 
the  viscosity  of  the  continuous  phase  is  taken  as  unity: 


where  17  is  the  coefficient  of  viscosity  of  the  system,  and  A  is  the 
ratio  of  the  volume  of  the  system  to  the  volume  of  disperse 
phase. 

In  the  case  of  emulsoid  sols  the  actual  volume  of  either  phase 
cannot  be  definitely  measured,  and  it  is  necessary  to  make  some 
assumption  that  will  give  a  basis  for  calculating  the  value  of  A. 
Hatschek  therefore  assumes  that  "at  any  given  temperature  the 
volume  of  disperse  phase  is  a  constant  multiple  of  the  volume  —  or 
weight  —  of  the  dissolved  substance."  This  assumption  has  been 
tested  as  follows.  A  transformation  of  the  previous  formula  may 
be  written: 

'« 


The  value  of  A   may  thus  easily  be  found,   and  since   A  = 

total  volume          ... 

—  ;  —    —  T-  —        —  r  —  this  value  should  bear  a  constant  ratio  to 
volume  disperse  phase 

total  volume  . 

the  expression  —  .  ,,    ,.  —         —  r  —  >  which  will  be  called  A  ' 
weight  disperse  phase 

e.g.,  "the  phase  ratio  must  be  a  constant  multiple  of  the  per- 


204 


GELATIN  AND  GLUE 


centage  contents."  It  must  be  pointed  out,  however,  that, 
inasmuch  as  the  geometric  structure  upon  which  formula  (3)  is 
based  cannot  arise  at  concentrations  of  dispersed  substance  plus 
hydrated  solvent  which  is  attached  thereto  of  less  than  about  50 
per  cent  of  the  total  volume,  the  formula  may  not  be  expected  to 
function  at  lower  concentrations  than  50  per  cent.  At  lower 
values  the  ratio  of  viscosity  to  dissolved  substance  is  approxi- 
mately linear  as  shown  by  formula  (2). 

Application  of  Hatschek's  Theory. — The  applicability  of  Hat- 
schek's formula  to  gelatin  has  been  rigidly  tested  by  the  author. 1 


4-          56         7         8         9         10 
PerCent  Gelatin  Concentration 

FIG.  31. — Variation  in  K  with  concentration  of  gelatin. 

It  was  found  that  the  value  of  A  '/A,  representing  the  volume 
occupied  by  unit  weight  of  the  dispersed  phase,  was  not  a  con- 
stant with  varying  concentration,  but  that  this  value  rose 
regularly  to  a  maximum,  and  thereafter  fell  regularly  with 
increasing  concentration.  This  is  shown  in  Fig.  31.  This  effect 
moreover  is  not  confined  to  gelatin,  but  is  found  to  obtain  in  the 
glycogen  sol  of  Botazzi  and  d'  Errico2  and  the  casein  sol  of  Chick 
and  Martin,3  reported  by  Hatschek.4  Inasmuch  as  these  varia- 
tions are  prefectly  regular  and  invariably  noted  they  may  not  be 

1  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  43  (1921)  1764. 

2  BOTAZZI  and  d'ERRico,  Pfluger's  Arch.  Physiol,  115  (1906),  359. 

3  CHICK  and  MARTIN,  Kolloid-Z.,  11  (1912),  102. 

4  HATSCHEK,  Trans.  Faraday  Soc.,  9  (1913),  80. 


GELATIN  AS  A  LYOPHILIC  COLLOID  205 

attributed  to  experimental  error,  but  must  be  fundamental. 
This  decrease  in  volume  per  unit  weight  of  dispersed  material 
after  the  concentration  has  reached  a  particular  value  may  be 
due  either  to  an  increasing  surface  tension  of  dispersion  medium  or 
to  a  reversal  of  phase.  It  is  apparent  that  by  increasing  the 
concentration  of  the  dispersed  particles,  the  film  of  continuous 
phase  separating  them  will  become  ever  thinner  and  thinner, 
and  in  so  doing  vastly  increase  the  free  surface  of  dispersion 
medium.  Any  such  increase  in  free  surface  will  be  opposed  by 
surface  tension  forces,  and  the  thinner  the  film  the  greater  will  be 
the  force  opposing  further  increase  in  surface.  This  force  may 
be  conceived  of  as  entering  into  competition  for  the  solvent  with 
the  force  which  induces  solvation  in  the  dispersed  particles. 
Writing  this  in  the  form  of  an  equilibrium: 

surface  tension  ^  solvation  potential. 

It  is  observed  that  increasing  the  dispersion  medium  favors  solva- 
tion, while  increasing  the  dispersed  phase  raises  the  surface  tension 
in  the  continuous  phase  and  thereby  decreases  the  solvation. 
This  means  that  the  volume  per  unit  weight  of  dispersed  phase 
will  decrease  after  the  concentration  has  passed  a  certain  opti- 
mum value,  which  accords  with  the  experimental  findings. 

The  extent  of  such  decrease  was  shown  by  the  author  to  be 
defined  approximately  by  the  equation: 


where  V°m  is  the  volume  of  the  dispersion  medium  at  the  point 
where  the  value  A'  /A  has  attained  its  maximum,  and  Vm  the 
volume  of  dispersion  medium  at  any  other  concentration,  s  being 
the  surface  tension  effect  on  the  decrease  in  volume  per  unit 
weight  of  dispersed  phase  at  increasing  concentrations. 

Hatschek1  regards  it  as  probable  that  the  failure  to  obtain  a 
constant  value  for  K  may  lie  in  the  nonconformity  of  the  gelatin 
molecules  to  the  conditions  of  the  hypothesis,  e.g.,  they  may  not 
be  spherical  and  produce  simple  polyhedra  upon  crowding.  Or 
the  assumption  that  the  continuous  phase  is  pure  dispersion 
medium,  and  that  all  the  colloid  is  in  the  disperse  phase,  associ- 
ated with  some  of  the  dispersion  medium,  may  be  an  undue  sim- 
plification of  the  conditions.  "It  is  probable,"  he  writes,  "that 
the  dispersion  medium  also  contains  some  of  the  colloid,  and  the 

1  E.  HATSCHEK,  personal  communication. 


206  GELATIN  AND  GLUE 

ratio  colloid  in  disperse  phase  to  colloid  in  continuous  phase  may 
become  smaller  with  increasing  concentration."  To  this  Fischer 
would  add  that  the  gelatin  may  be  either  liquid  or  solid. 

Robertson's  Theory. — Robertson1  explains  the  high  viscosities 
of  emulsoid  sols  in  a  slightly  different  manner.  He  recalls  the 
researches  of  Graham2  who  showed  that  the  velocity  of  diffusion 
of  crystalloids  through  gelatin  jellies  was  very  nearly  as  great 
as  through  water,  and  the  confirmation  of  such  results  by  Voight- 
lander3  and  Hufner.4  Bechhold  and  Ziegler5  found  the  gelatin 
jellies  to  retard  diffusion  of  crystalloids,  but  to  a  degree  incom- 
parably smaller  than  would  be  expected  from  the  viscosity. 
Dumanski6  has  further  shown  that  the  conductivities  of  inorganic 
salt  solutions  in  gelatin  jellies  is  only  slightly  less  than  those  of 
equally  concentrated  solutions  in  pure  water.  Robertson  further 
emphasizes  the  fact  that  the  conductivity  of  non-colloid  solutions, 
as  sugar,  glycerine,  etc.,  is  profoundly  influenced  by  slight 
increases  in  viscosity,  whereas  in  colloid  solutions  the  con- 
ductivity varies  normally  as  the  dilution  quite  irrespective  of  the 
much  greater  accompanying  variation  in  viscosity. 

In  accounting  for  these  anomalies  Robertson  turns  to  the 
net-structure  theory.7  He  argues,  that,  if  the  protein  or  colloid 
be  assumed  to  possess  a  structure  somewhat  similar  to  that  of  a 
tennis  net,  it  will  offer  practically  no  resistance  to  the  passage 
through  it  of  rapidly  moving  small  particles,  except  as  the  parti- 
cles may  occasionally  impinge  upon  the  meshes  of  the  net. 
Therefore  conductivity,  which  is  a  measure  of  the  rate  of  migra- 
tion of  ions,  would  be  but  very  slightly  modified  due  to  the 
presence  of  the  gelatin  "net."  On  the  other  hand,  a  viscosity 
measurement  would  be  considerably  influenced  by  such  a 
structure,  for  all  methods  of  measuring  viscosity  would  involve  a 
deformation  of  any  such  structure.  We  are  unable  by  viscosity 
measurements  to  differentiate  between  viscosity  resulting  from 
a  structure,  and  to  that  attributable  only  to  what  is  known  as  the 
internal  friction.  Some  objections  have  been  raised  to  this 

1  ROBERTSON,  "Physical  Chemistry  of  the  Proteins"  (1918),  320. 

2  GRAHAM,  Trans.  Roy.  Soc.,  London,  140  (1850),  1;  805;  141  (1951),  483. 

3  VOIGHTLANDER,  Z.  physik.  Chem.,  3  (1889),  316. 

4  HUFNER,  ibid.,  27  (1898),  227. 

5  BECHHOLD  and  ZIEGLER,  ibid.,  66  (1906),  105. 

6  DUMANSKI,  ibid.,  60  (1907),  553. 

7  Vide  Chap.  III. 


GELATIN  AS  A  LYOPHILIC  COLLOID  207 

conception  of  viscosity,  and  discussion  of  them  will  be  found  in 
Chap.  Ill 

The  Viscosity-plasticity  Relationship. — The  significance  of 
a  transitional  temperature  in  gelatin  " solutions"  has  recently 
been  receiving  much  attention  by  chemists. 

In  a  general  sense  it  has  long  been  recognized  that  whereas 
very  dilute  solutions  (1.0  per  cent)  of  pure  gelatin  would  gel  at 
low  temperatures  (10°C.),  yet  that  above  certain  temperatures, 
roughly  placed  at  about  35°C.,  gelation  would  not  take  place  at 
any  concentration.  Exceedingly  viscous  solutions  might  be 
obtained,  but  the  ability  of  these  to  congeal  to  a  jelly  was  not 
observed  above  this  temperature. 

In  a  sense,  the  melting  point  of  a  gelatin  or  glue  has  been  taken 
as  the  critical  temperature,  as  is  the  case  with  crystalloids,  but 
melting  point  is  not  at  all  easily  obtained  or  even  defined  when 
such  substances  as  gelatin  are  under  consideration.  Many 
attempts  have  been  made  however  to  determine  this  property. 
Chercheffski1  measured  the  temperature  at  which  small  cubes 
of  the  jelly  become  soft  enough  to  lose  their  cohesion.  Kissling2 
noted  the  temperature  at  which  the  surface  of  a  jelly  in  a  test 
tube  placed  horizontally  became  inclined.  Winkelblech3  shook 
a  glue  solution  in  cold  water  until  the  material  had  reached  a 
consistency  such  that  the  thermometer  placed  vertically  therein 
remained  stationary.  Kiittner  and  Ulrich-  describe  the  use  of 
Cambon's  fusiometer  which  consists  of  a  metallic  bowl  of  given 
dimensions  and  weight.  A  glue  is  allowed  to  gel  therein,  a  rod 
being  held  in  a  vertical  position  in  the  solution.  The  whole  is 
then  placed  in  a  beaker  of  warm  water,  suspended  by  the  rod, 
and  the  temperature  at  which  the  bowl  drops  from  the  rod  taken 
as  the  melting  point.  Herold5  allowed  a  thermometer  to  become 
congealed  in  a  test  tube  of  glue,  and  noted  the  temperature  at 
which  the  tube  fell  away  from  the  thermometer  when  suspended 
in  warm  water.  C.  R.  Smith6  determined  the  temperature  at 
which  a  gelatin  solution  maintained  a  stipulated  degree  of  vis- 
cosity, as  determined  by  the  " bubble"  methods.  Sheppard  and 

1  CHERCHEFFSKI,  Chem.  Ztg.,  25  (1901),  413. 

2  KISSLING,  Z.  angew.  Chem.,  16  (1903),  398. 

3  WINKELBLECH,  ibid.,  19  (1906),  1260. 

4  KUTTNER  and  ULRICH,  Z.  offent.  Chem.,  13  (1907),  121. 
6  HEROLD,  Chem.  Ztg.,  34  (1910),  203. 

6  C.  R.  SMITH,  J.  Am.  Chem.  Soc.,  41  (1919),  146;  /.  Ind.  Eng.  Chem.,  12 
(1920),  878. 


208  GELATIN  AND  GLUE 

Sweet1  have  determined  the  temperature  at  which  bubbles  of 
air  under  a  definite  low  pressure  cease  to  flow  through  a  gelatin 
sol. 

None  of  the  above  may  be  regarded  as  absolute  melting  point 
determinations  in  the  classical  conception  of  the  term.  A  very 
appreciable  time  factor  enters  into  the  above  determinations 
which  prevents  an  exact  coincidence  of  the  melting  and  solidifi- 
cation or  setting  points.  The  fact  must  be  met  that  in  such 
systems  as  these  the  transition  from  the  hydrosol  to  the  hydrogel 
condition  is  continuous,  as  far  as  our  ability  to  measure  the 
" points"  of  change  is  concerned,  and  to  quote  Sheppard,  "both 
the  'melting  point '  and  the  'setting  point '  are  more  or  less  arbi- 
trary conceptions,  and  their  determination  depends  mainly  upon 
standardized  experimental  conventions."  Sheppard  defines 
"melting  point"  and  "setting  point"  by  an  application  of  Clerk 
Maxwell's  elasticity  theory: 

7?        ^ 

l¥ 

where  E  =  the  elastic  modulus,  t]  —  coefficient  of  viscosity,  and 
T  =  time  of  relaxation,  i.e.,  time  for  a  deformation  to  fall  to  l/e 
of  its  initial  value.  He  argues  that  "the  'melting  point'  is  the 
temperature  at  which  the  elastic  modulus  becomes  very  small. 
Since  i\  remains  of  considerable  magnitude,  this  can  only  be  by  T 
becoming  very  large.  Hence,  both  'melting  point'  and  'solidi- 
fication point'  (setting  point)  might  be  defined  as  the  converg- 
ence temperature  at  which  the  'time  of  relaxation'  becomes 
infinite." 

Fischer2  accounts  for  the  differences  observed  between  the 
temperatures  of  coagulation  and  of  melting  by  his  theory  of 
mutual  solubility  of  the  two  phases  in  the  gelatin  water  system, 
and  the  influence  of  heat  upon  that  solubility.  At  high 
temperatures  he  assumes  liquid  hydrated  gelatin  dissolved  in  water. 
At  low  temperatures,  water  dissolved  in  solid  hydrated  gelatin. 
At  intermediate  temperatures  an  equilibrium  of  the  two  may 
exist,  and  time  is  necessary  for  the  attainment  of  equilibrium  in  a 
system  of  two  mutually  soluble  substances. 

The  actual  melting  point  temperatures  obtained  by  the  several 
methods  mentioned  above  lie  for  the  most  part  between  30°  and 

1  S.  E.  SHEPPARD  and  S.  SWEET,  J.  Ind.  Eng.  Chem.,  13  (1921),  423. 

2  MARTIN  FISCHER,  "Soaps  and  Proteins,"  New  York  (1921),  p.  70  et  seq. 


GELATIN  AS  A  LYOPHILIC  COLLOID  209 

35°C.  when  a  high  grade  of  gelatin  is  employed,  and  at  somewhat 
lower  temperatures  with  the  lower  grades  of  gelatins  and  glues. 
But  at  best  these  methods  are  based  upon  arbitrarily  selected 
1 '  experimental  conventions. ' ' 

In  a  study  upon  the  mutarotation  of  gelatin  C.  R.  Smith1  has 
shown  that  at  temperatures  above  33°  to  35°C.  the  specific  rota- 
tion of  gelatin  is  practically  constant  at  about  —123°,  while  at 
temperatures  below  15°C.  the  specific  rotation  is  practically 
constant  at  about  —266°.  At  all  temperatures  intermediate 
between  35°  and  15°C.  the  rotation  varies  between  these  two 
limits.  Smith  arrives  at  the  conclusion  that  gelatin  in  aqueous 
solution  exists  in  two  modifications :  the  one  stabile  at  tempera- 
tures above  33°  to  35°C.  which  he  denotes  as  Sol  form  A,  and  the 
other  stabile  at  temperatures  below  15°C.  which  he  denotes  as 
Gel  form  B.  "  Between  these  temperatures  a  condition  of 
equilibrium  between  the  two  forms  exists  and  the  mutarotation 
observed  seems  to  be  due  to  the  transformation  of  one  form  into 
the  other  by  a  reaction  which  is  reversible  with  temperature." 

Smith  showed  further  that  the  presence  of  only  0.60  to  1.00 
per  cent  of  Gel  form  B  was  required  in  order  to  effect  gelation. 
Thus  at  temperatures  of  10°  to  15°C.,  at  which  the  gelatin  is 
completely  in  the  gel  state,  a  0.60  to  1.00  per  cent  solution  will 
gel,  while  at  33°  to  35°C.,  since  the  gelatin  is  here  completely 
in  the  sol  state,  it  will  not  gel  at  any  concentration.  At  a  very 
slightly  lower  temperature  and  a  high  concentration  there  may 
be  just  enough  of  the  Gel  form  produced  to  result  in  gelation. 

The  temperature  33°  to  35°C.  is  therefore  regarded  by  Smith 
as  the  maximum  gelation  temperature,  and  by  virtue  of  the 
significance  attached  to  it  as  the  equilibrium  temperature  between 
the  sol  and  gel  forms,  it  may  be  regarded  with  somewhat  more 
reason  as  a  critical  temperature,  and  is  based  upon  somewhat 
sounder  principles  than  the  melting  point  determinations  (as 
applied  to  gelatin)  .'V ^ 

Bingham  and  Green2  have  made  exhaustive  studies  of  the  laws 
of  plastic  flow,  the  measurement  of  plastic  flow,  and  the  applica- 
tions of  plastic  flow  to  industrial  processes,  and  Bingham^has 

1  C.  R.  SMITH,  loc.  cit. 

2  E.  C.   BINGHAM,   U.  S.  Bureau  of  Standards  Bull.  13  (1916-17),  309; 
E.  C.  BINGHAM  and  H.  GREEN,  Proc.  Am.  Soc.  for  Test.  Mat.,  19  (1919); 
640;  H.  GREEN,  ibid.,  20  (1920),  451. 

3  E.  C.  BINGHAM,  ibid.,  18,  Pt.  II,  (1918),  373. 

14 


210 


GELATIN  AND  GLUE 


described  a  variable  pressure  method  for  the  measurement  of 
viscosity. 

It  has  been  shown  that  a  viscous  liquid  will  start  to  flow  no 
matter  how  small  a  pressure  is  applied.  With  plastic  materials 
no  flow  takes  place  until  after  the  pressure  has  exceeded  a  certain 
definite  value.  Bingham  points  out  that  when  viscosity  deter- 
minations are  made  by  noting  the  volume  of  outflow  of  the  liquid 
in  a  given  unit  of  time,  and  this  volume  plotted  against  a  variable 
but  rigidly  controlled  and  accurately  measured  pressure,  an 
extension  of  the  curve  to  the  axes  will  pass  through  the  origin, 
or  zero  point  of  the  axes,  provided  the  substance  obeys  the  laws 


100 
80 


40 


20 


TS~ 


^ 


Gelatin  No.l,20PerCerrt  Sol.,  W'reSO 
I     I     I     I     I     I          Ml 


—    ooooooooo    oooooo    o    o    o 

O^COOJ^OCD^-COOOVP      o^-corJ^O      ^-CO     rti 

-   oj   oj   cv   K>  ro    ^Tj-^j-toi^vp    S<^>    r- 
Vfscostty  (Angular  Deflection) 

FIG.  32. — Viscosity-plasticity  curves. 

of  a  truly  viscous  liquid,  but  that  the  extension  of  the  curve  will 
fall  upon  the  pressure  axis  at  a  finite  distance  (/)  from  the  volume 
axis  if  the  substance  is  a  plastic  solid.  This  distance,  (/),  Bing- 
ham calls  the  yield  value,  and  defines  it  as  the  force  required  to 
start  the  flow.  It  is  found  to  be  a  function  however  of  the  size 
of  the  capillary  used,  as  well  as  of  the  material  itself. 

It  seems  that  equally  comparable,  although  perhaps  less  sensi- 
tive measurements  for  the  determination  of  the  viscosity-plasti- 
city relations  may  be  made  by  the  use  of  a  torsional  viscosi meter 
of  the  MacMichael  type.  By  varying  the  speed  of  rotation  of 
the  cup  the  same  effect  is  produced  as  by  varying  the  pressure  in 
the  capillary  tube  type  of  instrument.  A  study  of  gelatin  solu- 
tions was  conducted  in  the  author's  laboratory  by  this  method.1 


R.  H.  BOGUE,  /.  Am.  Chem.  Soc.,  44  (1922),  1313. 


GELATIN  AS  A  LYOPHILIC  COLLOID  211 

Viscosity-plasticity  Studies. — The  procedure  employed  was  as  follows. 
Several  lots  of  the  highest  quality  of  granulated  gelatin  were  utilized 
in  the  tests.  These  were  made  up  accurately  into  10,  20,  and  25  per  cent 
solutions  by  soaking  in  cold  water  for  one  hour,  dissolving  in  a  water  bath 
at  70°C.,  again  making  up  accurately  all  water  lost  by  evaporation,  and 
with  no  delay,  introducing  into  the  cup  of  the  viscosimeter.  The  latter 
is  at  the  same  temperature  as  the  gelatin  solution,  and  is  immersed  in  the 
water  bath,  which  is  a  part  of  the  instrument,  at  a  temperature  of  70°C. 
The  heating  process  for  bringing  the  gelatin  into  solution  is  made  as  brief 
as  possible.  The  cover  of  the  instrument  is  kept  on  the  cup  to  prevent 
evaporation  during  the  measurements. 

The  solution  was  kept  thoroughly  stirred  by  lifting  the  plunger  up  and 
down,  and  the  temperature  permitted  (by  use  of  the  electric  heating  unit) 
to  fall  very  slowly.  The  viscosity  was  taken  intermittently  as  the  solution 
cooled  until  it  became  too  viscous  to  measure. 

The  velocity  of  rotation  of  the  cup  was  carefully  adjusted  before  and 
after  the  measurements.  Each  series,  as  above,  was  measured  at  the  same 
velocity  of  rotation  throughout  the  temperature  range  from  60°  to  31°C.,  or 
lower,  and  then  the  velocity  changed.  Speeds  from  5  to  100  r. p.m. were  used. 

The  whole  was  repeated  for  the  three  concentrations  used,  and  again 
repeated  with  the  employment  of  differently  sized  wires  in  the  instrument. 

An  example  of  the  data  obtained  is  shown  in  graph  form  in  Fig.  32. 

In  the  foregoing  figure  the  velocity  of  rotation  is  plotted 
against  angular  deflection. 

An  examination  of  these  curves  shows  that  by  continuing  each 
downward  until  it  intercepts  the  axis  two  conditions  are  made 
manifest.  In  one  of  these  conditions  the  origin  of  each  curve 
is  the  zero  point  of  the  axes.  In  general,  all  curves  plotted 
from  temperatures  higher  than  34°C.  are  of  this  type.  In  the 
other  condition;  the  origin  of  the  curves  lies  at  some  point  on  the 
viscosity  axis  at  a  varying  distance  from  the  ordinate  representing 
r.p.m.  The  lower  the  temperature  the  further  is  the  point  of 
interception  with  the  abscissa  removed  from  the  convergence 
point  of  the  axes. 

This  seems  to  mean,  arguing  from  the  geometry  of  the  graphs, 
that  in  those  cases  where  the  intercept  lies  on  the  abscissa  an 
infinitely  small  velocity  of  rotation  will  result  in  a  viscosity 
deflection  of  finite  magnitude.  That  is,  the  gelatin,  under  those 
conditions,  offers  a  permanent  and  fixed  resistance  to  deforma- 
tion. It  is  an  elastic  body;  it  possesses  a  measurable  degree  of 
rigidity;  and  deformation  may  not  occur  until  after  a  certain 
minimum  of  pressure  exerted  against  it,  has  been  exceeded. 
And  these  are,  as  a  matter  of  fact,  the  very  attributes  which  are 
characteristic  of  plastic  substances. 


212  GELATIN  AND  GLUE 

If  it  were  necessary  to  carry  the  analogy  further  we  could  say 
that  the  distance  from  the  origin  of  the  axes  to  the  point  of  inter- 
section corresponds  very  closely  to,  although  it  is  not  identical 
with,  the  yield  value,  /,  as  obtained  by  Bingham's  method. 
The  magnitude  of  this  distance  may  correctly  be  taken  as  a 
measure  of  the  plasticity  of  the  material. 

It  will  be  observed  that  at  velocities  of  rotation  above  60 
r.p.m.  there  is,  in  some  cases,  a  slight  bending  of  the  curves 
away  from  the  velocity  axis.  That  is,  at  the  higher  speeds  of 
rotation,  the  observed  viscosity  is  somewhat  greater  than  should 
obtain  if  the  lower  (straight)  portion  of  the  curve  may  be  regarded 
as  most  correctly  expressive  of  the  true  theoretical  values.  The 
reason  for  this  bending  is  undoubtedly  to  be  found  in  an  instru- 
mental error  by  which  eddy  currents  are  set  up  within  the  liquid 
when  the  velocity  exceeds  a  certain  value.  There  is  also  very 
probably  produced  at  the  higher  velocities  a  slipping  of  the 
liquid  along  the  sides  of  the  cup  causing  it  to  move  to  a  greater 
degree  en  masse  rather  than  by  the  telescopic  shear  of  truly 
viscous  flow. 

Above  a  certain  temperature  (at  any  given  concentration)  the 
curves  follow  the  laws  of  truly  viscous  flow,  e.g.,  they  converge, 
when  extrapolated  to  the  axes,  at  the  origin.  In  other  words  the 
observed  angular  deflection  is  directly  proportional  to  the  speed 
of  rotation.  But  at  a  given  temperature  (for  a  given  concentra- 
tion), and  at  all  temperatures  below  this  point,  the  curves 
follow  the  laws  of  plastic  flow  as  above  pointed  out.  Our 
"solution"  of  gelatin  behaves,  therefore,  as  a  viscous  liquid  at 
elevated  temperatures,  and  as  a  plastic  solid  at  low  temperatures 
(but  still  above  the  solidification  point). 

If  we  may  accept  C.  R.  Smith's  conclusions  that  above  33°  to 
35°C.  the  sol  form  only  may  exist,  while  below  that  temperature 
increasing  amounts  of  the  gel  form  are  in  equilibrium  with  the 
former  until  at  15°  the  gel  form  only  is  stabile,  then  it  seems  to 
follow  from  the  data  observed  that  gelatin  sol  is  a  viscous  liquid 
while  very  small  amounts  of  gelatin  gels  are  sufficient  to  impart 
to  the  "solution"  the  properties  of  plastic  flow.  (See  also  page 
148.) 

4.  THE  THEORY  OF  EMULSIONS 

Since  gelatin  and  glue  are  frequently  made  use  of  in  the  prepa- 
ration of  emulsions,  it  becomes  necessary  that  the  emulsion  condi- 


GELATIN  AS  A  LYOPHILIC  COLLOID  213 

tion  be  understood  and  the  several  theories  upon  emulsification 
be  presented. 

Technically,  an  emulsion  differs  from  an  emulsoid  only  in  the 
size  of  the  dispersed  particles.  It  consists  essentially  of  small 
droplets  of  one  liquid  dispersed  in  another  liquid.  In  order  to 
bring  this  condition  about,  however,  it  is  usually  necessary  that  a 
third  substance,  known  as  an  emulsifying  agent,  be  present. 
Emulsions  which  are  produced  without  an  emulsifying  agent  are 
invariably  of  only  transient  existence.  Violent  shaking,  for 
example,  of  an  oil  and  water  may  produce  an  emulsion,  but 
unless  some  stabilizing  substance  is  present,  it  will  break  immedi- 
ately when  the  shaking  is  stopped.  It  is  in  the  capacity  of  an 
emulsifying  agent  that  gelatin  or  glue  are  often  used. 

Types  of  Emulsions. — Emulsions  are  customarily  considered 
as  of  two  types:  (1)  the  oil  in  water  type,  which  consists  of  drop- 
lets of  oil  dispersed  in  water,  and  (2)  the  water  in  oil  type  which 
consists  of  droplets  of  water  dispersed  in  the  oil. 

The  two  types  may  be  distinguished  in  several  ways.  If 
water  is  the  external  phase,  the  emulsion  will  wet  substances  in 
the  usual  way,  while  if  oil  is  external  it  will  feel  oily  and  produce 
the  typical  grease  spot  on  paper.  If  a  drop  of  emulsion  is 
added  to  another  drop  of  the  external  phase,  it  will  disperse 
and  mix  freely,  but  on  its  addition  to  another  drop  of  the  internal 
phase  there  will  be  no  tendency  for  the  two  to  unite.  Robertson1 
suggested  applying  the  dye  Soudan  III  which  is  soluble  in  oil  but 
not  in  water.  On  shaking  with  an  emulsion  where  oil  is  the 
external  phase  the  entire  emulsion  will  be  stained  red  but  if  the 
emulsion  is  of  the  oil-in-water  type  only  the  oil  droplets  will  be 
colored.  Thomas2  does  not,  however,  regard  this  as  a  reliable 
means  of  determining  the  distribution  of  phases.  Newman3  has 
used  iodine  in  his  benzene-water  emulsions,  the  iodine  being 
soluble  in  the  benzene  and  not  in  the  water,  thus  confining  itself 
to  the  phase  in  which  the  benzene  is  present.  Methyl  orange  was 
similarly  used,  it  being  soluble  in  water  but  insoluble  in  benzene. 

Whether  the  water-in-oil  or  the  oil-in-water  type  of  emulsion 
will  be  formed  under  any  given  set  of  conditions  has  been  the 
subject  of  a  great  deal  of  discussion.  Walther  Ostwald4  in  1910 

1  ROBERTSON,  Kolloid-Z.,  1  (1910),  7. 

2  A.  W.  THOMAS,  personal  communication. 

3  NEWMAN,  J.  Phys.  Chem.,  18  (1914),  34. 

4  WALTHER  OSTWALD,  Kolloid-Z.,  6  (1910),  103;  7  (1910),  64. 


214  GELATIN  AND  GLUE 

urged  that  the  ratio  of  the  concentrations  of  the  two  phases  was 
the  determining  factor.  He  argued  that  for  any  two  immiscible 
substances  there  was  a  critical  concentration,  upon  one  side  of 
which  the  emulsion  would  be  of  one  type,  and  upon  the  other 
side,  the  other  type.  There  has  been  much  opposition  to  this 
point  of  view.  Bancroft1  maintained  that  an  elasticity  of  shape 
could  be  assumed  whereby  the  space  between  the  particles  might 
be  vanishingly  small,  and  Briggs  and  Schmidt2  have  shown  that  at 
a  given  concentration  either  type  could  be  produced  by  a  proper 
selection  of  emulsifying  agent. 

Bancroft's  Theory  of  Emulsion  Formation, — A  lowering  of  the 
surface  tension  of  one  of  the  two  liquids  has  long  been  regarded 
as  a  most  important  factor.  Plateau3  in  1870  and  Quincke*  in 
1888  suggested  the  influence  of  " surface  activity."  Bancroft5 
states  that  "if  the  surface  tension  between  Liquid  A  and  the 
emulsifying  agent  is  lower  than  the  surface  tension  between 
Liquid  B  and  the  emulsifying  agent,  Liquid  A  will  be  the  dis- 
persing and  Liquid  B  the  disperse  phase."  He  adds  that  an 
aqueous  colloid  (hydrophile)  as  an  emulsifying  agent  will  tend  to 
make  water  the  external  or  continuous  phase,  while  a  non-aqueous 
colloid  (hydrophobe)  will  tend  to  make  water  the  internal  or 
dispersed  phase.  This  idea  was  also  early  advanced  by  Fis- 
cher.6 Thus  gelatin,  being  a  hydrophile,  tends  to  produce  the 
oil-in-water  type  of  emulsion.  In  general,  if  the  emulsifying  agent 
is  wetted  more  easily  by  water  than  by  oil,  then  water  will  be  the 
external  phase,  while  if  the  emulsifier  is  more  easily  wetted  by 
the  oil,  then  the  oil  will  be  the  external  phase. 

In  order  that  a  substance  may  function  as  an  emulsifying 
agent,  it  must,  according  to  Pickering7  and  Bancroft,8  pass  into 
the  dineric  interface  (the  surface  separating  the  two  liquid  phases), 
and  form  a  coherent  film  there.  Unless  such  a  coherent  film 
is  formed  the  emulsion  will  crack,  and  if  it  does  not  pass  into  the 
dineric  interface  at  all,  no  film  will  be  formed  around  the  drop- 
lets of  the  dispersed  phase,  which  Bancroft  regards  as  funda- 

*  W.  D.  BANCROFT,  J.  Phys.  Chem.,  10  (1912),  179. 
2  BRIGGS  and  SCHMIDT,  ibid.,  19  (1915),  478. 
»S.   PLATEAU,  Ann.  Physik.,  141  (1870),  44. 
*G.  QUINCKE,  ibid.,  271  (1888),  580. 

5  W.  D.  BANCROFT,  /.  Phys.  Chem.,  17  (1913),  501. 

6  Cf.  MARTIN  FISCHER,  " Edema  and  Nephritis,"  New  York  (1915). 

7  S.  N.  PICKERING,  cit.  sup. 

s  W.  D.  BANCROFT,  /.  Phys.  Chem.,  19  (1915),  273. 


GELATIN  AS  A  LYOPHILIC  COLLOID  215 

mental  to  the  production  of  a  permanent  emulsion.  Thus  in  an 
emulsion  of  oil  and  water,  in  which  gelatin  is  used  as  the  emul- 
sifier, the  gelatin  would,  according  to  Bancroft,  concentrate  at  the 
dineric  interface  of  the  oil  and  water,  as  the  system  was  shaken 
and,  the  gelatin  being  a  hydrophile,  form  an  elastic  continuous 
film  around  each  little  droplet  of  oil.  If  the  emulsifier  is  a 
solid,  he  concludes  from  experiments  by  Hofman1  and  by  Des 
Condres2  that  "the  solid  particles  tend  to  go  into  the  water  phase 
if  they  adsorb  water  to  the  practical  exclusion  of  the  other  liquid ; 
they  tend  to  go  into  the  other  liquid  phase  if  they  adsorb  the 
other  liquid  to  the  practical  exclusion  of  the  water;  and  they 
tend  to  pass  into  the  dineric  interface  in  case  they  adsorb  the 
two  liquids  simultaneously." 

Pickering's  Theory. — These  conclusions  are  very  similar  to 
those  reached  by  Pickering3  in  an  exhaustive  study  on  emulsions 
in  1907.  Pickering  found  that  the  insoluble  basic  salts  of  iron, 
copper,  nickel,  and  aluminum,  and  even  clays,  lime,  silica,  and 
plaster  of  paris  functioned  as  good  emulsifying  agents  with 
respect  to  mineral  oil  and  water.  He  concluded  that  the  essential 
factor  in  emulsification  was  the  formation  of  a  solid  film  around 
the  dispersed  droplets,  and  that,  in  general,  the  smaller  the  size 
of  the  particles  of  the  emulsifier,  the  more  stabile  would  be  the 
resulting  emulsion. 

Clowes'  Theory. — Clowes4  found  that  he  could  bring  about  a 
reversal  of  phase  in  an  olive  oil-water  emulsion,  where  sodium 
oleate  was  the  emulsifying  agent,  by  the  addition  of  proper 
amounts  of  calcium  chloride.  That  is,  his  sodium  oleate  was 
converted  to  calcium  oleate,  and  whereas  the  sodium,  potassium, 
and  lithium  soaps  are  soluble  in  water,  and  not  in  oil,  thereby 
emulsifying  oil  in  water,  the  soaps  of  the  divalent  and  trivalent 
metals  were  soluble  in  oil  and  not  in  water,  and  thus  emulsified 
water  in  oil.  Clowes  adheres  to  the  Pickering-Bancroft  theory 
of  film  formation,  but  finds  that  the  nature  of  the  film  changes. 
Calcium  oleate  alone  gives  one  type  and  sodium  oleate  alone  the 
other  type.  At  some  particular  mixture,  as  represented  by  some 
particular  ratio  of  calcium  to  sodium  oleate  in  the  solution,  the 
effects  of  the  two  oleates  will  just  balance. 

1  HOFMAN,  Z.  physik.  Chem.,  83  (1913),  385. 

2  DES  CONDRES,  Arch.  Entwicklingsmechanik,  7  (1898),  325. 

3  PICKERING,  /.  Chem.  Soc.,  91  (1907),  2001;  Kolloid-Z.,  7  (1910),  15. 

4  G.  CLOWES,  /.  Phys.  Chem.,  20  (1916),  407. 


216  GELATIN  AND  GLUE 

Fischer's  Theory. — Martin  Fischer1  takes  exception  to  the 
tenuous  film  theory  above  described,  on  the  ground  that 
Pickering  makes  assumptions  which  are  not  justified.  For 
example,  Pickering,  in  explaining  the  stability  of  an  emulsion  of 
oil  in  soap,  needs  to  assume  the  soap  always  to  be  contaminated 
with  stearin  particles,  which  Fischer  shows  is  not  necessarily 
the  case.  Fischer's  theory,  in  his  own  words,  is  as  follows:  "An 
emulsion  is  stabilized  only  through  the  addition  of  a  lyophilic 
(hydrophilic)  colloid.  The  amount  of  colloid  necessary  is 
relatively  great.  It  must  be  sufficient,  at  least,  in  the  production 
of  an  emulsion,  to  bind  all  the  water  if  an  emulsion  showing  real 
permanence  is  to  be  produced.  Differently  expressed;  the 
production  of  a  lasting  emulsion,  as  of  oil  in  water,  is  really 
never  obtainable  through  the  division  of  the  former  into  the 
latter,  but  only  through  the  division  of  the  oil  into  a  hydrated 
(solvated)  colloid."  Thus  when  it  is  said  that  gelatin,  for 
example,  favors  the  formation  and  stabilization  of  an  oil-in-water 
emulsion,  the  theory  of  Fischer  would  have  it  that  the  gelatin  is 
a  hydrophilic  colloid  which,  with  water,  forms  a  colloid  hydrate, 
and  the  oil  is  dispersed  in  this  medium:  not  in  water  per  se. 

The  various  emulsifying  agents  are  of  different  degrees  of 
efficacy  in  their  emulsifying  power  essentially  on  account  of  the 
various  solvation  potentials  which  they  display.  High  viscosity 
is  of  great  advantage,  but  is  necessarily  secondary  to  the  property 
of  hydration.  ''Lasting  emulsions  of  oil  in  gelatin  are  obtainable 
only  by  dispersing  the  oil  in  a  gelatin  mixture  of  a  concentration 
which  is  just  fluid  at  the  temperature  at  which  the  experiment  is 
carried  out.  If  with  such  a  gelatin  colloid  the  temperature  is 
raised  (and  its  degree  of  hydration  thereby  decreased)  a  less 
permanent  emulsion  results.  On  the  other  hand,  an  emulsion 
of  oil  in  gelatin  remains  fixed  if  the  mixture  is  chilled  to  below  the 
gelation  point  of  the  gelatin." 

That  the  hydration  theory  of  Fischer  and  the  interfacial  film 
theory  of  Bancroft  and  Pickering  are  not  necessarily  irreconcilable 
to  each  other  has  been  urged  by  Fischer,  although  Bancroft  and 
Clowes  have  taken  exception  to  the  hydration  hypothesis. 
When  Clowes  considers  that  the  stabilizing  effect  of  sodium  oleate 
upon  an  oil-in-water  emulsion  is  due  to  a  lowering  of  the  surface 
tension  of  water,  Fischer  regards  the  stabilization  as  being  due 

1  M.  FISCHER  and  M.  HOOKER,  Science,  N.  S.,  43  (1916),  468;  "Fats  and 
Fatty  degeneration,"  New  York  (1917),  29. 


GELATIN  AS  A  LYOPHILIC  COLLOID  217 

to  a  division  of  the  oil  in  a  highly  hydratable  sodium  soap.  The 
destructive  action  of  calcium  upon  the  emulsion  Fischer  con- 
siders as  due,  not  to  complicated  changes  in  the  surface  film,  but 
to  the  fact  that  the  calcium  oleate  is  an  only  slightly  hydratable 
soap. 

Holmes'  Theory. — In  a  more  recent  contribution  to  the  subject 
of  emulsions  Holmes  and  Child1  affirm  that  too  great  viscosity 
is  just  as  prejudicial  to  emulsion  stability  as  too  little.  They 
maintain  that  the  maximum  lowering  of  surface  tension  should 
be  secured,  but  that  this  is  obtained  just  as  well  by  0.3-0.4  g.  of 
gelatin  per  100  c.c.  of  water  as  by  1.0  g.  Viscosity,  they  assert, 
is  the  leading  factor  in  oil-water  emulsification  where  gelatin  is 
used  "not  the  maximum,  but  the  most  favorable  viscosity," 
which  is  in  fact  "only  a  little  beyond  that  of  water."  They  find 
no  evidence  that  gelatin  particles  form  adhesive  layers  about  the 
oil  droplets,  nor  do  they  find  evidence  that  as  the  oil  content  is 
increased,  the  gelatin  content  must  also  be  increased  to  maintain 
the  original  stability  of  the  emulsion,  as  would  be  required  if 
adhesion  layers  were  formed  about  the  oil  droplets.  "One 
gelatin  content  in  a  given  volume  of  water  can  be  selected  that 
will  make  the  best  emulsion  for  all  oil  contents."  Their  findings 
in  general  tend  to  favor  the  hydration  hypothesis  of  Fischer,  but 
they  do  not  distinguish,  as  Fischer  does,  between  liquid  hydrated 
gelatin  and  solid  hydrated  gelatin.  An  optimum,  rather  than 
the  maximum  viscosity,  may  be  explained  by  the  fact  that 
medium  sized  particles  seem  to  be  susceptible  of  the  highest 
degree  of  hydration2  and  it  is  the  gelatin  possessing  the  highest 
degree  of  hydration  which  will  produce  the  best  emulsion.  As 
viscosity  is  a  measure  of  the  size  of  the  particles,  an  optimum 
rather  than  a  maximum  viscosity  would  be  expected  to  produce 
the  best  emulsion. 

Winkelblech's  Theory.— In  1906  Winkelblech3  found  that  if  a 
hydrocarbon,  such  as  benzine,  benzene,  chloroform,  etc.,  is 
shaken  with  water  in  which  is  dissolved  a  little  gelatin  or  glue, 
a  stiff  emulsion  is  produced.  If  the  gelatin  was  present  only  in 
very  small  amounts,  a  layer  of  small  bubbles  formed  at  the  inter- 
facial  surface  which,  on  breaking,  left  a  permanent  whitish 

1  H.  HOLMES  and  C.  CHILD,  J.  Am.  Chem.  Soc.,  42  (1920),  2049. 

2  Cf.  M.  Fischer,  J.  Am.  Chem.  Soc.,  40  (1918),  303;  R.  H.  BOGUE,  J. 
Ind.  Eng.  Chem.,  14  (1922),  32. 

3  WINKELBLECH,  Z.  angew.  Chem.,  19  (1906),  1953. 


218  GELATIN  AND  GLUE 

£ 

film.  He  could  just  detect  as  small  an  amount  as  0.06  mg.  of 
gelatin  in  10  c.c.  of  water  in  this  way,  and  suggested  the  applica- 
tion of  the  procedure  for  the  estimation  of  gelatin.  From  a 
study  of  this  paper  Bancroft1  concludes  that  it  furnishes  proof 
that  an  interfacial  substance  (a  substance  which  tends  to  pass 
into  the  dineric  interface  of  any  two  liquids)  may  be  separated 
from  its  suspension  in  one  liquid  by  shaking  with  another  liquid 
in  which  it  is  interfacial.  That  is,  gelatin,  in  suspension  in 
water,  may  be  separated  from  the  water  by  shaking  with  a 
hydrocarbon,  such  as  benzene,  in  which  case  the  gelatin  will 
become  concentrated  in  the  interfacial  film. 

The  Breaking  of  Emulsions. — The  breaking  of  an  emulsion 
necessarily  involves  the  reverse  of  the  processes  making  for 
stabilization.  The  most  apparent  means  would  be  the  destruc- 
tion in  some  way  of  the  efficacy  of  the  emulsifying  agent.  If 
the  tenuous  interfacial  film  of  Pickering  and  Bancroft  is  dissolved 
or  otherwise  destroyed  the  emulsion  will  of  course  break.  If 
the  hydrophilic  colloid  of  Fischer  is  diluted  beyond  the  point 
where  it  is  able  to  take  all  the  water  offered,  or  is  so  influenced 
by  external  conditions  that  its  original  capacity  for  holding  water 
is  sufficiently  reduced,  then  the  emulsion  will  break.  Dehydra- 
tion by  the  addition  of  a  salt  is  often  effective.  Thomas2  suggests 
seven  methods  by  which  emulsions  may  perhaps  be  broken: 

1.  Addition  of  excess  of  dispersed  phase. 

2.  Addition  of  a  liquid  in  which  the  two  liquids  phases  are  soluble. 

3.  Destruction  of  the  emulsifying  agent. 

4.  Filtration. 

5.  Heating. 

6.  Freezing. 

7.  Electrolyzing. 

1  W.  D.  BANCROFT,  J.  Phys.  Chem.,  19  (1915),  297. 

2  A.  W.  THOMAS,  J.  Ind.  Eng.  Chem.,  12  (1920),  177. 


CHAPTER  V 
GELATIN  AS  AN  AMPHOTERIC  COLLOID 

The    behavior    of    the    proteins    contradicts 

the    idea    that    the    chemistry    of    colloids 

differs    from    the    chemistry    of    crystalloids. 

Jacques  Loeb     (1920). 

PAGE 

I.  The  Amphoteric  Character  of  the  Proteins 219 

1.  The  Significance  of  Amphoteresis 219 

2.  Amphoteresis  in  the  Proteins 221 

3.  The  Mechanism  of  Protein-salt  Formation 223 

4.  The  Theories  of  Protein  lonization 225 

5.  The  Electrolysis  and  Electrophoresis  of  Proteins 227 

6.  The  Combining  Capacity  of  Proteins 229 

7.  Hydrolytic  Dissociation  of  Protein  Salts 232 

8.  The  Dominant  Influence  of  the  Dibasic  and  Diacid  Protein 
Radicals  in  lonization 234 

9.  Recapitulation 235 

II.  The  Effect  of  Inorganic  lonogens  upon  Proteins 236 

1.  Precipitation  and  Coagulation 236 

2.  Robertson's  Theory  of  Protein-salt  Formation 239 

3.  Objections  to  Robertson's  Theory  of  Protein-salt  Formation .  .  241 

4.  Ion  Series  in  Protein  Precipitation,  Swelling,  etc 243 

5.  Applications  of  the  Laws  of  Classical  Chemistry  to  the  Protein- 
salt  Equilibrium 245 

6.  The  Isoelectric  Point  of  Gelatin 246 

7.  Loeb's  Method  for  the  Study  of  the  Gelatin-salt  Equilibrium..  247 

8.  Gelatin  Salts  and  Metal  Gelatinates. . : 248 

9.  The  Influence  of  Valency  upon  Protein-salt  Formation 253 

10.  The  Depressing  Action  of  Salts 260 

11.  The  Micelle  Theory  of  McBain 264 


I.  THE  AMPHOTERIC  CHARACTER  OF  THE  PROTEINS 

1.  The  Significance  of  Amphoteresis. — Whenever  any  chem- 
ical substance  is  capable  of  reacting  with  an  acid,  with  the 
formation  of  a  salt,  it  follows  that  the  substance  contains  basic 
groups  susceptible  of  neutralization,  and  if  it  reacts  with  a  base, 
acid  groups  must  be  present.  But  a  large  number  of  substances 
are  capable  of  exhibiting  either  basic  or  acidic  properties  depend- 
ing upon  the  conditions  to  which  they  are  subjected.  Probably 

219 


\ 


220  GELATIN  AND  GLUE 

all  of  the  oxygen  acids  are  of  this  class.1  Aluminum  hydroxide 
may  be  cited  as  typical.  This  substance  is  nearly  insoluble  in 
water,  but  readily  dissolves  in  acids  to  form  an  aluminum  salt, 
and  in  bases  to  form  a  metal  aluminate.  The  ionization  equilib- 
rium is  represented  as  follows:  * 

A1+++  +  3 (OH)-  <=>  A1(OH)8  *±  3H+  +  (A1O3)  =; 
and  3H+  +  (A1O3)S  <=>  H+  +  (A102)-  +  H20. 

The  term  amphoteric  was  given  by  Bredig2  to  all  substances 
possessing  this  power  of  combination  with  the  ions  of  either  an 
acid  or  a  base. 

The  action  of  the  acid  or  the  base  upon  amphoteric  substances 
of  this  type  is  very  simply  explained  by  the  law  of  mass  action, 
and  solubility  product,  of  Guldberg  and  Waage. 

According  to  the  law  of  mass  action, 

[AI+++]  X 


and 

[ff+]8  X  [A  10 


where  Kb  is  the  ionization  constant  for  the  basic  ionization  of  the 
aluminum  hydroxide,  and  Ka  the  ionization  constant  for  the  acid 
ionization  of  the  same.  The  brackets  signify  concentrations  in 
all  cases.  So  long  as  the  aluminum  hydroxide  is  present  in  solid 
form,  the  amount  of  this  substance  in  solution  will  be  constant, 
depending  upon  its  solubility  at  that  temperature,  and,  therefore, 
the  product  of  the  concentration  of  the  Al+++  ions  and  the 
concentration  of  the  (OH~)3  ions  will  be  a  constant.  But  if  now 
an  acid  is  added  the  concentration  of  the  hydroxyl  ions  must  be 
greatly  reduced,  for,  by  the  same  law 

[H+]  X  [OH-]  -  Kw, 

the  ionization  constant  for  water,  and  any  increase  in  hydrogen 
ions  must  therefore  result  in  a  decrease  in  the  hydroxyl  ions  in 
order  that  their  product  may  remain  constant.  But  as  the 
hydroxyl  ions  decrease,  more  aluminum  ions  must  be  produced 
in  order  that  [A1+++]  X  [OH~]3  remain  constant.  This  can  take 
place  only  through  the  dissociation  of  more  aluminum  hydroxide, 

1  J.  STIEGLITZ,  "Qualitative  Chemical  Analysis,"  New  York  (1916),  vol. 
1,  p.  175. 

2G.  BREDIG,  Z.  Electrochem.,  6  (1899),  33. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  221 

and  upon  continuing  the  process  the  latter  must  eventually  be 
brought  entirely  into  solution. 

The  addition  of  a  base  would,  in  an  entirely  similar  way, 
depress  the  hydrogen  ions,  and  result  in  an  increase  in  the 
aluminate  ions  which  would  likewise  result  in  solution  of  the 
aluminum  hydroxide. 

This  same  type  of  amphoteresis  is  found  in  the  organic  acids, 
as  shown  for  example  in  the  ability  of  acetic  acid  to  substitute 
either  the  hydrogen  or  the  hydroxyl  of  its  carboxyl  group  : 

CH3COOH  +  NaOH  -»  CH3COONa  +  H2O; 
and          3CH3COOH  +  PC13  ->  3CH3COC1  +  P(OH)3. 

2.  Amphoteresis  in  the  Proteins.  —  A  different  condition  is 
encountered  however  in  certain  organic  compounds,  wherein  the 
basic  and  the  acidic  properties  emanate  from  distinctly  different 
groups  within  the  molecule.  As  typical  of  this  latter  class  the 
amino-acids  are  most  important.  The  —  NH2  group  is  distinctly 
basic  and  will  combine  readily  with  acids,  while  the  —  COOH 
group  is  the  common  organic  acid  group,  and  combines  with 
bases.  Thus  in  amino  acetic  acid  or  glycine: 


CH  +HC1    ->   CH2\ 

XCOOH  XCOOH; 


and  CH2(  +  NaOH  ->  CH  +  H2O. 

XCOOH  XCOONa 

We  know  that  proteins  are  made  up  of  many  amino-acids  and 
if  it  could  be  shown  that  each  of  these  constituent  acids  carried 
its  amino  and  carboxyl  in  an  uncombined  state,  as  in  the  simple 
acid  illustrated,  then  no  further  explanation  would  be  required 
to  account  for  the  amphoteric  behavior  of  the  proteins.  Van 
Slyke  and  Birchard1  have  pointed  out  however  that  only  a  very 
small  portion  of  the  total  nitrogen  of  proteins  is  present  in  the  free 
condition.  They  report  the  following  data: 

TABLE  38.  —  PERCENTAGE  OF  TOTAL  NITROGEN  PRESENT  IN  FREE  AMINO 

GROUPS 
Haemoglobin  .................   6.0     Edestin  ......................    1.8 

Casein  .......................   5.5     Gliadin  ......................    1.1 

Hsemocyanin  ................   4.3     Zein  .........................   0.0 

Gelatin  ......................   3.1 

i  D.  D.  VAN  SLYKE  and  F.  J.  BIRCHARD,  J.  Biol.  Chem.,  16  (1913),  539. 


222  GELATIN  AND  GLUE 

After  a  complete  hydrolysis,  however,  in  which  process  the  acids 
are  liberated  from  their  combinations,  the  nitrogen  present  in 
free  amino  groups  ranges  from  60  to  80  per  cent  of  the  total 
nitrogen. 

.  /  As  acids  combine  with  the  proteins  in  much  greater  proportion 

than  is  represented  by  the  figures  in  the  table,  it  is  obvious  that 
still  other  groups  beside  the  free  amino  radicals  are  capable  of 
reaction  with  acids.  In  fact  Van  Slyke  and  Birchard  have  shown 
that  the  free  amino  nitrogen  in  proteins  corresponds  exactly 
to  one  half  of  the  nitrogen  represented  by  the  lysine  alone  that  is 
present.  Lysine  contains  an  a  and  an  co  amino  group,  and  as  the 
time  required  for  the  latter  to  interact  with  nitrous  acid  (30 
minutes)  is  the  same  as  that  required  by  the  protein,  but  is  much 
longer  than  is  taken  by  the  a  group  (3  minutes),  it  seems  highly 
probable  that  the  free  amino  nitrogen  of  an  unaltered  protein  is 
attributable  only  to  the  o>  amino  nitrogen  of  lysine.  Zein,  which 
contains  no  lysine,  yields  no  amino  nitrogen  with  nitrous  acid. 
Any  hydrolysis  will  however  result  in  the  formation  of  amino- 
acids,  and  then  all  a  groups  as  well  as  some  others  will  become 
free  and  respond  to  the  nitrous  acid  reaction. 

Inasmuch  as  the  amino  groups  are  very  readily  attacked  by 
nitrous  acid  when  in  the  form  of  — NH2,  but  are  not  affected 
while  in  the  protein  molecule,  it  follows  that  they  must  be  in  some 
kind  of  combination  in  the  latter.  The  exact  manner  in  which 
combination  is  effected  is  a  question  that  has  long  been  a  subject 
of  speculation.  The  most  commonly  accepted  type  is  that  of  a 
simple  condensation  between  the  amino  group  of  one  acid  and 
the  carboxyl  group  of  another.  In  this  manner  the  peptid 
glycylglycine  is  formed  from  the  condensation  of  two  molecules 
of  glycine: 

NH2CH2CO!OH  +  H!HNCH2GOOH  -* 

NH2CH2COHNCH2COOH  +  H2O. 

The  peptids  and  polypeptids  are  therefore  essentially  amino- 
acids,  and  are  capable  of  reacting  with  acids  or  bases  through 
their  terminal  amino  and  carboxyl  groups  respectively.  But  as 
previously  stated  such  reaction  is  not  confined  to  these  groups. 

This  has  been  demonstrated  in  a  number  of  ways.  Vernon1 
showed  that  the  capacity  of  a  hydrolyzed"  protein  to  neutralize 
bases, was  only  slightly  greater  than  that  of  the  unaltered  protein, 

1  H.  M.  VERNON,  /.  PhysioL,  31  (1904),  346. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  223 

and  Blasel  and  Matula1  and  Pauli  and  Hirschfeld2  have  prepared 
deaminized  gelatin  by  allowing  nitrous  acid  to  react  with  the 
protein,  and  found  that  acids  combined  with  the  product  to 
nearly  the  same  degree  as  with  the  untreated  gelatin.  As  there 
can  be  no  terminal  amino  groups  in  the  deaminized  product,  it  is 
obvious  that  the  acid  reacts  with  some  internal  groups.  And  as 
the  internal  — COHN —  groups  are  transformed  upon  hydrolysis 
to  — COOH  and  — NH2  groups  with  no  marked  increase  in  basic 
or  acidic  combining  capacity  it  seems  probable  that  the  — CO- 
HN—  groups  also  are  capable  of  neutralization,  and  responsible 
for  the  reactivity  of  the  proteins  with  electrolytes. 

Many  other  investigations  make  it  imperative  that  this  con- 
clusion be  accepted.  Osborne3  has  shown  that  edestin  combines 
with  acids  in  such  proportion  that  a  very  large  percentage  of  its 
neutralizing  power  must  be  derived  from  other  than  free  — NH2 
groups.  Osborne  and  Leavenworth4  found  that  edestin  combines 
with  cupric  hydroxide  in  exact  proportion  to  the  amount  of 
—COHN —  groups  which  are  present  in  the  unaltered  edestin 
molecule. 

3.  The  Mechanism  of  Protein -salt  Formation. — An  hypothesis 
for  the  mechanism  of  the  reaction  by  which  the  — COHN — 
groups  may  react  with  acids  and  bases  has  been  postulated  by 
Robertson.5  He  points  out  that  both  a  keto  and  an  enol  form 
of  that  group  may  exist,  as 

Keto  form  H     O 

!    I! 

H2N.CH2.CO— HN.CH2.COOH,  or  R— N— C— R; 

and 

Enol  form  OH 

H2N.CH2.C(OH)  =  N.CH2.COOH,  or  R— N  =  C— R. 

Of  these  the  latter  is  much  the  more  probable  as  it  permits  of  a 
combination  with  either  acids  or  bases,  while  the  former  may 
conceivably  neutralize  only  acids. 

1  L.  BLASEL  and  J.  MATULA,  Biochem.  Z.,  68  (1914),  417. 

2  W.  PAULI  and  M.  HERSCHFELD,  ibid.,  62  (1914),  245. 

3  T.  B.  OSBORNE,  /.  Am.  Chem.  Soc.,  24  (1902),  39. 

4  T.  B.  OSBORNE  and  C.  S.  LEAVENWORTH,  J.  Biol.  Chem.,  28  (1916),  109. 
5T.  B.  ROBERTSON,  "The  Physical  Chemistry  of  the  Proteins,"  N.  Y. 

(1918),  24-31. 


224 


GELATIN  AND  GLUE 


The  enol  form  may  react,  according  to  Robertson,  with  acids 
or  bases  in  the  following  ways: 


OH 


ONa 


Ri— N  =  C— R2  +  NaOH  ->  Rj— N  =  C— R2,  which  ionizes 

H       OH  ONa 

\/  I 

to  [Rj— N  =  ]=  +  [  =  C— R2]++;  and  (1) 

OH  H        Cl  OH 


Ri— N  =  C— R2  +  HC1  -»  Ri— N  =  C— R2,    which   ionizes 
H      Cl  OH 

V          ! 

to  [Ri— N  =  ]-  +  [  =  C— RJ++  (2) 

The  possible  combinations  with  the  diamino  acids  is  increased 
as  follows : 


(3) 


OH                                                                  OH 

++ 

H     OH 

1 

1 

V 

/C  =  N—  R2 

/°  = 

=  N—  R2 

Ri(                           -f  KOH  -f  H2O  -» 

-|- 

XJ  =  N—  Rs 

"!\c  = 

=  N—  Rs 

1 

1 

/\ 

OH 

OK 

H        OH 

OH 

OK~ 

-H- 

H          OH 

1 

1      • 

\/ 

/C  =  N—  R2 

xC- 

=  N—  R2 

R/                            +  2KOH  —  > 
\C  =  N—  Rs 

R/ 
\C  = 

+ 

=  N—  Rs 

1 

1 

/\ 

OH 

OK 

H         OH 

OH 

OH 

% 

~H         Cl~" 

1 

1 

\/ 

/C  =  RJ 

/C  = 

=  N—  R2 

Bif                          +  HC1  +  H2O  —  > 

RK 

-j- 

\C  =  Rj 

\C  = 

=  N—  Rs 

1 

1 

/\ 

OH 

OH 

H         OH 

OH 

OH~ 

-H- 

H         Cl 

1 

| 

\/ 

/C  =  R2 

R/                           +  2HC1  —  > 

R/C  = 

+ 

=  N—  R2 

\C  =  R. 

\C  = 

=  N—  Rs 

1 

I 

/\ 

OH 

OH 

H         Cl 

(4) 


(5) 


(6) 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  225 

Of  these,  number  (3)  is  not  known  to  exist,  but  all  of  the  other 
types  are  believed  to  have  been  observed. 

Owing  to  the  presence  of  both  a  positive  and  a  negative  group 
in  the  same  molecule  it  is  conceivable  that  these  may  neutralize 
each  other,  forming  an  internal  salt: 


/NH2  /NH3+  +  OH- 

R/  +  H20  ->  R(  ->  R(    \  +  H20, 

XCOOH  XCOO~  +  H+  XCOO 

NH 


and  R      \    ->  R      \  +  H2O. 
XCOO         XCO 

It  may  also  happen  that  in  the  combination  of  two  amino-acids 
interaction  may  take  place  between  both  of  the  acid  and  basic 
groups  of  both  acids,  also  resulting  in  a  ring  compound: 


^/ 

R\ 


HOOC, 

+  XR^R/  XR  +  2H2O. 

COOH       H2N    / 


Still  other  possible  combinations  have  been  suggested,  but 
those  listed  above  cover  the  more  important  known  cases. 

Schryver1  has  suggested  that  the  difference  in  the  solubility 
and  other  properties  between  the  globulins  and  the  albumins  may 
lie  in  the  relative  position  of  the  reactive  carboxyl  and  amino 
groups  in  the  molecule.  When  the  steric  structure  of  the  mole- 
cule is  such  that  the  above  mentioned  groups  may  react  with 
each  other  to  form  internal  anhydrides,  such  as  are  described 
above,  and  as  is  known  to  be  possible  in  most  amino-acids,  then 
the  resulting  molecule  is  water  soluble,  and  belongs  to  the  albumin 
class.  When,  however,  the  structure  does  not  permit  of  such 
anhydride  formation,  then  reaction  occurs  between  the  amino 
group  of  one  molecule  and  the  carboxyl  group  of  another  mole- 
cule, with  the  formation  of  a  compound  of  doubled  molecular 
weight.  This  is  insoluble,  and  belongs  to  the  globulin  class. 

4.  The  Theories  of  Protein  lonization. — It  is  of  special  signifi- 
cance to  observe  that  in  his  depiction  of  the  mechanism  of  the 
ionization  of  proteins  Robertson  regards  the  ionic  separation  as 
occurring  between  two  parts  of  the  protein  molecule  rather  than 
between  protein  on  the  one  hand  and  the  inorganic  cation  or 

1  S.  B.  SCHRYVER,  Proc.  Roy.  Soc.  (London),  83  (1910),  .96;  Kolloid-Z.,  8 
(1911),  233. 

15 


226  GELATIN  AND  GLUE 

anion  on  the  other.  This  view  is  in  the  nature  "of  a  departure 
from  that  which  had  previously  been  believed  to  obtain  in 
protein  ionization,  but  has  been  regarded  by  its  champions  as  a 
necessary  step  in  the  evolution  of  our  introspective  knowledge 
of  proteins.  So  long  as  it  was  acceded  that  neutralization 
occurred  only  at  the  terminal  amino  or  carboxyl  groups  of  the 
catenary  molecule,  it  was  necessary  to  grant,  a  priori,  that  the 
ionic  break  should  take  place  at  the  same  point: 

HOOC.R.NH2  +  HC1  -»  HOOC.R.NH3C1  -» 

HOOC.R.NH3+  +  Gl- 
and H2N.R.COOH  +  NaOH  -»  H2N.R.COONa  +  H2O-> 

HoN.R.COO-  +  Na+. 

But  with  the  accumulation  of  data  which  proved  both  directly 
and  indirectly  that  interaction  with  acids  and  bases  was  not 
confined  to  the  terminal  amino  and  carboxyl  groups,  but  was 
even  more  evident  with  the  internal  — COHN —  groups,  it 
became  necessary  for  investigators  to  turn  their  attention  more 
analytically  upon  the  point  of  ionic  rupture.  As  a  result  Robert- 
son1 has  accumulated  a  number  of  experiments  both  by  himself 
and  other  workers  which  in  his  belief  leave  little  occasion  for 
further  doubt  upon  this  point. 

Bugarsky  and  Liebermann2  have  shown  by  potentiometric 
means  that  "the  number  of  Cl~  ions  bound  by  a  given  mass  of 
protein  dissolved  in  dilute  hydrochloric  acid  is  exactly  equal  to 
the  number  of  H+  ions  which  it  binds." 

Oryng  and  Pauli3  observed  that  gelatin,  both  normal  and 
deaminized,  in  solution  in  potassium  chloride,  combined  with  a 
definite  proportion  of  Cl~  ions,  and  that  this  amount  was  increased 
greatly  upon  the  addition  of  acids. 

Blasel  and  Matula1  found  that  deaminized  gelatin  retained 
the  power  of  binding  Cl~  ions  from  solutions  of  hydrochloric 
acid  despite  the  absence  of  the  terminal  amino  groups. 

Robertson  has  prepared  caseinates  of  the  alkalies  and  alkaline 
earths  that  are  neutral  or  even  acid  to  litmus,  but  finds  neverthe- 
less that  these  caseinates,  which  can  contain  no  free  base,  are  still 
excellent  conductors  of  electricity,  a  neutral  solution,  for  example, 

1  T.  B.  ROBERTSON,  lib.  cit.,  167. 

2  S.  BUGARSKY  and  L.  LIEBERMANN,  Arch.  ges.  Physiol.,  72  (1898),  51. 

3  T.  ORYNG  and  W.  PAULI,  Biochem.  Z.,  70  (1915),  368. 
4L.  BLASEL  and  J.  MATULA,  ibid.,  58  (1914),  417. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  227 

showing  a  conductivity  of  92.7  X  10~3  reciprocal  ohms  per  equi- 
valent of  base  neutralized  at  30°C.  In  another  experiment 
Robertson  finds  that  the  conductivity  of  the  potassium  caseinate 
alone,  in  solution  of  potassium  chloride  of  varying  concentration 
from  0  to  0.3  N,  is  constant  within  the  limits  of  accuracy  of  the 
experiment.  Obviously,  if  dissociation  occurred  between  the 
casein  on  the  one  hand,  and  the  potassium  on  the  other,  a  decided 
increase  in  conductivity  must  result  as  more  potassium  is  intro- 
duced into  the  molecule,  and  the  constancy  of  the  above  data 
can  be  interpreted  in  no  other  way  but  that  the  ionization  of 
the  potassium  caseinate  does  not  yield  potassium  ions,  or  else 
(Fischer's  view)  that  there  is  no  ionization  at  all. 

By  employing  data  of  Kohlrausch  and  Holborn,1  Robertson 
has  calculated  the  equivalent  conductivity  of  a  number  of  basic 
protein  salts  at  infinite  dilution  and  finds  that  in  many  cases 
this  maximum  value  is  smaller  than  the  equivalent  conductivity 
of  the  inorganic  ion  alone.  Now  if  dissociation  took  place 
between  the  protein  and  the  inorganic  radical,  then  the  maximum 
conductivity  must  be  the  sum  of  the  equivalent  conductivity 
of  the  inorganic  ion  and  that  of  the  protein  ion.  It  must  in  all 
cases  therefore  be  greater  than  the  conductivity  of  the  inorganic 
ion  by  a  quantity  representative  of  the  conductivity  of  the 
protein  ion.  Since  however  it  is  found  in  many  cases  to  be  less, 
the  inference  is  that  dissociation  cannot  have  taken  place  in  the 
manner  postulated,  i.e.,  between  protein  and  inorganic  ion,  but 
must  rather  have  been  between  two  portions  of  the  protein  itself. 

5.  The  Electrolysis  and  Electrophoresis  of  Proteins. — Many 
experiments  have  demonstrated  that  certain  proteins  appear  to 
migrate,  under  the  influence  of  the  electric  current,  towards  the 
cathode  or  towards  the  anode,  depending  upon  the  condition  of 
solution  of  the  protein.  Thus  Hardy2  has  shown  that  dialyzed 
egg  albumin  migrated  under  electrical  tension  to  the  cathode  if  a 
little  acid  was  present,  and  to  the  anode  if  alkali  was  present. 
Similar  results  have  been  obtained  with  gelatin  and  a  number  of 
other  proteins,  the  only  difference  noted  being  in  the  exact 
degree  of  acidity  or  alkalinity  required  to  bring  about  a  given 
migration. 

The  fact  that  the  protein  appears  to  move  as  a  whole  and, 

1  F.  KOHLRAUSCH  and  L.  HOLBORN,  "Das  Leitvermogen  der  Electrolyte," 
Leipzig  (1898). 

2  W.  B.  HARDY,  /.  Physiol.,  24  (1899),  288;  33  (1905),  286. 


228  GELATIN  AND  GLUE 

under  a  given  set  of  conditions,  in  one  direction  only,  suggests  a 
refutation  of  Robertson's  theories  above  cited,  but  Robertson 
explains  the  observation  as  being  apparent  rather  than  real. 
According  to  his  views  the  protein  should  split,  a  part  migrating 
towards  the  cathode  and  a  part  towards  the  anode  regardless  of 
the  state  of  acidity  or  alkalinity  of  the  solution.  He  further 
maintains  that  this  is,  in  fact,  what  happens,  but  that  the  differ- 
ent types  of  change  taking  place  at  the  electrodes  makes  it  appear 
as  if  the  migration  were  in  one  direction  only.  Thus,  for  example, 
free  uncombined  protein  is  insoluble.  In  the  electrolysis  of  a 
protein  dissolved  in  a  base  the  anion,  after  migrating  to  the 
anode  and  neutralizing  any  base  that  may  be  present,  will  be 
precipitated  as  insoluble  protein.  The  cation,  on  the  other 
hand,  would  carry  to  the  cathode  an  excess  of  base  and,  as  protein 
is  soluble  in  alkaline  solutions,  precipitation  could  not  occur. 
Similarly,  in  the  presence  of  an  acid,  the  free  uncombined  protein 
must  be  precipitated  at  the  cathode,  but  not  at  the  anode,  as  at 
the  latter  point  an  excess  of  acid  would  result  in  its  solution. 

In  another  experiment  Robertson  shows  that  the  loss  of  casein 
from  the  anodal  region  of  an  alkaline  solution  of  casein  is,  upon 
electrolysis  for  2  hours  at  30°,  about  twice  as  great  as  that  from 
the  cathodal  region,  while  if  the  cations  consisted  of  potassium 
ions  the  loss  in  casein  from  the  anodal  region  would  be  at  least 
four  times  that  from  the  cathodal  region,  since  the  equivalent 
velocity  of  the  potassium  ions  is  at  least  four  times  that  of  the 
more  cumbersome  casein  ions. 

Objection  is  raised  to  Robertson's  views  of  ionization  by  Pauli, 
Samec,  and  Strauss1  upon  the  ground  that  (1)  the  hypothesis 
involves  the  dissociation  of  proteins  into  groups  that  are  not 
known  to  have  an  ex  parte  existence;  (2)  electrophoresis  experi- 
ments reveal  the  presence  of  but  one  protein  ion:  and  (3)  under 
certain  conditions  the  H+  and  Cl~  ions  may  be  bound  in  unequal 
proportions  by  proteins  in  hydrochloric  acid  solution. 

The  previous  discussion  has  for  the  most  part  expressed 
Robertson's  views  upon  these  objections.  He  urges  that  the  first 
point  is  beside  the  question,  for  the  ions  of  common  inorganic 
electrolytes  are  likewise  incapable  of  existance  when  separated 
from  the  electrical  field  in  which  they  are  found.  He  recalls,  in 
answer  to  the  second  point,  that  Stirling  and  Brito2  have  obtained 

1  PAULI,  SAMEC,  and  STRAUSS,  Biochem.  Z.,  59  (1914),  470. 

2  STIRLING  and  BRITO,  J.  Anat.  Physiol.,  16  (1882),  446. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  229 

a  simultaneous  deposition  of  crystals  of  haemoglobin  at  both 
electrodes  by  the  passage  of  a  direct  current  through  the  solution, 
and  that  Howell1  has  obtained,  with  fibrinogen,  an  increase  in 
concentration  at  both  poles.  That  an  inequality  in  the  binding 
of  Cl~  and  H+  ions  may  occur  is  taken  to  signify  only  a  difference 
in  affinity  of  the  nitrogen  atom  for  the  respective  ions  of  the 
hydrochloric  acid.  This  difference  seems  to  be  dependent  upon 
the  proportion  of  hydrochloric  acid  to  protein,  and  not  at  all 
upon  the  dilution  of  the  system. 

Robertson  raises  a  further  objection  to  his  hypothesis  based 
upon  the  unprecidented  breaking  of  a  double  bond  between  a 
carbon  and  a  nitrogen  atom  in  the  process  of  ionization.  He 
answers  it  however  by  calling  attention  to  the  findings  of  Gom- 
berg2  that  "the  precise  point  within  a  molecule  at  which  ioniza- 
tion may  occur  is  determined  by  the  strains  to  which  the  molecule 
is  subjected,  and  that  when  the  strain  is  unusually  great  the 
break  involved  in  ionization  may  occur  at  points  which  resist  the 
tension  due  to  strains  of  normal  magnitude."  He  concludes 
with  the  very  pertinent  argument  that  "the  additional  strains 
which  a  molecule  of  acid  or  base  introduces  into  the  molecule 
are  not  merely  those  commensurate  with  and  attributable  to  its 
weight,  but  also  strains  of  electrostatic  origin,  since  the  salt 
which  is  formed  unquestionably  undergoes  ionization.  It  may 
very  possibly  be  true  that  the  first  step  in  salt  formation  consists 
in  the  neutralization  of  end  — NH2  or  — COOH  groups,  but  that 
the  ionization  of  the  compound  formed,  leading  to  the  develop- 
ment of  electrostatic  tension  at  the  very  places  at  which  it 
must  exert  the  greatest  strain,  namely  the  extremities  of  the 
molecule,  results  in  the  splitting  of  the  otherwise  stable  linkage 

I 

— C=N —  and   the  redistribution   of  the   components   of  the 
molecule  and  the  strains  to  which  it  is  subjected."3 

6.  The  Combining  Capacity  of  Proteins.— A  measurement  of 
the  exact  combining  capacity  of  proteins  for  acids  and  bases  is  a 
subject  that  presents  many  technical  difficulties.  A  few  salient 
facts  have  however  been  observed.  Pauli  and  Hirschfeld4  studied 
the  combining  capacities  of  normal  and  deaminized  gelatin  (in 

1  HOWELL,  Am.  J.  Physiol.,  40  (1916),  526. 

2  M.  GOMBERG,  /.  Ind.  Eng.  Chem.,  6  (1914),  33. 

3  T.  B.  ROBERTSON,  lib.  cit.,  192. 

« W.  PAULI  and  M.  HIRSCHFELD,  Biochem.  Z.,  62  (1914),  245. 


230 


GELATIN  AND  GLUE 


addition  to  several  other  proteins)  for  acids  by  electrometric 
determination  of  the  hydrogen  ion  concentrations1  in  the  presence 
of  varying  amounts  of  the  acids.  They  found  that  the  extent  of 
dilution  of  the  system  had  no  material  effect  upon  the  maximum 
combining  capacity,  but  that  the  amount  of  acid  bound  depended 
rather  upon  the  ratio  of  acid  to  protein.  It  is  especially  signifi- 
cant that,  for  a  given  amount  of  gelatin,  the  percentage  of  bound 


0.015 


0.01 


*£  0.005 


0.01 


0.04 


0.05 


0.02  0.03 

Acid  Added 
FIG.  33. — Relation  between  acid  added  and  acid  bound.     (Pauli  and  Hirschfeld.") 

acid  increases  as  the  amount  of  acid  actually  present  increases. 
This  is  well  shown  by  Fig.  33,  taken  from  the  above  mentioned 
report. 

Similar  results  have  been  obtained  by  Robertson2  with  casein 
in  solution  with  potassium  hydroxide.  He  finds  that  potassium 
hydroxide  combines  with  casein  in  increasing  amount  as  the  ratio 
of  the  alkali  to  the  casein  in  the  system  increases.  By  interpolat- 
ing from  his  data  he  postulates  that  a  least  five  different  com- 
pounds of  the  base  with  the  casein  are  capable  of  existence,  and 
that  with  the  addition  of  each  equivalent  of  potassium  hydroxide  a 
new  set  of  — COHN —  groups  opens  up  in  ionization.  Thus,  in 
the  first  reaction,  the  casein  molecule  is  broken  into  two  ions  by  the 
introduction  of  one  equivalent  (11.4  X  10~5  equivalents  of  KOH 
per  gram  of  casein)  of  the  base.  A  second  portion  breaks  these 
into  four  ions,  and  so  on  until  at  the  maximum  combining  capa- 
city 16  equivalents  of  the  base  have  been  added,  and  16  — CO- 
HN—  groups  have  been  opened  up. 

This  same  process  is  ascribed  to  the  ionization  of  all  proteins. 
In  proteins  that  are  soluble  it  is  not  possible  to  obtain  directly 

1  The  determination  of  hydrogen  ion  concentration  by  electrometric  means 
is  discussed  in  the  Appendix. 

2  T.  B.  ROBERTSON,  lib.  tit.,  195. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  231 

the  minimum  equivalent  of  a  base  or  acid  that  will  produce  the 
first  ionic  break  in  the  molecule,  but  the  general  similarity  in 
the  action  of  the  several  proteins  makes  it  seem  probable  that 
the  nature  of  ionization  is  the  same  in  all  cases. 

The  equilibrium  between  gelatin  and  acids  and  bases  has  been 
studied  in  some  detail  by  Miss  Lloyd.1  She  found  that  the 
degree  of  swelling  varied  with  the  ratio  dry  weight  of  gelatin  to 
volume  of  0.001N  HC1  where  only  the  volume  was  a  variable. 
She  incorrectly  ascribes  this  to  the  mass  relation.  The  significant 
reason  for  the  variation  in  swelling  probably  lies  in  the  alteration 
in  the  pH  of  the  solution.  The  smaller  the  volume  of  solution 
the  less  the  total  hydrogen  ions  present,  and  the  more  rapidly 
will  the  pH  change  with  time.  The  swelling  is  defined  in  this 
instance  by  the  pH. 

Miss  Lloyd  found  further  that  gelatin  would  dissolve  com- 
pletely, in  the  course  of  a  few  days,  in  solutions  of  hydrochloric 
acid  and  of  sodium  hydroxide  that  were  0.01N  or  stronger;  the 
more  rapidly  on  the  alkaline  side,  but  with  the  greater  preliminary 
swelling  on  the  acid  side.  The  gelatin  chloride  reaction  was 
found  to  differ  fundamentally  from  the  sodium  gelatinate  reac- 
tion, however,  in  that  the  former  was  reversible  while  the  latter 
was  not.  This  was  demonstrated  by  the  following  procedure. 
The  gelatin  chloride  solution  was  neutralized,  the  gelatin  precipi- 
tated with  saturated  ammonium  sulphate  or  alcohol,  dissolved 
in  water  and  allowed  to  stand  in  the  cold.  A  gel  was  produced 
possessing  all  of  the  properties  of  the  original  gelatin  gel.  A 
similar  treatment  of  the  sodium  gelatinate  gave  a  solution  which 
would  not  gel  upon  cooling.  Miss  Lloyd  accounts  for  the  differ- 
ence on  the  ground  of  some  structure  alteration,  as  actual  degra- 
dation of  the  molecule  was  shown  not  to  have  occurred.  (No 
differences  were  observed  between  the  free  amino-acid  and  free 
ammonia  content  of  a  solution  in  sodium  hydroxide  and  one  in 
water.)  She  regards  it  as  probable  that  under  the  action  of 
acids  gelatin  goes  to  the  keto-form,  and  under  the  action  of  bases 
to  the  enol-form.  "This  would  conform  with  the  observation 
that  the  free  acid  from  sodium  gelatinate  differs  in  properties 
from  the  free  base  of  gelatin  hydrochloride. 

By  assuming  a  molecular  weight  for  gelatin  of  about  10,000, 
Miss  Lloyd  finds  that  the  basisity  of  gelatin  at  a  pH  of  2.5 
(at  which  point  gelatin  appears  to  be  completely  neutralized  by 
1  D.  J.  LLOYD,  Biochem.  J.,  14  (1920),  147. 


232 


GELATIN  AND  GLUE 


hydrochloric  acid)  is  8  while  at  a  pH  of  13  (at  which  point  the 
acid  valencies  are  satisfied)  the  acidity  is  28.  By  employing 
Berrar's  figures  the  latter  value  is  reduced  to  13. 

7.  Hydrolytic  Dissociation  of  Protein  Salts. — It  seems  to  have 
been  a  rather  generally  accepted  view  that  the  salts  of  gelatin 
and  the  other  proteins  with  inorganic  cations  or  anions  were 
easily  dissociated  hydrolytically  with  water.  In  fact  the  mass 
law  of  Guldberg  and  Waage  would  demand  that  such  should  be 
the  case  provided  that  ionization  took  place  between  the  organic 
complex  on  the  one  hand  and  the  inorganic  ion  on  the  other. 
That  is,  if  the  protein  reacted  with  a  base  according  to  the 
equation : 

H2N .  R .  COOH  +  KOH  ->  H2N .  R .  COO~  +  K+  +  H2O, 
or  with  an  acid  according  to  the  equation : 

HOOC .  R .  NH2  +  HC1  ->  HOOC .  R .  NH3+  +  Cl~, 

then  hydrolytic  dissociation  would  be  expected  to  follow.  This 
is  obvious,  for  the  inorganic  base  or  acid  is  in  each  case  highly 
ionized  while  the  organic  acid  or  base,  that  is,  the  uncombined 
protein,  is  very  weak  and  but  feebly  ionized.  In  the  presence, 
therefore,  of  an  excess  of  water  the  undissociated  protein  mole- 
cule would  tend  to  be  produced  to  the  ever  increasing  exclusion 
of  the  ionized  moiety.  In  other  words,  the  actual  percentage  of 
combined  inorganic  cation  or  anion  would  steadily  and  rapidly 
decrease  with  increasing  dilution.  That  such  a  decrease  does 
not  take  place  is  indicated  however  by  the  findings  of  some  inves- 
tigators. The  percentage  of  combined  base  or  acid  is  uninflu- 
enced, within  the  limits  of  experimental  error,  by  dilution,  but 
is  determined  by  the  relative  concentration  of  protein  and  inor- 
ganic ion.  This  may  be  interpreted  to  signify  that  ionization 
cannot  take  place  in  the  manner  just  illustrated,  but  must  be 
independent  of  the  elements  of  water.  In  accordance  with  the 
hypothesis  of  intramolecular  ionization : 


OH 


R— C   =   N— R  +  KOH 
OH 


and  R— C  =  N— R  +  HC1 


OK 

| 

R— C  = 
OH 

! 

R— C  = 


H       OH 


=  N— R 
H        Cl 


=  N— R 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  233 

no  water  enters  into  the  reaction,  and  consequently  water  would 
be  without  influence  on  ionization  upon  dilution  of  the  system. 
Robertson  regards  this  evidence,  i.e.,  the  non-dependence  of  the 
composition  of  protein  salts  upon  their  dilution,  as  of  the  greatest 
importance  in  proving  that  the  terminal  — COOH  and  — NH2 
groups  are  not  responsible  for  the  formation  of  such  salts. 

Theoretical  evidence  of  the  independence  of  the  composition 
of  protein  salts  upon  the  dilution  is  obtained  in  the  case  of  a 
number  of  proteins  by  the  application  of  the  Ostwald  dilution 
law  for  binary  electrolytes.  Robertson1  writes  the  equation: 

m  =  Ax  +•  Bx2, 

where  m  is  the  equivalent  concentration  of  the  base  or  acid  bound 
by  the  protein,  x,  the  specific  conductivity  in  reciprocal  ohms, 
and  A  and  B  constants,  respectively  equal  to: 

1.037  X  10~2          ,     1.075  X  10~4 

p(u  +  v)  Kp(u  +  vY' 

in  which  p  is  the  number  of  equivalents  of  protein  salt  to  which 
each  equivalent  of  neutralized  acid  or  base  gives  rise,  and  u  and 
v  are  average  equivalent  migration  velocities  in  centimeters  per 
second  under  unit  potential  gradient,  of  the  cations  and  anions 
respectively.  K  is  the  dissociation  constant.  On  applying  the 
above  formula  to  a  number  of  proteins  under  a  number  of  differ- 
ent conditions,  the  observed  and  the  calculated  values  of  m 
are  found  to  be  in  very  close  agreement,  which  shows  that, 
"for  a  given  combination  of  acid  or  base  with  protein,  containing 
a  given  proportion  of  the  acid  or  base,  the  number  of  equivalents 
of  protein  salt  to  which  one  equivalent  of  neutralized  acid  or 
base  gives  rise  is  independent  of  the  dilution." 

By  further  applying  the  formula  to  compounds  of  the  diacid 
bases  it  is  found  that  the  ratio  of  the  value  of  p(u  +  v)  for  mon- 
acid  bases  to  its  value  for  diacid  bases  is  very  close  to  2:1. 
This  is  shown  to  signify  that  an  equivalent  of  a  monacid  base 
gives  rise  to  twice  the  number  of  equivalents  of  protein  salt  as 
an  equivalent  of  a  diacid  base.  It  has  been  pointed  out  that 
one  equivalent  of  a  monacid  base  yields,  on  combination  with 
protein,  two  equivalents  of  the  protein  salt,  according  to  the 
equation : 

1  T.  B.  ROBERTSON,  lib.  ciL,  220. 


234 


OH 


GELATIN  AND  GLUE 

OK     1++      [H       OHl  = 


R— C  =  N— R  +  KOH 


R—  C  = 


=  N—  R 


If  the  diacid  bases  reacted  in  a  similar  manner  we  should  expect 
the  equation  to  be  as  follows: 

R— C  — 


OH 


2R— C  =  N— R  +  Ca(OH)2 


° 


\ 


V 


Ca 


R—  C  = 


H        OH 

\/ 

=  N— R 


in  which  case  one  equivalent  of  the  diacid  base  would  also  give 
rise  to  two  equivalents  of  the  protein  salt.  The  data  above 
referred  to  show  that  this  cannot  be  the  case,  and  an  internal 
neutralization  may  be  assumed  of  two  of  the  positive  and  two  of 
the  negative  valencies  forming  ions  of  the  type : 


—  CON 


Ca 


H      OH 


— CO+      H 

\ 

or  Ca   ^N  —  . 

/\ 
— CO+    OH 


This  necessitates  the  assumption  that  the  ions  produced  by  the 
alkaline  earths  are  twice  the  weight  of  those  produced  by  the 
alkalies  with  proteins.  In  this  connection  it  is  worthy  of  notice 
that  differences  in  the  properties  of  these  two  types  of  protein 
salts  have  been  observed  by  Loeb  and  others  which  also  tend  to 
confirm  this  conclusion.1 

8.  The  Dominant  Influence  of  the  Dibasic  and  Diacid  Protein 
Radicals  in  lonization. — Evidence  is  also  available  which  tends 
to  indicate  that  the  diamino  radicals  and  the  dicarboxylic  acid 
radicals  present  in  the  proteins  are  the  active  agents  in  accom- 
plishing salt  formation  with  acids  and  with  bases  respectively.2 


1  For  example,  a  greater  insolubility  of  such  salts. 

2  A.  KOSSEL,  Z,  physiol.  Chem.,  26  (1898),  165. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


235 


For  example  the  freezing  point  of  solutions  of  casein  in  a  base  is 
unaltered  by  varying  the  concentration  of  the  casein,  provided 
the  amount  of  base  present  is  constant.  That  is,  a  given  quantity 
of  a  base  always  gives  rise  to  the  same  number  of  protein  ions, 
whether  the  base  is  combined  with  more  or  with  less  protein.  It 
must  follow  that  each  molecule  of  alkali  gives  rise  to  one  ion  of  the 
protein.  If  the  reaction  were  concerned  with  only  one  — CO- 
HN —  group,  i.e.,  a  monocarboxylic  acid,  one  molecule  of  the  mono- 
acid  base  would  give  two  protein  ions,  as: 


OH  OK 


R— C  =  N— R  +  KOH  -»     R— C  = 


H 


OHh 


+ 


=  N— R 


but  if  a  dicarboxylic  acid  were  involved,  the  number  of  protein 
ions  would  be  identical  with  the  molecules  of  base  added,  as 


OH 


R(  )R  +  2KOH 

xc  =  w 

\ 

OH 


OK 


R: 


OK 


H       OH 


R 

=  w 
/\ 

H       OH 


Similarly,  the  freezing  point  of  ovomucoid  appears  to  be 
unaltered  by  varying  the  amount  of  acid  (hydrochloric)  added, 
and  consequently  the  number  of  ions  per  unit  volume  of  the 
system  is  unchanged.  The  relation  between  the  number  of 
molecules  of  acid  added,  and  the  number  of  ions  formed  is  also 
found  to  be  in  the  ratio  of  1 : 1  in  the  case  of  salmin.  These  data 
find  expression  in  Robertson's  scheme  of  ionization  as  follows: 


OH 

/C=N\ 
R  R  +  2HC1 


OH 


OH 

I 


OH 


H       Cl 


:R 


H     ci 


=  N' 


9.  Recapitulation. — To  summarize   the   preceding  section  of 
this  chapter,  evidence  has  been  presented  which  indicates  that: 


236  GELATIN  AND  GLUE 

1.  Proteins  are  amphoteric  compounds  capable  of  combining 
with  either  acids  or  bases  to  form  protein  salts.     The  salts  thus 
formed  yield,  in  water,  true  or  colloidal  solutions  which  contain 
ions,    together    with    nonionized    hydrated    colloid    aggregates. 

2.  lonization  of  these  salts  takes  place,  not  primarily  between 
the  protein  and  the  inorganic  ion  by  a  reaction  with  terminal 
— NH2  or  — COOH  groups,  but  between  nearly  equal  portions  of 
the  protein  molecule,   by  a  break  in  the  internal  — COHN— 
groups. 

3.  Protein  salts  are  relatively  stabile  and  not  easily,  or  but 
slowly,  dissociated  hydrolytically. 

4.  The  extent  of  salt  formation  is  determined  by  the  relative 
proportions  of  acid  or  base  present,  and  but  little  by  the  degree 
of  dilution  of  the  system. 

5.  An  equivalent  of  a  monoacid  base  yields  twice  the  number  of 
equivalents  of  a  protein  salt  as  an  equivalent  of  a  diacid  base. 

6.  Successive  additions  of  acid  or  base  to  a  protein  open  up 
additional  — COHN —  groups  situated  at  or  near  the  centre  of  the 
molecules  or  ions  reacted  upon. 

7.  The  ionization  of  the  proteins  takes  place  essentially  at  the 
points  of  union  of  the  diamino  and  of  the  dicarboxylic  acid 
groups. 


II.  THE  EFFECT  OF  INORGANIC  IONOGENS  UPON  PROTEINS 

1.  Precipitation  and  Coagulation. — According  to  Hardy1  and 
a  number  of  later  investigators  the  reaction  resulting  from  the 
precipitation  of  a  protein  by  a  salt  may  be  of  two  types.  The 
first  type  is  entirely  similar  to  the  reaction  between  solutions  of 
two  electrolytes  wherein  the  product  of  the  reaction  is  insoluble, 
such  as  the  precipitation  resulting  from  the  interaction  of  solu- 
tions of  sodium  sulphate  and  barium  chloride.  The  reaction 
involves  a  combination  of  ions  with  the  formation  of  a  sparingly 
soluble  compound.  Such  a  reaction  may  be  illustrated  by  the 
interaction  of  gelatin  and  phosphotungstic  acid,  resulting  in  the 
formation  of  insoluble  gelatin  phosphotungstate.  Representing 
the  gelatin  by  A  and  the  phosphotungstic  radical  by  B  the  reac- 
tion may  be  expressed  very  simply: 

i  W.  B.  HARDY,  J.  Physiol,  33  (1905),  251. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  237 

AOH  +  HB  ->  AB  +  H2O, 
or  perhaps  more  accurately, 

/NH2  /NH3B 

A'  +  HB  ->  A^ 

X30OH  X2OOH. 

The  reaction  will  obviously  be  subject  to  the  limitations  defined 
by  the  Mass  Law  and  the  formation  of  a  precipitate  in  any  case 
must  be  determined  by  the  solubility  product  of  the  salt  formed. 
That  is,  in  any  reaction  such  as: 

AOH  +  HB  -*  AB  +  H2O, 
the  ratios: 

[A]+  X  [B]-  ,  [AB]solution 

=  **i 


r  A  T>I  **i  T  A  -ri-,  , 

[AB]  [AB]Soiid 

must  obtain.  Combining  these  we  find,  in  a  saturated  solution 
at  a  given  temperature  : 

[A]  +  X  [B]~   =   Ksoiubiiity  product. 

^ 

If,  now,  the  product  of  the  above  ion  concentrations  is  made 
greater  than  the  value  of  the  solubility  product  constant  for  that 
substance,  equilibrium  can  be  maintained  only  by  the  precipita- 
tion of  the  undissociated  salt.  This  means  that  a  definite 
concentration  of  precipitant  must  be  added  to  any  protein  at  any 
temperature  before  precipitation  will  result,  and  the  greater  the 
insolubility  of  the  resulting  salt,  the  less  the  amount  of  precipitant 
required. 

The  other  type  of  reaction  by  which  inorganic  salts  may  pre- 
cipitate protein  is  of  a  nature  that  strongly  suggests  dehydration, 
as  first  suggested  by  Hofmeister  in  1889-90.  Jones  and  his 
pupils1  have  found  that  inorganic  molecules  and  ions  possess  the 
power  of  forming  hydrates  or  solvates  in  the  presence  of  water, 
especially  at  low  temperatures.  At  high  temperatures  these  are 
decomposed. 

G..M.  Smith2  showed  that  in  the  case  of  the  alkali  metals  and 
the  halogens  the  hydration  increased  in  the  order  of  decreasing 
ionic  weight.  The  following  table  gives  his  figures  at  0°C. 

1  H.  C.  JONES  and  K.  OTA,  Am.  Chem.  J.,  22  (1899),  5;  H.  C.  JONES  and 
H.  S.  UHLER,  ibid.,  34  (1905),  291;  H.  C.  JONES,  Z.  physik.  Chem.,  74  (1910), 
325. 

2  G.  M.  SMITH,  /.  Am.  Chem.  Soc.,  37  (1915),  729. 


238  GELATIN  AND  GLUE 

TABLE  39. — HYDRATION  OP  INORGANIC  IONS 


Molecules 

Molecules 

Ion 

Ionic 
weight 

of 
hydrated 

Ion 

Ionic 
weight 

of 
hydrated 

water 

water 

Cs 

133 

3  7 

I 

127 

4  3 

Rb  

85 

6.4 

Br  

80 

6.7 

K  

39 

9.6 

N03  

62 

8.9 

Na  

23 

16.9 

C103  

83 

9.3 

Li  

7 

24.0 

Cl  

35 

9.6 

When  more  than  one  salt  is  present  in  the  solution  the  various 
molecules  or  ions  compete  for  the  water,  and  if  the  latter  is 
present  in  insufficient  amount,  each  will  take  up  the  water  in 
proportion  to  its  solvate  potential.  In  this  respect  the  action  is 
analogous  to  the  distribution  by  differential  solubility  of  a  solute 
in  two  or  more  solvents. 

There  is  an  abundance  of  evidence  leading  to  the  belief  that 
proteins  are  exceptional  in  their  tendency  to  form  solvation 
compounds  with  water.  Since  this  is  the  case  it  would  be 
expected  that  in  solutions  containing  both  proteins  and  inorganic 
salts  there  would  exist  a  pronounced  competition  on  the  part 
of  the  two  substances  for  the  solvent.  By  increasing  the  relative 
proportion  of  salt  to  protein  it  is  conceivable  that  the  latter  might 
be  forced  to  give  up  water  to  the  point  where  it  would  no  longer 
be  soluble  in  the  solution,  and  precipitation  would  result. 

It  seems  very  doubtful  however  if  this  process  ordinarily  takes 
place  to  the  exclusion  of  the  one  previously  described.  Taking 
gelatin  as  an  example,  it  appears  that  magnesium  sulphate  may 
react  in  three  ways.  If  the  solution  is  alkaline,  precipitation 
takes  place  only  at  high  concentrations.  If  the  solution  is 
neutral,  precipitation  is  more  pronounced.  If  the  solution  is 
acid  to  a  definite  optimum,  then  precipitation  reaches  its  maxi- 
mum, but  declines  thereafter  with  further  additions  of  acid.  The 
explanation  is  probably  as  follows.  In  alkaline  solutions  magne- 
sium gelatinate  will  be  formed.1  This  is  apparently  sufficiently 
soluble  in  alkaline  solutions  so  that  a  rather  high  concentration 
of  the  magnesium  sulphate  must  be  present  in  order  to  produce 
precipitation.  This  is  finally  brought  about  by  two  simultane- 
ous processes.  The  available  solvent  is  being  removed  by 

1  Vide  later  sections  in  this  chapter. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  239 

increasing  additions  of  the  reagent,  and  the  product  of  the  con- 
centration of  the  magnesium  and  the  gelatinate  ions  is  as  regu- 
larly increasing,  until  eventually  it  exceeds  the  solubility  product 
for  magnesium  gelatinate,  and  precipitation  results. 

The  same  procedure  occurs  in  an  acid  medium  except  that  the 
gelatin  salt  is,  in  this  case,  gelatin  sulphate.  Apparently 
the  molecular  solubility  of  this  substance  is  less  than  that  of 
the  magnesium  gelatinate,  for  precipitation  occurs  at  lower 
concentrations  of  the  precipitant.1 

The  point  of  maximum  precipitation  appears  to  coincide  with 
the  isoelectric  point  of  gelatin2  which  is  found  to  be  in  a  slightly 
acid  medium  at  a  pH  of  4.7.  In  this  condition  the  gelatin  reacts 
with  neither  ion  of  the  salt,  and  precipitation  is  brought  about 
entirely  by  dehydration.  Even  in  the  absence  of  any  electrolyte 
the  isoelectric  gelatin  is  sufficiently  anhydrous  to  precipitate 
spontaneously  at  low  temperatures. 

It  has  been  suggested  that  the  two  types  of  precipitation  of 
proteins,  as  above  described,  be  distinguished  by  the  terms 
precipitation  and  coagulation,  the  former  referring  to  the  inter- 
action between  ions,  and  the  latter  to  the  producton  of  an  insolu- 
ble material  through  the  agency  of  dehydration. 

2.  Robertson's  Theory  of  Protein-salt  Formation.  —  Robertson3 
regards  the  dehydration  attendent  upon  coagulation  as  resulting 
in  the  formation  of  protein  anhydrides.  This  reaction  involves 
only  the  terminal  —  NH2  and  —  COOH  groups,  so  that  any 
combinations  of  the  protein  with  inorganic  anions  or  cations, 
since  that  combination  is  effected  at  the  internal  —  COHN  — 
groups,  would  not  be  affected  by  anhydride  formation.  The 
following  types  of  anhydride  formation  may  occur: 


x  .v  xN(OH)Cv 

Rv  ,     Rv  ,R     ,     Rv  yR. 

^CO,  XCOHNX  XC(OH)NX 

The  mechanism  of  precipitation  is  accounted  for  by  assuming 
that  in  a  solution  of  an  acid  protein,  the  cation  only  of  an  added 
inorganic  salt  will  enter  into  combination  with  the  protein, 
liberating  an  acid,  while,  in  a  solution  of  an  alkaline  protein. 
both  ions  of  the  added  salt  enter  into  combination,  liberating 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  106. 

2  Vide  Appendix. 

3  T.  B.  ROBERTSON,  lib.  cit.,  129. 


240  GELATIN  AND  GLUE 

only  water.     These  reactions  may  be  pictured  by  the  following 
equations  : 

Acid  protein  plus  salt 

OHH      Cl 


H2N.R.C    =   N.R.COOH  +  2NaN03  <=* 

ONaNa    Cl 


H2N.R.C  =      N.R.COOH  +  2HNO3; 

Alkaline  protein  plus  salt 


ONa  H      OH 

I 


H2N.R.C     =    N.R.COOH  +  NaCl  <=> 

ONa  Na    Cl 


H2N.R.C  =      N.R.COOH +  H2O. 

It  will  be  observed  that  the  same  salt  is  formed  in  both  cases. 
Internal  neutralization  may  take  place  forming  the  compound: 

ONa       Na   Cl 


H3N.R.C     =         N.R.COO, 

I , —I 

and  in  the  presence  of  a  large  amount  of  water  a  certain  degree  of 
hydrolytic  dissociation  may  occur: 

ONa  ONa 

Na    Cl  H      Cl 

V  \/ 

H2N.R.C  =  N.R.COOH  +  H2O  <=>  H2N.R.C  =  N.R.COOH    + 

NaOH. 

Robertson  believes  that  the  above  explanation  accounts  for  a 
number  of  experimental  observations,  among  which  may  be 
cited  the  following: 

1.  The  addition  of  neutral  salts  to  an  acid  protein  increases 
the  acidity  of  the  solution,  while  the  addition  of  neutral  salts  to 
an  alkaline  protein  does  not  increase  the  alkalinity  of  the  solution. 

2.  Cations    are    responsible    for   the    precipitation   of   acid- 
proteins,  anions  for  alkali -proteins. 

3.  The    precipitation    of    proteins    by  salts  is  more  readily 
brought  about  in  acid  than  in  alkaline  solutions. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  241 

3.  Objections  to  Robertson's  Theory  of  Protein-salt  Forma- 
tion. —  In  the  light  of  the  recent  work  of  Loeb1  however  the 
hypothesis  of  Robertson  does  not  appear  to  be,  in  all  respects, 
adequate.  Loeb  has  shown  that  an  acid  gelatin,  in  the  presence, 
for  example,  of  a  solution  of  silver  nitrate,  will  combine  with 
the  anion  only,  and  that  only  the  merest  traces  of  the  silver  ion 
enter  into  combination.  If,  however,  the  gelatin  is  alkaline 
considerable  portions  of  the  silver  ion  become  combined  with  the 
protein.  According  to  Robertson's  view  the  final  product  should 
be  the  same  in  both  instances,  and  in  both  cases  should  contain 
the  silver  ion.  His  formula  would  be: 

OAg    Ag    N03 

\/ 

H2N.R.C     =     N.R.COOH, 

or  the  above  salt  in  an  ionized  condition.  That  this  is  not  the 
case  has  been  demonstrated. 

In  another  experiment  sodium  bromide  was  added  to  gelatin  at 
different  hydrogen  ion  concentrations,  and  in  this  case  it  was 
established  that  the  acid  gelatin  combined  with  the  anion  in 
considerable  proportions,  while  only  traces  of  bromide  were  found 
combined  with  the  alkali  gelatin.  Taken  together  these  experi- 
ments show  that  acid-gelatin  is  capable  of  combination  with  the 
anions  but  not  the  cations  of  inorganic  salt  solutions,  while 
alkali-gelatin  is  capable  of  combination  with  the  cations,  but 
not  the  anions  of  such  solutions.  These  results  are  not  in 
conformity  with  Robertson's  hypothesis  of  the  action  of  salts 
upon  proteins. 

Pauli2  has  suggested  that  the  reaction  between  acid-  and  alkali- 
protein  and  inorganic  salt  solutions  takes  place  according  to  the 
following  equations: 


Acid-protein  +  NaNO 


3 

/  H  NH2/  H 

\C1  /        \C1 

+NaN03    -»     R\  +  HN03; 

COOH  COONa 

1  JACQUES  LOEB,  J.  Gen.  Physiol,  1  (1918-19),  39;  237;  363;  483;  559. 

2  W.  PAULI  and  H.  HANDOVSKY,  Biochem.  Z.,  18  (1909),  340;  24  (1910), 
239;  W.  PAULI  and  R.  WAGNER,  ibid.,  27  (1910),  296. 

16 


242 


GELATIN  AND  GLUE 


NH 


H 


Alkali-protein  +  KC1 

NH2/K 
/        \C1 


"\  +KC1      ->         R(  +  H20. 

COONa  COONa 

These  equations  are  no  more  satisfactory  in  explaining  the  find- 
ings of  Loeb  than  are  those  of  Robertson,  for  they  assume  a 
combination  between  acid-gelatin  and  inorganic  cation,  and 
between  alkali-gelatin  and  inorganic  anion,  which  has  been 
shown  not  to  occur. 

It  seems  probable  that  the  reactions  may  be  represented  in 
conformity  with  all  of  the  observed  facts  by  the  following 
equations : 

Acid-gelatin  +  AgNO3  (X  =  any  monovalent  anion) 


OH  " 
H    X    1 

\    /     1 

i 

H    X 

\  / 

\/      1 
H2N.R.C  =_ 

-1 

"•; 

\/ 
_=N.R.C 

( 

H     N03 

\    / 

^00 
)H' 

H 

i 

+  2AgNO3  <=> 

H     N03 
\   / 

V 

H2N.R.C 

1  

T 

\/ 
=  N.R.COOH 

2AgX; 

Alkali-gelatin  +  AgNO3  (M  =  any  monovalent  cation) 
OM  1++  |~H      OH 


H2N.R.C  =1  +  I  =  N.R.COOM  I       +  2AgNO- 


OAg 
H2N.R.C  - 


•H- 


H       OH 


=  N.R.COOAgJ   +  2MN03; 


OH 

X  I 


H 
\ 

H2N.R.C 


Acid-Gelatin  +  NaBr 


=  N.R.COOH     +  2NaX; 


=  N.R.COOH 

OH 
H      Br 


H2N.R.  C  = 


OM 

| 

H2N.R.C  = 


GELATIN  AS  AN.  AMPHOTERIC  COLLOID  243 

Alkali-gelatin  +  NaBr 
H     OH 


=  N.R.COOM     +  2NaBr 


ONa|++        H     OH 


H2N.R.C  =      +      =N.R.COONa     +  2MBr. 


These  equations,  while  fitting  equally  well  with  Robertson's 
reactions  in  his  general  scheme  of  protein  ionization,  account 
more  satisfactorily  for  the  facts  observed  in  the  interactions 
between  proteins  and  inorganic  salts  than  the  formulas  given  by 
him.  The  observation  that  the  acid-gelatin  interacts  only  with 
the  anions  of  an  added  salt,  while  the  alkali-gelatin  reacts  only 
with  the  cations,  is  accounted  for  by  these  equations. 

4.  Ion  Series  in  Protein  Precipitation,  Swelling,  etc.  —  In 
1888  Hofmeister1  enunciated  what  has  since  come  to  be  known 
as  the  Hofmeister  series  of  ion  reactivity  with  proteins.  He 
studied  a  number  of  proteins  and  other  colloids  including  gelatin, 
egg-albumin,  serum-albumin,  sodium  oleate,  and  ferric  hydroxide, 
and  found  that  the  order  of  influence  of  inorganic  radicals  upon 
them  was  approximately  the  same  for  each  of  the  substances 
studied.  Different  inorganic  ions  behaved  very  differently. 
but  their  coagulating  power  or  gelation  effect  upon  proteins  and 
other  colloids  was  nearly  always  found  to  be  in  the  same  order. 

Having  established  this  uniformity  of  behavior  of  inorganic 
ions  in  the  precipitation  of  proteins  Hofmeister  next  proceeded 
to  study  the  effect  of  similar  ions  upon  the  swelling  of  proteins. 
He  found  that  the  swelling  of  the  different  proteins  was  likewise 
affected  in  a  similar  sense  by  the  inorganic  radicals,  and  he 
arranged  the  series  in  the  following  sequence,  beginning  with 
the  lowest  degree  of  swelling:  sulphates  <  citrates  <  tartrates  < 
acetates  <  alcohol  <  cane  sugar  <  grape  sugar  <  distilled  water  < 
chlorides  <  chlorates  <  nitrates  <  bromides.  This  order  was  noted 
to  be  nearly  identical  with  the  order  of  efficacy  of  the  salts  in 
coagulation,  the  first  three  or  four  in  the  series  being  the  most 
ready  precipitants  of  the  proteins. 

1  F.  HOFMEISTER,  Arch,  exptl.  Path.  Pharm.,  24  (1888),  247;  26  (1888), 
1;  27  (1890),  395;  28  (1891),  210;  Z.  physiol.  Chem.,  14  (1890),  165. 


244 


GELATIN  AND  GLUE 


The  researches  of  Hofmeister  were  continued  by  Pauli1  who 
studied,  in  addition  to  the  coagulation  and  the  swelling  of 
proteins,  changes  in  their  viscosity  and  the  temperatures  of  their 
gelatinization  and  melting,  as  affected  by  inorganic  ions.  His 
results  confirm  those  of  Hofmeister  by  placing  the  order  of  the 
effect  of  ions  upon  the  gelatinizing  and  melting  points  of  gelatin 
parallel  with  their  power  of  coagulating  gelatin,  and  of  inhibiting 
the  swelling  of  gelatin  plates.  The  anions  of  the  salts  were  the 
most  effective,  but  the  cations  were  not  without  influence,  and 
an  ionic  sequence  was  also  recorded  for  them.  The  following 
table  taken  from  Pauli  expresses  the  results  obtained  with 
proteins : 

TABLE  40. — EFFECT  OF  INORGANIC  IONS  ON  PROPERTIES  OF  PROTEINS 


Lowering  of  gela- 
tinization point  of 
gelatin     (Pauli) 

Increasing  swell- 
ing effect  on  gela- 
tin   (Hofmeister) 

Decreasing  coag- 
ulating effect  on 
gelatin   (Pauli) 

Increasing  coag- 
ulating effect  on 
egg  globulin 
(Pauli) 

Decreasing  effect 
of    acids   on    vis- 
cosity of  blood 
serum  (Pauli) 

Sulphates 
Citrates 
Tartrates 
Acetates 
Chlorides 
Chlorates 
Nitrates 

Sulphates 
Citrates 
Tartrates 
Acetates 
Chlorides 
Chlorates 
Nitrates 

Sulphates 
Citrates 
Tartrates 
Acetates 
Chlorides 

Ammonium 
Potassium 
Sodium 
Lithium 
Barium 
Magnesium 

Hydrochloric 
Monochloracetic 
Oxalic 
Dichloracetic 
Citric 
Acetic 

Bromides 
Iodides 

Bromides 



Trichloracetic 

Mixtures  of  salts  were  observed  to  produce  an  effect  equal  to  the 
algebraic  sum  of  the  individual  effects  resultant  from  the  com- 
ponent ions  represented. 

The  first  researches  tending  to  throw  doubt  upon  the  validity 
of  the  Hofmeister  and  Pauli  series  were  performed  by  Posternak2 
in  1901.  By  employing  a  vegetable  protein  obtained  from  the 
seeds  of  Picea  excelsa,  he  succeeded  in  demonstrating  that 
the  order  of  effectiveness  of  the  inorganic  ions  in  coagulating  the 
protein  in  a  slightly  acid  solution  was  exactly  reversed  in  a 
slightly  alkaline  solution.  The  condition  of  acidity  or  alkalinity 
had  not  previously  been  considered.  Pauli3  soon  verified  these 

1  W.  PAULI,  Arch.  ges.  Physiol.,  71  (1898),  333;  78  (1899),  315;  W.  PAULI 
and  P.  RONA,  Beitr.  physiol.  path.  Chem.,  2  (1902),  1. 

2  S.  POSTERNAK,  Ann.  Inst.  Pasteur,  16  (1901),  85;  169;  451;  570. 

3  W.  PAULI,  Beitr.  physiol  path.  Chem.,  3  (1903),  225. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  245 

findings  with  egg-white  and  other  proteins,  and  later  investi- 
gators have  confirmed  their  results.  Ions  which  most  strongly 
induce  coagulation  in  an  acid-protein  are,  in  an  alkali-protein, 
the  least  effective,  while  those  ions  that  are  nearly  without  influ- 
ence in  an  acid-protein  will,  in  an  alkali-protein,  be  most 
effective. 

5.  Application  of  the  Laws  of  Classical  Chemistry  to  the 
Protein-salt  Equilibrium. — The  investigations  of  Loeb1  con- 
stitute, however,  the  most  emphatic  argument  against  the  validity 
of  the  Hofmeister-Pauli  ion  series.  Loeb  has  long  been  a  leader 
in  the  classical  school  of  chemistry,  and  takes  serious  exception  to 
the  tendency  of  some  of  the  modern  investigators  to  explain  the 
reactions  which  obtain  in  colloidal  solutions  as  of  a  special  type 
dependent  upon  adsorption,  degree  of  dispersion,  and  the  like, 
rather  than  as  special  cases  which  may  be  adequately  explained 
by  an  application  of  the  older  and  more  firmly  established  laws  of 
solutions  and  of  the  purely  chemical  forces  of  primary  and  sec- 
ondary valency.  S0rensen2  has  said  "the  properties  of  colloidal 
solutions  can  be  most  efficiently  inquired  into  by  application,  as 
far  as  possible,  of  the  same  views  and  methods  as  those  generally 
applied  to  true  solutions,"  and  Loeb3  affirms  that  "the  variation 
of  the  physical  properties  of  gelatin  under  the  influence  [for 
example]  of  hydrobromic  acid  is  an  unequivocal  function  of  the 
number  of  gelatin  bromide  molecules  formed,  and  colloidal 
speculations  not  based  on  the  laws  of  classical  chemistry  are 
neither  needed  nor  warranted."  Pauli  and  Robertson  have  also, 
as  the  previous  sections  of  this  chapter  reveal,  favored  a  chemical 
conception  of  the  reactions  of  the  proteins,  but  their  experiments 
were  inconclusive  in  proving  such  a  conception. 

It  will  be  recalled  that  the  ion  series  above  described  were 
capable  of  revealing  no  chemical  relationship  whatsoever  that 
could  be  interpreted  in  accordance  with  the  hitherto  known  laws 
of  valency.  Thus  divalent  sulphate,  monovalent  acetate,  and 
monovalent  alcohol  were  found  by  Hofmeister  to  react  in  a 
similar  way  in  inhibiting  swelling  or  in  producing  coagulation; 
other  monovalent  radicals  increased  the  swelling  or  inhibited 
coagulation.  Pauli 's  series  on  the  viscosity  of  blood  albumin  is 

1  J.  LOEB,  J.  Gen.  PhysioL,  1  (1918-19),  39;  237;  363;  483;  559;  2  (1919-20) ; 
87;  3  (1920-21),  85;  247;  Science,  62  (1920),  449. 

2  S.  S0RENSEN,  CompL  rend.  trav.  lab.  Carlsberg,  12  (1917),  369. 

3  J.  LOEB,  J.  Gen.  PhysioL,  1  (1918-19),  378. 


246  GELATIN  AND  GLUE 

equally  unexplainable  as,  in  the  series,  the  strong  monobasic 
acid  hydrochloric  is  followed  by  the  weak  monochloracetic  acid, 
the  dibasic  oxalic  acid,  the  tribasic  citric  acid,  and  this  in  turn 
by  the  monobasic  acetic  acid,  etc.  So  long  as  these  series  were 
accepted  it  was  impossible  to  prove  the  existence  of  a  definite 
stoichiometrical  relationship  in  the  interaction  of  inorganic  ions 
with  proteins. 

Loeb  has  attacked  the  problem  from  a  new  angle.  Previous 
investigators  had  studied  the  effect  of  inorganic  ions  by  adding 
equivalent  amounts  of  the  ionogens  to  proteins  prepared  in  a 
standard  way,  and  had  entirely  failed  to  take  cognizance  of  a 
most  important  variable  in  the  system,  namely,  the  hydrogen  ion 
concentration.  Loeb  used  protein  solutions  of  uniform  hydro- 
gen ion  concentration  for  his  comparisons,  and  by  so  doing  dis- 
covered an  entirely  different  relationship,  i.e.,  that  " acids, 
alkalies,  and  neutral  salts  combine  with  proteins  by  the  same 
chemical  forces  of  primary  valence  by  which  they  combine  with 
crystalloids,  and  that,  moreover,  the  influence  of  the  different 
ions  upon  the  physical  properties  of  proteins  can  be  predicted 
from  the  general  combining  ratios  of  these  ions."  The  principal 
investigations  which  have  led  to  these  conclusions  will  be  pre- 
sented in  the  following  sections.  The  argument  reveals  evidence 
that  goes  far  toward  indicating  the  nonexistence  or  at  least  the 
inadequacy  of  the  Hofmeister  and  Pauli  series,  and  formulates 
the  gelatin-salt  equilibrium  in  terms  of  hydrogen  ion  concentra- 
tion and  valence. 

6.  The  Isoelectric  Point  of  Gelatin. — Ordinary  gelatin  is 
usually  found  to  be  nearly  neutral,  that  is,  it  possesses  a  hydrogen 
ion  concentration  of  about  CH  =  10~7  or,  in  terms  of  S0rensen's 
logarithmic  symbol,1  pH  =  7.  This  value  will  vary  somewhat, 
but  the  highest  grades  of  gelatin  are  usually  of  this  order  of 
hydrogen  ion  concentration.  If,  now,  such  a  gelatin  be  pul- 
verized and  treated  in  solution  with  any  neutral  salt  it  will  be 
found  that  the  gelatin  reacts  with  the  cations  but  is  unaffected  by 
the  anions.  On  the  other  hand,  if  the  gelatin  is  first  treated  with 
an  acid,  and  the  excess  of  acid  removed  as  completely  as  possible 
by  washing  with  water,  then  it  is  observed  that  the  gelatin  will 
react  with  the  anions,  but  will  not  be  affected  by  the  cations.  If 
the  original  gelatin  is  first  treated  with  a  base,  and  the  excess 
likewise  removed,  the  gelatin  will  react  only  with  the  cations. 

1  See  Appendix  for  a  discussion  of  hydrogen  ion  concentration  and  pH. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  247 

Neutral  gelatin  possesses  therefore  the  reacting  properties  of  the 
alkali-treated  substance,  but  these  are  quite  opposite  from  the 
properties  of  the  acid-treated  material. 

There  must,  obviously,  be  some  point  in  the  hydrogen  ion 
concentration  that  is  intermediate  between  the  alkali  and  the 
acid  conditions  at  which  the  combination  of  the  gelatin  with 
anions  and  cations  would  be  equal  or  negative.  Since  neutral 
gelatin  reacts  as  an  alkali  gelatin,  this  point  must  have  a  pH  value 
less  than  7.0.  That  is,  the  gelatin  must  be  on  the  acid  side  of 
neutrality  at  this  point  of  equal  or  zero  reactivity  with  anions 
and  cations. 

Under  the  influence  of  an  electric  potential  gelatin,  in  water  and 
in  alkaline  solutions,  is  found  to  migrate  toward  the  anode. 
That  is,  the  neutral  gelatin  appears  to  give  off  hydrogen  ions,  and 
conducts  itself  as  if  it  were  an  acid.  In  alkaline  solutions,  e.g., 
in  the  presence  of  sodium  hydroxide,  it  appears  as  if  the  gelatin 
had  undergone  a  simple  neutralization,  hydrogen  and  hydroxyl 
ions  having  combined,  and  sodium  gelatinate  in  an  ionized  state 
remaining.  The  gelatin  ion,  being  the  anion,  migrates  therefore 
to  the  anode.  In  acid  solution,  e.g.,  in  the  presence  of  hydro- 
chloric acid,  the  gelatin  is  found,  however,  to  migrate  to  the 
cathode.  That  is,  it  conducts  itself  as  if  it  had  ionized,  liberating 
hydroxyl  ions  which  had  combined  with  the  hydrogen  ions  of 
the  acid,  and  left  in  the  solution  the  salt,  gelatin  chloride,  also 
in  an  ionized  state.  The  gelatin,  being  in  this  case  the  cation, 
migrates  to  the  cathode.  By  starting  with  an  alkaline  or  neutral 
gelatin  and  slowly  adding  acid,  or  by  starting  with  an  acid-protein 
and  adding  alkali,  a  point  is  eventually  reached  at  which  no  migra- 
tion of  gelatin  is  observed.  This  is  known  as  the  isoelectric  point. 
It  was  found  by  Michaelis,1  and  corroborated  by  other  investi- 
gators, to  be,  for  gelatin,  CH  =  2  X  10~5,  or  in  terms  of  S0rensen's 
symbol  pH  4.7. 

7.  Loeb's  Method  for  the  Study  of  the  Gelatin-salt  Equili- 
brium.— It  seemed  probable  that  the  point  of  equivalency  in  the 
reactivity  of  gelatin  for  anions  and  cations  might  be  identical 
with  the  isoelectric  'point.  This  was  demonstrated  by  Loeb2  to 
be  the  case.  One  gram  portions  of  gelatin,  pulverized  so  as  to 
pass  a  60  mesh,  but  to  be  retained  by  an  80  mesh  sieve,  were 
treated  with  100  c.c.  portions  of  nitric  acid  or  hydrochloric  acid 

1  MICHAELIS,  "Die  Wasserstoffionenkonzentration,"  Berlin  (1914). 

2  J.  LOEB,  loc.  cit. 


248  GELATIN  AND  GLUE 

for  an  hour  at  15°C.  The  concentrations  of  the  acid  used  varied 
from  N/8  to  N/8192,  and  water  served  as  a  control.  The 
several  portions  were  then  filtered  and  washed  with  2  or  3 
perfusions  of  cold  distilled  water  to  remove  the  excess  of  acid 
which  remained  in  the  film  about  the  granules  of  gelatin.  With 
series  prepared  in  this  manner  a  large  number  of  tests  were 
made.  The  swelling  was  measured  directly  by  the  height  in 
millimeters  to  which  the  swollen  particles  rose  in  the  cylindrical 
funnels  used.  The  several  portions  were  then  placed  in  beakers, 
melted,  made  up  to  100  c.c.,  i.e.,  1  per  cent  solutions,  and  the 
following  determinations  made: 


The  conductivity       ™,  at  24°C. 
\  ohms  / 

The  osmotic  pressure  expressed  in  millimeters  height  to  which 
the  1  per  cent  gelatin  solution  rose  in  the  manometer  tube.1 

The  alcohol  number,  i.e.,  the  c.c.  of  95  per  cent  alcohol  required 
to  produce  a  precipitate  in  5  c.c.  of  the  gelatin  solution  at  20°C. 

The  viscosity. 

The  hydrogen  ion  concentration  determined  by  the  colorimetric 
method2  of  S0rensen  and  Clark.3 

8.  Gelatin  Salts  and  Metal  Gelatinates.—  The  results  obtained 
with  hydrochloric  acid  are  shown  in  Fig.  34.  *. 

The  most  obvious  point  about  these  curves  is  the  remarkable 
similarity  revealed.  The  curve  for  viscosity  is  not  included, 
but  is  stated  to  be  parallel  to  that  for  the  osmotic  pressure.  It 
is  observed  that  all  of  the  properties  mentioned  are  high  on  the 
acid  side  of  the  isoelectric  point,  i.e.,  in  the  region  of  gelatin 
chloride;  that  they  reach  a  minimum  at  the  isoelectric  point; 
and  that  they  again  rise,  but  to  a  lesser  extent,  on  the  alkaline 
side  of  that  point,  i.e.,  in  the  region  of  hydrogen  gelatinate. 
The  curve  for  conductivity  is  especially  significant  since  it  is  a 
direct  expression  of  the  degree  of  electrolytic  dissociation  of  the 
gelatin  in  the  solution.  At  the  isoelectric  point  the  gelatin  is 
unquestionably  but  very  slightly  dissociated,  as  the  conductivity 
is  there  nearly  zero.  On  the  acid  side  the  salt,  gelatin  chloride, 

1  Vide  page  103,  for  the  technique  of  the  osmotic  pressure  determination. 

2  Vide  Appendix,  page  599,  for  a  discussion  of  the  colorimetric  method  for 
the  determination  of  hydrogen  ion  concentration. 

3  Cf.  W.  M.  CLARK,  "The  Determination  of  Hydrogen  Ions,"  Baltimore 
(1920),  38. 

<J.  LOEB,  loc.  cit.,  1  (1918),  44. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


249 


is  apparently  highly  dissociated,  as  indicated  by  the  high  con- 
ductivity. On  the  opposite  side  of  the  isoelectric  point,  the  weak 
acid,  hydrogen  gelatinate,  is,  as  would  be  expected,  dissociated, 


IdoelecUic  point 
Region  ot  Gelattn-Cl  J,  Region  o(  Gelatin- fi 


za< 


HCI  concentre 


67    71 


FIG.  34.  —  Curves  of  the  conductivity,  osmotic  pressure,  swelling,  and  alcohol 
number  of  gelatin  previously  treated  with  various  concentrations  of  HC1  and 
then  freed  from  excess  of  HC1  by  washing  with  water. 

but  to  a  lesser  degree,  as  weak  acids  do  not  dissociate  to  the  same 
degree  as  salts. 


250 


GELATIN  AND  GLUE 


Fischer1  explains  these  observations  as  follows:  The  gelatin 
and  acid  combine  to  form  a  gelatinate.  This  has  a  greater 
solubility  for  water — hence  the  increase  in  swelling  and  increase 
in  viscosity;  also  greater  solubility  in  water — hence  the  increase 
in  osmotic  pressure.  The  dissociation  value  is  also  higher  than 
the  degree  of  hydrolysis — hence  the  greater  conductivity  and  the 
greater  concentration  of  free  H  or  OH  ions. 


Region  of  _ 
Gelatin-NOj 


Isodecfric 


of 

-Gelatinate 


90 
80 
70 
60 
50 
40 
30 
20 
10 
0 
6 
6 
4 
2 
0 

\ 

* 

\ 

Nj 

\ 

\ 

1 

\ 

\ 

/ 

s* 

\ 

\/ 

T 

To 

ral 

SWf 

II  in 

t 

cc 

y 

?cc 

rnb'n 

led 

^ 

Wl 

hO. 

-bgn 

•gel 

tin 

/ 

r 

_J 

Remain  clear  in  light  Turn  biacK  in  light 

COnc.U5ed     8"     J6     32  64   128  SlF  C6  J024  SJ2  lOW  2048  4096  6i92 

pH  3.6    3.8  3.8  3.9  4.1    4.3  4.6  4.7   50  5.3  5.7  6.1    61!  6.4 


AgNOa 


FIG.  35. — Gelatin  treated  with  different  concentrations  of  HNOs,  from  M/8  to 
M/8192,  washed,  and  then  treated  with  the  same  concentration  of  AgNOa, 
(M/16),  and  then  washed  again. 

The  parallelism  between  the  several  curves  is  so  perfect  that 
Loeb  was  led  to  conclude  that  the  degree  of  conductivity,  i.e., 
the  ionization  of  the  gelatin,  determines  to  a  large  extent  many 
of  the  other  properties  of  the  gelatin  solution.  The  colloid 
behavior,  however,  of  soaps  in  alcohol,  benzene,  etc.,  or  of  rubber 
in  carbon  disulphide  or  toluol  or  gasoline  does  not  appear  to  be 
explainable  by  any  theory  involving  ionization. 

Upon  modifying  the  experiment  by  the  addition  of  sodium 
sulphate,  in  equal  concentration  to  all  samples,  sodium  gelatinate 
would  be  formed  on  the  alkaline  side  of  the  isoelectric  point. 


1  MARTIN  FISCHER,  personal  communication. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


251 


This,  being  a  salt,  would  be  expected  to  produce  a  high  ioniza- 
tion,  hence  a  high  conductivity,  and  all  other  properties  would 
likewise  be  expected  to  be  higher  than  in  the  lesser  ionized  hydro- 
gen gelatinate  (free  gelatin).  This  is  in  fact  found  to  be  the  case. 
When  silver  nitrate  is  used  in  place  of  sodium  sulphate,  by 
adding  equal  amounts  to  each  of  the  acid-treated  gelatins,  the 
acid  used  being  in  this  case  nitric  acid,  we  would  now  predict 
that  those  gelatins  which  have  a  pH  value  less  than  4.7  would 
react  with  the  anion  only.  That  is,  gelatin  nitrate  would 


FIG.  36. — Photograph  of  the  gelatin  solutions  whose  curves  are  contained  in 
Fig.  35,  taken  a  week  after  the  experiment  was  made.  (By  permission  of  Jacques 
Loeb.) 

be  produced  at  pH<4.7.  In  those  proteins  where  pH  is 
greater  than  4.7  we  would  expect  only  the  cation  to  react,  with 
the  formation  of  silver  gelatinate.  When  pH  =  4.7  no  reaction 
with  either  ion  would  take  place.  That  this  is  the  case  has  been 
previously  stated.  The  experiment  was  performed  in  the  dark, 
but  within  a  few  minutes  after  exposure  to  the  light  the  samples 
that  had  a  pH  greater  than  4.7  (in  this  instance  5.0)  had  turned 
dark  brown,  due  to  the  reducing  action  of  the  light  upon  the 
silver  salt,  while  on  the  more  acid  side  the  liquids  remained  clear. 
At  the  isoelectric  point  precipitation  due  to  insolubility  of  the 
gelatin  had  taken  place,  but  no  silver  was  present.  The  accom- 
panying curves  and  photograph1  make  clear  this  striking 
experiment. 

Many  other  salts  containing  a  readily  detectable  cation  were 
tested  with  identical  results.  Thus  when  the  gelatins  which 

1  J.  LOEB,  loc.  tit.,  1  (1918),  240. 


252  GELATIN  AND  GLUE 

have  been  made  to  varying  pH  by  treatment  with  hydrochloric 
acid  are  further  treated  with  nickel  chloride,  and  the  excess  of 
salt  washed  out  with  cold  water,  the  presence  of  nickel  may 
easily  be  determined  in  all  solutions  with  a  pH>4.7,  and  shown 
to  be  absent  at  pH  ^4.7.  Dimethylglyoxime  produces  a  crimson 
color  in  the  presence  of  nickel,  and  may  be  used  with  striking 
results  for  this  test.  Copper  salts,  when  added  in  the  above  man- 
ner, produce  a  blue  solution  with  the  gelatin  portions  of  a  pH 
greater  than  4 . 7,  but  show  no  trace  of  existence  in  the  solutions 
of  a  pH  less  than  4.7. 

Similar  experiments  may  be  made  which  serve  equally  well  to 
demonstrate  the  combination  of  gelatin  with  the  anion  of  salts 
at  a  pH  value  of  less  than  4.7.  For  this  purpose  the  gelatin  is 
treated,  after  the  acid  treatment  to  obtain  varying  hydrogen  ion 
concentrations,  with  salts  in  which  the  anion  may  be  easily  shown 
to  be  present  or  absent.  Potassium  ferrocyanide  answers  the 
purpose  very  well.  After  the  salt  has  been  allowed  to  react  for 
an  hour,  the  gelatins  are  filtered,  washed  as  before  specified  to 
remove  all  excess  of  salt,  and  made  up  to  1  per  cent  solutions. 
In  the  course  of  a  few  days,  in  those  samples  of  pH  <  4.7  the  gelatin 
solutions  turn  blue,  due  to  the  formation  of  the  ferriferro  salt, 
while  all  others  remain  colorless.  This  shows  that  only  when 
the  gelatin  has  a  pH<4.7  does  it  interact  with  anions  of  added 
salts.  Other  salts,  as  potassium  sulphocyanide,  which  turns  red 
upon  the  addition  of  a  ferric  salt,  may  be  employed  with  equal 
success  to  demonstrate  this  type  of  combination. 

It  seems  conclusively  demonstrated,  therefore,  that  gelatin  is 
an  ampholyte  which  may  ionize  either  as  an  acid  or  as  a  base, 
that  is,  it  may  give  off  an  excess  of  hydroxyl  ions,  or  an  excess  of 
hydrogen  ions,  depending  upon  the  hydrogen  ion  concentration 
of  its  solution.  Its  reactions  with  acids,  bases,  and  salts  are 
then  entirely  parallel  with  those  of  the  inorganic  ampholytes. 
As  isoelectric  gelatin  it  is  insoluble  and  unionized.  With  acids 
it  reacts  forming  a  gelatin  salt,  and  its  combination  with  other 
salts  is  then  confined  to  the  anions  of  the  latter.  With  bases  it 
reacts  to  form  metal  gelatinate,  and  on  interacting  with  other 
salts  can  then  combine  only  with  the  cations. 

At  any  given  condition  of  equilibrium  the  amount  of  gelatin 
chloride  or  of  potassium  gelatinate  formed  will  according  to  the 
Mass  Law  be  proportional  to  the  concentrations  of  hydrochloric 
acid  and  of  potassium  hydroxide  respectively  that  are  present. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


253 


This  also  is  shown  to  obtain  by  the  curves  in  Fig.  34,  and  by 
numerous  other  experiments.  This  point  was  further  demon- 
strated by  the  addition  of  hydrochloric  acid  in  small  amounts  to 
isoelectric  gelatin  and  the  simultaneous  determination  of  the 
hydrogen  ion  concentration.  It  was  shown  that  the  same 
amount  of  chlorine  was  always  in  combination  with  a  given  mass 
of  gelatin  for  the  same  pH.  This  means  that  if  the  concentra- 
tion and  pH  of  a  gelatine  are  known,  the  amount  of  chlorine  in  com- 
bination with  it  may  readily  be  calculated.  This  holds  equally 
well  for  bases,  for  at  the  same  pH  the  amount  of  cation  in  com- 
bination is  always  the  same. 

9.  The  Influence  of  Valency  upon  Protein-salt  Formation.— 
The  most  potent  argument  in  favor  of  a  purely  chemical  con- 
ception of  protein-salt  formation  may  be  found  in  the  relative 
behavior  of  inorganic  anions  and  cations  of  different  valency. 
In  the  weak  dibasic  and  tribasic  acids  such  as  oxalic,  citric,  phos- 
phoric, etc.,  it  has  been  shown1  that  the  primary  ionization  occurs 
much  more  readily  than  the  secondary,  and  this  in  turn  more 
readily  than  the  tertiary.  In  oxalic  acid,  for  example,  the  pri- 
mary ionization: 

COOH 


COOH 


COO- 


is  about  seven  hundred  and  sixty  times  the  secondary  ionization. 
"COOH    ~       [COO— "- 

*  +  H+ 

coo—        |_co°— _ 

The  primary  ionization  of  phosphoric  acid: 


/OH 

O  =  P^OH 
\OH 


/O- 
O  =  P— OH 

NOH 


is  about  fifty  thousand  times  the  secondary  ionization : 


O  =  P^O- 

NOH 


O  =  P^-OH 
\OH 


and  this  in  turn  about  five  hundred  thousand  times  the  tertiary 
ionization: 


1  See  Appendix,  page  579,  for  ionization  of  acids  and  bapes. 


254 


GELATIN  AND  GLUE 


In  the  stronger  sulphuric  acid  however  the  primary  ionization: 


°v 


CY 


H 


OH 


C)< 


-o—  |- 


\)H 


is  only  about  thirty  three  times  the  secondary  ionization : 


i: 

40 
38 

1  34 
&32 

tl  30 
S  26 
§  26 

c* 

B  22 
$  20 
*5  16 

1* 

s  14 

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1  10 

1: 

4 

2 

ft 

V 

X  \)H_ 

0\/°~r 

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\ 

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s. 

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s. 

<^>>«. 

V 

pM  20   22   24    26  2.8   3.0   3.2  3.4   3.6   38  40  42  4.4  46  46 
FlG    37^^curves  for  the  number  of  cc.  of  0.1N  HNOs,  H2SO4,    oxalic,  and 
phosphoric  acids  required  to  bring  1  gm.  of  isoelectric  gelatin  to  different  pH 
(in  100  cc.  of  solution). 

It  would  therefore  be  expected  that  in  interaction  with  the 
proteins  the  former  weak  acids  would  behave  as  if  they  were 
monobasic,  one  hydrogen  only  being  effective,  while  in  the  strong 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


255 


sulphuric  acid  both  hydrogen  ions  would  be  effective  and  the 
acid  would  react  as  dibasic.  This  was  found  to  be  the  case  by  the 
following  experiment.  Tenth  normal  solutions  of  several  acids 
of  varying  basicity  were  added  in  varying  amounts  to  gelatin 
that  had  been  rendered  isoelectric,  and  the  pH  of  1  per  cent 
solutions  determined  for  each  addition.  Upon  plotting  the 
pH  value  against  the  c.c.  of  the  N/10  acid  added  it  was  observed 
that  the  curves  were  identical  for  all  of  the  monobasic  acids  and 
the  strong  dibasic  sulphuric  acid,  but  that  the  curve  for  oxalic 


3  OH  47  52   £7    62   6.7    72   7.7  62   6.7 

8 


92    9.7  102  107  112  1L7 


FIG.  38.— Curves  for  the  number  of  cc.  of  0.1N  NaOH,  KOH,  Ba(OH)2,  and 
Ca(OH)2  required  to  bring  1  gm.  of  isoelectric  gelatin  to  different  pH  (in  100  cc. 
of  solution).  All  four  curves  are  identical. 

acid  showed  that  about  twice,  and  for  phosphoric  acid  about 
three  times,  the  volume  were  required  to  produce  a  given  pH  as 
for  the  monobasic  acids.  This  is  shown  in  Figure  37. 1  If,  how- 
ever, the  oxalic  and  phosphoric  acids  are  treated  as  if  they  were 
monobasic,  and  added  therefore  in  equivalent  molecular  propor- 
tions, the  curves  for  all  were  identical. 

On  applying  this  principle  to  bases  we  should  expect  the  strong 
diacid  bases  calcium  and  barium  hydroxides  to  act  as  diacid 
rather  than  as  monacid  bases.  On  adding  tenth  normal  solutions 
of  these,  and  of  sodium  and  potassium  hydroxides,  to  isoelectric 
gelatin,  it  was  found  that  the  curves  plotted  as  before  were 
identical,  as  shown  in  Figure  38. l 

i  J.  LOEB,  loc.  cit.,  3  (1920),  100;  104. 


256 


GELATIN  AND  GLUE 


The  following  table  shows  the  c.c.  of  0.01N  acid  in  combination 
with  10  c.c.  of  a  1  per  cent  gelatin  solution  at  different  pH:1 

TABLE  41. — COMBINATION  OF  GELATIN  WITH  ACIDS 


pH 

3.1 

3.2 

3.3 

3.4 

3.5 

3.7 

3.9 

4.1 

4.2 

4.3 

Nitric  acid  

4.35 

4.1 

3.6 

3.2 

2.85 

2.45 

1.9 

1.45 

0.75 

Oxalic  acid          

9.6 

8.75 

7  6 

6.7 

6  00 

4  3 

3  0 

1  65 

Phosphoric  acid   . 

12.4 

10.4 

9.8 

9  00 

7  4 

5  8 

4  5 

2  6 

2  1 

Loeb  therefore  reaches  the  conclusion  that  "the  ratios  in  which 
the  ions  combine  with  proteins  are  identical  with  the  ratios  in 


325 

300 

2.75 

250 

225 

200 

175 

150 

125 

•00 

75 

50 

25 

0 


05T)ot 


cPres; 


u  re 


pH  2.3    Z5    21     29    3.1      33    35   37    39    4.1     43   45  47 

H2504  • 
H£r° 

FIG.  39.— Osmotic  pressure  curves  for  gelatin  sulfate  and  gelatin  bromide. 
Abscissae  represent  pH;  ordinates,  osmotic  pressure;  showing  that  for  the  same 
pH  the  osmotic  pressure  is  higher  when  HBr  than  when  HzSO4  is  added  to  gelatin. 

which  the  same  ions  combine  with  crystalloids.  Or,  in  other 
words,  the  forces  by  which  gelatin  and  egg  albumin  (and  prob- 
ably proteins  in  general)  combine  with  acids  or  alkalies  are  the 
purely  chemical  forces  of  primary  valence." 

If  this  is  the  case,  as  seems  most  highly  probable,  then  since, 
as  has  been  shown,  the  viscosity,  osmotic  pressure,  swelling,  and 
alcohol  number  all  reveal  curves  that  are  parallel  to  the  curve, 

1  J.  LOEB,  Science,  62  (1920),  454. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


257 


325 
300 
275 
250 
225 
200 
175 
150 
125 
100 
75 
50 
25 
0 
3 
2 
J 
n 

x^ 

/ 

/ 

/ 

/ 

/ 

/ 

n 

I 

^ 

^ 

—  ' 

/  J 

r^ 

w 

f\ 

OS 

notii  Pr, 

SSUI 

e 

Cor 

duct  vit\ 

f\r\A 

-Q 

ims 

'- 

, 

o 

}  • 

« 

0 

•* 

pH  43   5.1    5.4  5.7  6.0  6.3  6.6  &9   7.2  75  78  8.1    8.4  8.7  9.0 

oBaf)H)2  •NaOH 

FIG.  40. — Showing  that  while  the  curves  for  conductivity  of  sodium  and  barium 
gelatinate  are  practically  identical,  the  curves  for  the  osmotic  pressures  are  very 
different. 


1  Z  3  4  5  6 

FIG.  41.— Influence  of  HC1,  HNO3,  H3PO4,  H2SO4,  trichloracetic,  and  oxalic 

acids  on  the  swelling  of  gelatin. 
17 


258 


GELATIN  AND  GLUE 


for  conductivity,  the  properties  enumerated  must  be  dependent 
upon  and  a  function  of  the  ionization  of  the  protein  which,  in 
turn,  has  been  shown  to  be  defined  by  the  hydrogen  ion  concen- 
tration. And  it  has  furthermore  been  shown  that  all  those  acids 
whose  anions  combine  as  monovalent  ions,  and  those  bases  whose 
cations  combine  as  monovalent  ions,  raise  these  several  prop- 


pH  4 


6 


10 


U 


12 


FIG.  42. — Curves  for  the  effect  of  different  bases  on  swelling.  Those  for  LiOH, 
NaOH,  KOH,  and  NH4OH  are  practically  identical  and  about  twice  as  high  as 
those  for  Ca(OH)2  and  Ba(OH)2. 


erties  of  the  proteins  much  more  than  those  acids  and  bases  whose 
anions  and  cations  respectively  combine  as  bivalent  ions  for  the 
same  pH.  Numerous  experiments  have  confirmed  this  general 
rule.  Figures  39  and  40  show  the  relative  effects  of  monovalent 
and  divalent  acids  and  bases  respectively  upon  the  osmotic 
pressure  of  gelatin  solutions  at  different  pH  values.1 

Figures  41  and  42  show  the  effects  of  such  acids  and  bases  upon 
the  swelling  of  gelatin.2 

Figures  43  and  44  show  similar  effects  upon  the  viscosity  of 
gelatin  solutions.3 

In  connection  with  the  ionization  theories  of  the  proteins,  the 
theory  advanced  by  Procter  and  Wilson  to  account  for  the  swell- 
ing of  gelatin,  and  the  mathematical  confirmation  of  that  theory 

1  J.  LOEB,  loc.  ciL,  1  (1918-19),  567;  493. 

2  J.  LOEB,  ibid.,  3  (1920),  253;  256. 

3  J.  LOEB,  loc.  cit.,  3  (1920),  101;  104. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


259 


pH  1  2  3 

FIG.  43. — The  curves  of  specific  viscosity  of  1  per  cent  solution  of  originally 
isoelectric  gelatin  brought  to  different  pH  by  different  acids. 


pH     4  5  C  7  8  9  10          U          1* 

Fio.  44. — Curves  for  specific  viscosity  of  Na  and  Ca  gelatinate  for  different  pH. 


260  GELATIN  AND  GLUE 

by  Wilson  should  be  recalled.1  They  assumed  and  demonstrated 
a  chemical  combination  between  the  gelatin  and  one  of  the  ions 
of  the  electrolyte  in  which  it  was  immersed,  and  accounted 
stoichiometrically  for  the  swelling  phenomena  observed  by  a 
mathematical  treatment  of  the  ion  relations  involved.  Wilson 
regards  this  as  the  strongest  kind  of  evidence  that  with  gelatin 
and  acids  we  are  dealing  with  ionic  reactions,  and  in  acid  solution 
the  positive  electrical  charge  on  gelatin  is  due  to  the  ionization 
of  a  gelatin  salt  of  that  acid. 

10.  The  Depressing  Action  of  Salts. — It  has  been  observed  by 
a  number  of  investigators  that  the  action  of  neutral  salts  upon 
the  physical  properties  of  proteins  differs  from  that  of  acids 
and  bases.  Pauli2  regards  the  combinations  of  neutral  salts 
with  electrically  neutral  protein  as  adsorption  compounds,  while 
reaction  with  acids  and  bases  he  regards  as  true  salt  formation. 
Lillie3  has  stated  that,  while  acids  and  bases  increase,  salts 
depress  the  osmotic  pressure  of  gelatin.  Loeb4  has  reported  that 
neither  of  these  statements  is  correct  on  account  of  the  failure 
of  the  writers  to  take  into  account  the  hydrogen  ion  concentration 
of  the  solutions.  He  asserts  that  "when  acids  or  alkalies  are 
added  to  isoelectric  gelatin  both  ions  of  the  acid  or  alkali  influ- 
ence the  physical  properties  of  proteins,  but  in  an  opposite 
direction.  When  we  add  acid  to  isoelectric  protein  the  hydrogen 
ions  increase  but  the  anions  depress  the  osmotic  pressure  and  vis- 
cosity of  the  protein  solution  (and  this  depressing  action  increases 
with  the  valency  of  the  anion  of  the  acid) .  As  long  as  little  acid 
is  added  to  isoelectric  protein  the  augmenting  action  of  the  hydro- 
gen ion  on  these  properties  increases  more  rapidly  with  increasing 
concentration  of  the  acid  than  the  depressing  action  of  the  anion; 
while  when  the  pH  of  the  solution  falls  below  3.3  or  3.0  the  reverse 
is  the  case.  This  causes  the  drop  in  the  curves  for  osmotic 
pressure,  viscosity,  and  swelling  below  a  pH  of  3.0. 

"When  we  add  alkali  to  isoelectric  protein  the  OH  ions  (or  the 
diminution  of  the  concentration  of  hydrogen  ions)  effect  an 
increase  in  the  osmotic  pressure,  viscosity,  etc.,  of  the  solution 
of  metal  proteinate  while  the  cation  of  the  alkali  depresses  these 
properties  with  a  force  increasing  with  the  valency  of  the  cation. 

1  Vide  pages  176  to  182. 

2  W.  PAULI,  Fortschr.  naturwiss.  Forschung,  4  (1912),  223. 

3  R.  S.  LILLIE,  Am.  J.  PhysioL,  20  (1907-08),  127. 

4  J.  LOEB,  /.  Gen.  PhysioL,  3  (1921),  391. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  261 

In  the  lowest  concentrations  of  the  alkali  added  the  augmenting 
action  of  the  OH  ion  on  the  physical  properties  of  the  metal 
proteinate  increases  more  rapidly  with  the  concentration  than 
the  depressing  effects  of  the  cation  of  the  alkali;  while  in  higher 
concentrations,  i.e.,  as  soon  as  the  pH  becomes  about  10.0  or 
11.0,  the  reverse  is  the  case. 

"When,  however,  neutral  salts  are  added  to  protein  solutions 
we  no  longer  notice  an  opposite  effect  of  the  oppositely  charged 
ions.  When  neutral  salts  are  added  to  isoelectric  gelatin  no 
effect  is  noticed  as  long  as  the  concentration  of  salt  does  not 
reach  the  value  required  for  precipitation.  When  neutral  salt  is 
added  to  a  protein  solution  on  either  side  of  its  isoelectric  point 
only  a  depressing  action  of  that  ion  which  has  the  opposite  sign 
of  charge  as  the  protein  ion  is  observed.  No  augmenting  action 
of  the  ion  with  the  same  sign  of  charge  as  the  protein  is 
noticeable." 

In  other  words,  Loeb  has  pointed  out  an  undisputed  fact,  and 
has  given  it  a  new  explanation.  He  notes  that  the  addition  of 
any  neutral  salt  to  a  pure  gelatin  of  any  pH  (provided  the  addi- 
tion of  such  salt  does  not  involve  an  alteration  in  the  pH  of  the 
solution)  results  in  a  depression  of  the  osmotic  pressure,  viscosity, 
etc.,  of  the  solution.  He  regards  the  hydrogen  and  the  hydroxyl 
ions  as  unique  in  that  they  alone  are  possessed  of  the  power  to 
increase  these  properties,  while  all  other  ions  depress  them.  This 
peculiarity  is  attributable  to  the  power  of  these  ions  to  produce 
ionized  gelatin,  either  as  the  gelatin-acid  salt,  or  as  the  metal 
gelatinate.  When  acid  is  added  to  isoelectric  gelatin,  gelatin- 
acid  salt  is  produced  and  for  this  reason  the  osmotic  pressure, 
viscosity,  etc.,  increase  rapidly.  The  anion  of  the  acid  is 
constantly  exerting  its  depressing  influence  upon  these  properties, 
but  the  extent  of  such  effect  is  not  as  great  as  the  increasing  effect 
due  to  the  ionization,  hence  these  properties  increase.  "Near 
the  isoelectric  point  the  amount  of  gelatin-acid  salt  formed 
increases  very  rapidly  with  the  addition  of  acid,  but  when  the  pH 
approaches  3.0  the  addition  of  the  same  amount  of  acid  which 
near  the  isoelectric  point  caused  a  considerable  change  (increase  in 
ionization)  now  causes  only  a  slight  change,  while  when  the  pH 
falls  below  3.0  the  depressing  influence  of  the  anion  continues 
with  increasing  concentration  of  the  electrolyte." 

Thus  if  we  start  with  gelatin  at  a  pH  of  4.0  and  add  increasing 
amounts  of  hydrochloric  acid,  the  viscosity  will  rise  at  first  due 


262 


GELATIN  AND  GLUE 


to  an  increasing  ionization  caused  by  the  hydrogen  ion.  The 
depressing  effect  of  the  chloride  ion,  which  has  been  acting  all  the 
time,  becomes  manifest  when  pH  becomes  smaller  than  3.0, 
at  which  time  the  viscosity  begins  to  fall.  If  sodium  chloride 
were  used  in  place  of  the  acid,  the  sodium  ion,  since  it  cannot 
produce  an  increase  in  the  ionization  of  the  gelatin,  is  entirely 
without  effect,  while  the  chloride  ion,  being  depressing,  results 
in  a  decrease  in  viscosity  from  the  start.  This  is  shown  in  Fig. 
45.  If  calcium  chloride  and  lanthanum  chloride  are  used  in  place 


Concentration 

FIG.  45. — Difference  in  the  effect  of  different  concentrations  of  NaCl  and  of 
HC1  on  the  specific  viscosity  of  a  1  per  cent  solution  of  gelatin  chloride  of  pH  4.0. 
In  the  case  of  Nad  we  observe  only  the  depressing  effect  of  the  Cl  ion;  in  the 
case  of  HC1  we  notice  an  augmenting  effect  of  the  H  ion  and  a  depressing  effect 
of  the  Ci  ion,  the  latter  prevailing  as  soon  as  the  concentration  of  acid  added  is 
>N/256. 

of  sodium  chloride,  and  in  molecularly  equivalent  quantities, 
the  depressive  effect  of  the  chloride  will  be  in  the  order  LaCl3  > 
CaCl2>NaCl,  i.e.,  in  the  order  of  the  amounts  of  Cl  present, 
but  if  used  in  equi-normal  quantities,  i.e.,  amounts  containing 
the  same  quantity  of  Cl,  the  effects  are  identical.  If  sodium 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


263 


sulphate  and  sodium  ferrocyanide  are  used  in  amounts  which  are 
also  equivalent  in  respect  to  their  anions  (equimolar).  it  is  found 
that  the  order  of  depression  is  Na4Fe(CN)6>Na2S04>NaCl. 
That  is,  the  depression  is  in  the  order  of  the  valence  of  the 
anion. 

The  above  salt  effect  is  based  upon  the  maintaining  of  a  con- 
stant pH.     If  a  salt  is  added  which  results  in  a  change  in  the 


Concentration 

FIG.  46. — The  depressing  effect  of  different  salts  with  monovalent  anion 
(NaCl,  NaH2PO4,  NaCNS,  NaH  tartrate,  and  NaH2  citrate)  on  the  specific 
viscosity  of  1  per  cent  solution  of  gelatin  chloride  of  pH  3.0.  The  effect  of  NaCl 
and  NaH2PO4  are  identical  since  the  pH  is  not  altered  by  the  addition  of  these 
salts.  The  depression  in  the  values  for  the  specific  viscosity  is  greater  in  the 
case  of  Na  acetate  than  in  the  case  of  NaCl  for  the  reason  that  the  Na  acetate 
raises  the  pH  of  the  gelatin  solution. 

pH,  then  the  ionization  factor  would  have  also  to  be  considered. 
Thus  the  addition  of  sodium  acetate  to  gelatin  chloride  would, 
through  the  hydrolytic  dissociation  of  the  salt,  make  the  solution 
slightly  more  alkaline,  and  have  an  effect  similar  to  the  addition 
of  a  small  amount  of  an  alkali.  This  would  obviously  bring  the 
gelatin  chloride  nearer  to  the  isoelectric  point,  or,  in  other  words, 


264  GELATIN  AND  GLUE 

repress  the  ionization.  Thus  the  depression  in  viscosity  due  to 
the  acetate  ion  would  be  augmented  by  the  decrease  in  ionization, 
and  a  sharper  fall  with  increasing  concentration  would  result. 
This  is  shown  to  be  the  case  from  Fig.  46. 

If,  on  the  other  hand,  a  salt  which  hydrolyzed  with  the  libera- 
tion of  hydrogen  ions  as,  for  example,  aluminium  chloride,  stannic 
chloride,  etc.,  were  to  be  added  to  a  gelatin  chloride  of  a  pH  of 
4.0,  it  would  be  expected  that  the  depressing  effect  of  the  chloride 
ions  would  be  opposed  by  the  effect  of  an  increased  ionization, 
and  an  actual  increase  in  viscosity  would  probably  be  observed. 
Copper  chloride  would  produce  a  lesser  degree  of  ionization,  and 
whether  the  mean  of  the  opposing  effects  would  produce  an 
actual  rise  or  decrease  in  viscosity  would  depend  on  the  exact 
ratio  of  the  two  influences. 

We  have  discussed  thus  far  only  the  reactions  and  relations  on 
the  acid  side  of  the  isoelectric  point.  But  what  has  been  said 
above  applies  mutatis  mutandis  on  the  alkaline  side.  Sodium 
hydroxide,  for  example,  increases  the  ionization  of  gelatin  as 
sodium  gelatinate.  This  increase  in  ionization  results  in  an 
increase  in  viscosity,  etc.  The  sodium  ion  is  acting  as  a  depressing 
agent,  but  not  until  the  pH  reaches  about  10.0  does  it  exceed  the 
opposite  influence  of  the  hydroxyl  ions.  Sodium  chloride  pro- 
duces a  depressing  action  throughout,  due  to  the  sodium  ion, 
since  no  effect  is  produced  upon  ionization.  Any  other  salt 
solutions  containing  equivalent  amounts  of  any  monovalent  cation 
act  similarly.  Divalent  and  trivalent  cations  exert  increased 
effects,  while  anions  are  without  influence.  Salts  like  sodium 
acetate  or  sodium  silicate,  since  on  the  alkaline  side  of  the  isoelectric 
point  they  increase  the  ionization,  would  be  expected  to  oppose 
the  depressing  effect  of  the  sodium  ion,  and  an  actual  increase  in 
viscosity,  etc.,  might  be  observed.  The  author1  has  found  this  to 
be  the  case  with  sodium  silicates,  and,  which  is  of  great  importance 
in  the  present  theory,  the  degree  of  increase  produced  by  seven 
silicates  of  varying  composition  was  found  to  be  proportional  to 
the  degree  of  hydrolytic  dissociation  which  these  silicates 
underwent  in  dilute  solution.  Curves  showing  a  maximum  of 
viscosity  and  swelling  at  a  pH  of  about  9.0  are  thus  obtained. 
These  are  shown  in  Fig.  47. 

11.  The  Micelle  Theory  of  McBain. — The  data  that  have  been 
given  upon  the  conductivity  and  osmotic  pressure  of  gelatin 

1  R.  H.  BOGUE,  J.  Ind.  Eng.  Chem.,  14  (1922),  32. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID 


265 


when  dissolved  in  dilute  acids  or  bases,  and  the  theories  advanced 
to  account  for  such  action,  find  excellent  confirmation  and  sup- 
port in  the  micelle  theory  postulated  by  McBain  and  his  pupils1 


Solid 

Semi- 
Solid 

Liquid 

^i.e»o 

'8 

o 
.** 

?  1.50 

d> 
;g 

^  1.40 

o 
O 

50 

i>45 

•f  40 
w 

£  35 
<*- 

5  30 

•£  o* 
w   25 

—    on 

\   v 

J 

C.LLY  CO 

N  SI  57  El 

vcr 

=^ 

I 

z 

\ 

VISCO 

SITY 

/ 

/ 

\ 

^ 

/ 

/ 

/ 

X 

\ 

/ 

SWELL 

ING  y 

/ 

/ 

/ 

/ 

±    L° 
.5 

/ 

6  7  8  9 

Value  of  Gelatin  Solution 


10 


FIG.  47.  —  The  jelly  consistency,  viscosity,  and  swelling  of  gelatin  at  varying  pH 

values. 


to  account  particularly  for  the  behavior  observed  with  soap 
solutions.  The  importance  of  their  findings  has,  however,  far 
outdistanced  the  original  investigations,  and  the  conclusions 
which  were  derived  from  an  intensive  study  of  sodium  palmitate 
appear  to  be  equally  applicable  to  the  large  group  of  colloidal 
electrolytes  of  which  the  proteins  are  important  members. 

1  J.  W.  McBAiN  and  TAYLOR,  Ber.,  43  (1910),  321;  Z.  physik.  Chem.,  76 
(1912),  179;  J.  W.  McBAiN  and  C.  S.  SALMON,  /.  Am.  Chem.  Soc.,  42 
(1920),  426. 


GELATIN  AND  GLUE 

McBain1  defines  colloidal  electrolytes  as  "salts  in  which  one  of 
the  ions  has  been  replaced  by  a  heavily  charged,  heavily  hydrated 
ionic  micelle  which  exhibits  equivalent  conductivity  that  is  not 
only  comparable  with  that  of  a  true  ion  but  may  even  amount  to 
several  times  that  of  the  simple  ions  from  which  it  has  been 
derived.  In  other  words,  this  ionic  micelle  is  a  typical  but  very 
highly  charged  colloidal  particle  of  very  great  conductivity." 
Of  particular  significance  is  the  relation  defined  between  con- 
ductivity and  osmotic  pressure.  "The  conductivity  of  such  a 
colloidal  electrolyte  is  quite  comparable  with  that  of  an  ordinary 
electrolyte.  On  the  other  hand,  since  the  ionic  micelle  exhibits 
only  the  osmotic  effect  characteristic  of  an  ordinary  colloid, 
the  total  osmotic  activity  of  the  colloidal  electrolyte  is  cor- 
respondingly deficient  and  may  be  distinctly  less  than  that  of  a 
non-electrolyte.  Thus  high  conductivity  goes  hand  in  hand 
with  only  moderate  osmotic  effects." 

The  principal  evidence  for  the  existence  of  the  ionic  micelle  is 
based  upon  a  comparison  of  conductivity  and  osmotic  data. 
For  example,  in  concentrated  solutions  of  the  higher  soaps  the 
osmotic  activity  is  often  only  about  half  that  required  to  explain 
the  conductivity.  Whereas  the  conductivity  is  nearly  as  great 
as  that  of  an  inorganic  salt,  the  osmotic  pressure,  on  the  other 
hand,  is  only  about  half  that  of  a  non-electrolyte  such  as  sucrose. 
The  osmotic  activity  appears  therefore  to  correspond  with  that 
of  the  inorganic  ion  only,  while  "the  other  half  of  the  current 
must  be  carried  by  an  ion  that  is  colloidal  so  as  not  to  exhibit 
appreciable  osmotic  activity,  and  that  nevertheless  retains  the 
sum  total  of  the  electrical  charges  of  the  ions  from  which  it  was 
derived.  This  is  the  ionic  micelle."  Taking  potassium  laurate 
as  an  example,  when  the  entire  osmotic  pressure  is  attributed  to 
the  potassium  ion,  about  half  of  the  conductivity  must  be 
ascribed  to  the  colloid.  But  this  portion  of  the  conductivity 
ascribed  to  the  ionic  micelle  of  potassium  laurate  is  about  three 
times  greater  than  could  be  exhibited  by  the  separate  laurate 
ions  had  they  retained  an  independent  existence.  This  McBain 
accounts  for  by  pointing  out  that,  as  predicted  from  Stoke's 
Law,  the  resistance  offered  to  a  particle  increases  directly  with 
its  diameter,  and  that  when  a  number  of  small  particles  coalesce, 
the  diameter  of  the  large  particle  does  not  increase  in  the  same 

1  J.  W.  McBAiN,  "Third  Report  on  Colloid  Chemistry,"  British  Assoc. 
for  the  Adv.  of  Science  (1920),  2. 


GELATIN  AS  AN  AMPHOTERIC  COLLOID  267 

ratio  as  the  electric  charge,  the  latter  being  equal  to  the  sum  of 
the  charges  added. 

The  formula  ascribed  to  the  ionic  micelle  of  sodium  palmitate  is 
written  : 

(NaP)z.(P-)n.(H20)ro. 

The  exponents  x,  n,  and  m,  which  indicate  the  ratio  of  the  com- 
ponents of  the  micelle,  may  vary  continuously  with  change  in 
concentration  or  temperature  or  upon  the  addition  of  salts. 

It  does  not  appear  to  be  a  difficult  matter  to  apply  the  micelle 
theory  to  the  reactions  of  gelatin  and  to  reconcile  its  existence 
with  the  experimental  findings  of  Loeb,  Procter,  Wilson,  Robert- 
son, and  others.  Consider  the  salt  sodium  gelatinate.  The 
formula  of  the  ionic  micelle  would  be: 


in  which  (NaG)»  represents  a  variable  amount  of  undissociated 
molecules  of  sodium  gelatinate,  and  (G~)n  a  variable  number  of 
negatively  charged  gelatin  ions.  These  are  combined  with  a 
variable  amount  of  water  of  hydration.  The  sodium  gelatinate 
may  be  represented  by: 

/NH2 

R\ 
XCOONa 

and  the  negatively  charged  gelatin  ions  by: 


R 


/NH2     1 
\COO-J 


There  would  also  be  present  in  the  system,  according  to  Loeb,  a 
variable  amount  of  isoelectric  gelatin  and  of  sodium  hydroxide. 
The  former  might  or  might  not  become  a  part  of  the  ionic  micelle, 
but  its  presence  could  not  materially  modify  the  nature  of  that 
body.  The  sodium  hydroxide  would  constitute  the  determining 
influence  in  the  equilibrium  which  should  define  the  relative 
values  of  x,  n  and  m.  By  the  application  of  the  Mass  Law,  any 
increase  in  the  concentration  of  the  sodium  ion,  whether  by  the 
addition  of  sodium  hydroxide,  or  a  sodium  salt,  would  decrease 
the  concentration  of  the  gelatin  ion  and  increase  the  concentra- 
tion of  the  unionized  sodium  gelatinate.  This  would  seem  at 


268  GELATIN  AND  GLUE 

first  glance  to  contradict  Loeb's1  findings  that  the  conductivity 
of  the  gelatin  salt  increases  regularly  with  acid  or  base  additions, 
for  the  conductivity  must  be  ascribed  to  the  ionized  portion  of 
the  gelatin,  and  as  acid  or  base  are  added,  this  ionized  portion 
should  decrease.  A  more  detailed  inspection  of  the  reaction  will 
show,  however,  that  the  objection  is  not  well  made.  In  the  first 
place,  in  the  salt  sodium  gelatinate, 

NaG  <=>  Na+  +  G~ 
and 

[Na+]  X  [G~] 
[NaG] 

As^[Na+]  is  increased,  [G~]  must  decrease,  but  the  product 
lNa+]  X  [G~~]  must  remain  constant,  and  the  electric 
current  will  be  carried  equally  well  by  a  high  sodium  and 
low  gelatin  ion  concentration  as  by  equivalent  amounts  of  the  two. 
Furthermore,  it  must  not  be  lost  sight  of  that  isoelectric  gelatin 
is  present  in  equilibrium  with  the  other  components  represented, 
and  the  sodium  hydroxide  establishes  not  only  the  equilibrium 
between  ionized  and  unionized  sodium  gelatinate,  but  also  be- 
tween isoelectric  gelatin  and  sodium  gelatinate.  So,  concomi- 
tantly  with  a  decrease  in  ionization  of  the  already  existing  sodium 
gelatinate,  there  is  also  taking  place  a  continuous  increase  in  the 
total  amount  of  sodium  gelatinate  (ionized  and  unionized)  in  the 
system. 

If  we  consider  now  the  salt,  gelatin  chloride,  the  conclusions 
will  be  nearly  the  same.  The  formula  of  the  ionic  micelle  would 
be: 


where  the  equilibrium  between  the  ionized  and  unionized  gelatin 
chloride  could  be  represented  by: 

yNH3-    l  + 
Rv 


COOH 


\ 


COOH 


The  effect  of  further  additions  of  acid  would  be  similar  to  those 
found  to  obtain  with  further  additions  of  base  to  the  sodium 
gelatinate. 

1  J.  LOEB,  J.  Gen.  Physiol,  3  (1920),  260. 


PART  II 
TECHNOLOGICAL  ASPECTS 

THE   SECOND   GREAT   MISSION    OP   SCIENCE   IS   TO  APPLY  ALIKE  TO 

THE   SERVICE   OF   MAN   THE   KNOWLEDGE   OF   THE   AGES  AND  THE 

LIGHT     OF    THE     YOUNGEST   DAY,   THAT   LIFE    MAY    BE   RICHER   IN 

OPPORTUNITY 


PART  II 
TECHNOLOGICAL  ASPECTS 

CHAPTER  VI 

THE  MANUFACTURE  OF  GLUE  AND  GELATIN 

BY  RALPH  C.  SnuEY1  M.S. 

Glue  is  cooked  from  the  hides  of  bulls. 
(Pliny,  about  50  A.D.) 

PAGE 

I.  Raw  Materials 271 

II.  Manufacture 277 

1.  Hide  Glue r  277 

Preparation  and  Preservation  of  Stock 277 

Soaking  and  Washing 278 

Liming 279 

Washing  and  Deliming 281 

The  Boiling  Process. . 282 

Clarification  and  Filtration 287 

Evaporation 290 

Chilling  and  Spreading 292 

Drying 294 

2.  Bone  Glue 300 

3.  Ossein 303 

4.  Gelatin 306 

III.  By-Products 307 

IV.  A  Select  Bibliography  on  the  Manufacture  of  Glue  and  Gelatin. . .    313 
The  glue  making  process,  in  its  simplest  terms,  consists  of 

nothing  more  than  the  production  of  a  concentrated  soup  stock 
or  consomme*  from  certain  animal  refuse,  and  its  subsequent 
purification  and  drying  for  the  market. 

I.  RAW  MATERIALS 

The  raw  materials  used  in  the  making  of  glue  and  gelatin  are, 
in  the  general  order  of  their  glue  making  value:  skin  or  hide, 
connective  tissue,  cartilage,  and  bone.  Muscle  tissue  is  of  no 
value.  Horns  and  hoofs  contain  no  gelatin.  However,  the 

1  Chemical  Engineer  with  the  Redmanol  Chemical  Products  Company  of 
Chicago.  Formerly  with  the  Armour  Glue  Works  of  Chicago. 

271 


272  GELATIN  AND  GLUE 

horn  piths,  the  inner  bony  core  of  the  horns,  are  an  important 
source  of  ossein. 

Hide  is,  of  course,  so  valuable  to  the  tanner  that  only  those 
parts  which  he  cannot  use  find  their  way  to  the  glue  maker. 
Calf  skin,  although  higher  in  mucin-like  substances,  yields  the 
best  and  clearest  gelatin.  Kip  stock,  or  the  skin  from  almost 
mature  calves,  comes  next,  and  finally  the  hide  from  the  mature 
cattle.  Skins  of  other  animals  are  handled  similarly  to  those  of 
cattle,  and  for  the  sake  of  brevity  those  of  cattle  alone  will  be 
treated  in  this  chapter.  Notable  among  other  sources  are: 
sheep,  goat,  deer,  coney,  pig,  dog,  horse  and  in  fact  the  scraps 
from  the  hides  and  pelts  of  all  animals  which  find  their  way  into 
any  manufacturing  process.  The  term  hide  is  applied  to  the 
skins  of  the  larger  animals  while  that  of  the  smaller  animals  may 
carry  the  term  skin  or  simply  stock.  Coney  stock  is  the  par- 
tially dehaired  and  shredded  skins  of  rabbits,  a  by-production 
from  the  hat  maker. 

The  scrap  from  the  tanner  is  often  sorted  into  classes  which 
allow  the  glue  maker  to  get  much  more  uniform  treatment  than 
if  everything  were  handled  together.  Hide  pieces  are  the 
portions  trimmed  off  in  preparing  the  hide  for  the  tanner.  The 
pates  or  faces  are  prized  by  the  glue  maker,  but  the  mucous  mem- 
branes of  the  mouth  and  nose  swell  rapidly,  and  being  easily 
overheated  are  apt  to  show  abnormal  losses.  Ears  are  harder, 
have  considerable  adhering  flesh  and  because  of  the  varying 
thickness  do  not  lime  uniformly.  The  hair  of  the  interior  of  the 
ear  is  removed  and  used  in  making  "camel's  hair'7  brushes. 
Tails  if  stripped  of  the  bone  give  a  satisfactory  product. 

After  the  tanner  has  unhaired  and  limed  his  hides  he  passes 
them  through  the  fleshing  machine,  a  machine  having  spiral 
blades  mounted  on  a  roller  for  the  purpose  of  removing  all 
adhering  flesh  and  fat  left  by  the  butcher.  These  trimmings, 
known  as  fleshings,  as  well  as  splits,  or  still  deeper  trimmings 
taken  off  by  splitting  knives,  give  glues  similar  to  hides  but  of 
somewhat  lower  grade. 

Skivings  are  shavings  taken  off  later  in  the  tanning  process  by  a 
machine  somewhat  similar  to  a  planer  for  the  purpose  of  giving 
the  leather  a  uniform  thickness  and  appearance.  As  the  tanning 
process  seems  to  be  as  much  physical  as  chemical  it  ought  to  be 
possible  to  remove  the  tanning  material  and  use  the  recovered 
product  for  glue.  Many  patents  have  been  taken  out  for  such 


MANUFACTURE  OF  GLUE  AND  GELATIN  273 

processes,  but  very  few  of  them  are  being  worked  at  present. 
Alum  tanned  and  chrome  tanned  leathers  are  more  easily  han- 
dled than  those  produced  with  tannin.  In  general  the  processes 
consist  of  strong  alkali  treatment  with  either  caustic  or  carbonate 
of  soda,  followed  by  acid  treatment  and  finally  bleaching  with 
hydrogen  peroxide  or  a  strong  oxidizing  agent. 

Raw  hide  scraps,  lace  leather,  worn  out  raw  hide  pinions  and 
"loom  pickers"  can  be  handled  the  same  as  other  scrap. 

Sinews  or  tendons  trimmed  off  by  the  butcher  require  slower 
treatment  than  hide  stock  but  yield  a  very  good  grade  of  glue. 

Cartilaginous  material  may  be  handled  with  either  sinews  or 
bone:  this  is  generally  determined  by  the  material  with  which  it 
is  associated. 

Bones  from  different  parts  of  the  body  vary  somewhat  in 
composition.  The  soft  bones  of  the  head  and  shoulders  yield 
more  glue  than  the  thigh  bones  and  thick  parts  of  the  vertebrae. 
In  general  the  tubular  bones  are  poorer  in  glue  than  the  flat 
bones.  Young  animals  have  less  mineral  matter  in  the  bones 
than  do  older  animals,  but  they  also  have  relatively  more  water. 
In  the  teeth,  the  dentine  and  cement  are  true  bony  structure 
while  the  enamel  is  very  high  in  lime  and  magnesia,  relatively 
low  in  phosphates,  and  yields  practically  no  gelatin. 

In  the  order  of  their  value  for  producing  glue  of  good  color  and 
test,  they  may  be  classed  as  green  or  fresh  bone,  steam,  pickle, 
country  and  junk  bones. 

Green  bone,  as  its  name  implies,  is  the  fresh  bone  direct  from 
the  butcher,  and  consists  of  the  heads  and  parts  removed  in 
preparing  the  carcass  for  market.  To  prevent  deterioration  it 
must  receive  immediate  attention. 

Steamed  bone  or  packer  bone  is  fresh  bone  which  has  been 
subjected  to  a  preliminary  cooking  by  the  packer,  perhaps  before 
the  complete  removal  of  the  flesh. 

Pickle  bone  is  somewhat  similar  to  steam  bone,  but  having  been 
in  pickle  requires  more  careful  washing  to  remove  the  added 
salts. 

Country  bone  is  the  scrap  from  the  small  butcher  shops  and 
varies  considerably  in  quality. 

Junk  bone  is  what  its  name  signifies — old,  dry  or  partly  decom- 
posed bone,  anything  the  junk  dealer  may  pick  up. 

The  following  tables,  supplied  through  the  courtesy  of  Mr. 
Heicke  of  Armour  &  Co.,  show  average  yields  of  glue,  grease,  and 

18 


274 


GELATIN  AND  GLUE 


tankage  obtained  from  various  market  grades  of  raw  materials 
over  a  period  of  about  20  years. 


TABLE  42. — YIELDS  OF  GLUE,  GREASE,  TANKAGE  AND  PER  CENT  OF  WATER 

IN  HIDE  STOCK 


Raw  material 

Water 
per  cent 

Glue 
per  cent 

Grease 
per  cent 

Tankage 
per  cent 

Ossein,  dry  

10 

70-80 

5 

Calf  stock  gr  salted 

30-40 

16 

2  00 

9 

Calf  stock  gr.  limed  
Calf  stock  gr  dry 

50-70 
10 

14 
35-40 

1.00 
1  00 

5 
5-10 

Calf  splits  gr  limed 

50-70 

14 

1  00 

5 

Kipstock  gr.  salted  
Cattle  hide  stock  gr.  salted  
Cattle  hide  stock  gr.  limed  
Cattle  hide  stock  dry  
Ears  (cattle)  gr.  salted  
Ears  (cattle)  gr.  limed  
Ears  (cattle)  dry  
Hide  splits  gr  limed 

30-40 
30-40 
50-70 
10 
35 
50-60 
10-15 
50-70 

18 
18 
14 
35 
13-14 
12 
35 
14 

3.00 
3.00 
2-4 
1.00 
4-5 

rt' 

8 
10 
8 
5-10 
5 
5 
10-20 
5 

Rawhide  dry  

10 

40-50 

5 

Cotton  pickers  dry  
Lace  leather 

10 
40 

50 
25 

1.00 
12-15 

8 
5 

Sheepskin  gr  limed 

60 

7.00 

7  00 

5-10 

Goatskin  gr.  limed  
Horsehide  gr  salted 

60 
40 

7-9 

12 

1-2 
5 

7 
10-20 

Buffalo  hide  dry  .... 

10-20 

45-50 

2 

10-20 

Alligator  hide  gr    

30-40 

40 

15-20 

Rabbitskin  French  dry  

10 

50-55 

20 

Rabbi  tskin  Australian  
Rabbitskin  English 

10 
10 

50-55 
50 

25 
16 

Rabbitskin  Chappings  

10 
50-60 

30-40 
20-25 

15 

20-30 
10 

50-60 

14 

10-15 

5 

Fleshings  cow,  gr.  limed  :  .  . 
Fleshings  calf,  limed  
Fleshings  horse  limed 

50-70 
50-70 
50-70 

8-12 
7-8 
6-8 

5-12 
2-3 
1-2 

5-10 
10 
15 

Fleshings  cow,  dry 

15 

20-25 

5-25 

10-20 

Fleshings  German 

10 

27-33 

1.00 

Fleshings  Italian  
Fleshings  India  

12 
15 

40 
35 

2.50 

20-30 
20 

Gara  Gara  Blanca  dry  S.  A  
Gara  Nigra  dry  S.  A  
Chrome  splits  
Chrome  shavings  

10-15 
10-15 
40-50 
40-50 

40 
32 
25 
18-20 

3 
1 

15-20 
20 

Sinews  green  

50-60 

16-18 

2-3 

6 

35-50 

22-24 

2-3 

7-8 

10-15 

40-50 

3-4 

10-15 

Sinews  dry  bone  (Calcutta)  

10-15 

35-40 

30 

In  studying  these  figures  it  is  necessary  to  consider  the  wide 
variations  in  composition  due  to:  1,  the  condition  of  the  animals, 
caused  by  climate,  region,  feeding,  age  and  handling;  2,  the 


MANUFACTURE  OF  GLUE  AND  GELATIN 


275 


TABLE  43. — YIELD  OF  GLUE,  GREASE,  TANKAGE  AND  PER  CENT  OF  WATER 

IN  BONE  STOCK 


Raw  material 

Water 
per  cent 

Glue 
per  cent 

Grease 
per  cent 

Tankage 
per  cent 

HN3    in 
tankage 
per  cent 

Boiled 

Cattle  skulls  green  •  
Cattle  jaws  green  
Cattle  feet  green.  
Cattle  rib  bones  

40-60 
40-60 
40-60 
40-60 

12 
12 
14 
10 

10 
5 
12 
10 

28 
45 
32 
30 

2.5 
2.5 
2.5 
2  5 

open 
open 
open 

Cattle  knuckles  
Cattle  skulls  dry  
Cattle  jaws  dry  

50 
10 
10 

10 
18-20 
18-20 

20 
1 
1 

28 
66 
66 

2.5 
0.75 
0  75 

open 
pressure 

Cattle  knuckles  dry  
Pig  heads  green  . 

10 
50-60 

18-20 
8 

1 
10 

66 

28 

0.75 
2  00 

pressure 

Pig  feet  green  
Calf  heads  green  
Calf  feet  green 

50-60 
50-60 
40-50 

14 

8 

g 

14 
6 

3 

20 
28 
30 

3.00 
2.00 
2  0o 

open 
open 

Sheep  heads  green/  
Hornpiths  green  ,  
Hornpiths  dry  

50-60 
35-40 
10-12 

6 
18 
23 

10 

25 
32 
66 

3.00 
2.5 
0  75 

open 
open 

Country  bones  dry  
Junk  bones  dry  

10-15 
10-15 

15 
15 

2-4 

2-4 

50-60 
50-60 

0.75 
1  00 

pressure 

condition  and  treatment  of  the  stock  in  preparation  for  market; 
and  3,  handling  of  the  stock  in  the  factory,  which  last  will  be 
treated  in  discussion  of  variations  in  process.  The  percentage 
moisture  in  the  stock  as  received  is  the  factor  of  widest  variation 
and  is  indicative  of  the  amount  of  concentration  to  which  the 
stock  has  been  subjected  in  preparation  for  transportation  and 
storage.  "Bone  dry"  condition  of  stock  means  roughly  10  per 
cent  moisture  except  in  the  presence  of  appreciable  quantities  of 
flesh,  when  the  moisture  content  runs  higher.  The  dry  cattle 
skulls,  jaws  and  knuckles  referred  to  in  Table  43  have  been  grease 
extracted  and  dried  before  being  marketed. 

Secondary  Raw  Materials. — Water  is  undoubtedly  the  most 
important  of  all  secondary  raw  materials  of  the  glue  maker,  the 
amount  used  often  running  into  millions  of  gallons  per  day. 
As  to  quality,  the  best  is  none  too  good,  and  for  the  gelatin  maker 
distilled  water  is  almost  a  necessity.  In  the  washing  process 
described  later  it  is  necessary  to  have  water  of  low  total  salt 
content  to  obtain  the  maximum  swelling.  This  is  more  impor- 
tant by  far  than  to  have  the  water  "soft"  or  low  in  lime.  The 
ordinary  softening  processes  are  of  value  only  in  so  far  as  they 
produce  a  bright  and  sparkling  water  or  reduce  the  bacterial 
content  if  coagulation  methods  are  used.  The  presence  of 


276  GELATIN  AND  GLUE 

bacteria  in  the  final  wash  water  is  to  be  guarded  against,  for 
practically  neutral  glue  stock  is  an  excellent  medium  for  bacterial 
growth,  and  if  kept  for  any  time  after  washing,  the  stock  is 
bound  to  deteriorate  to  a  certain  extent.  It  is  therefore  often 
desirable  to  sterilize  the  water,  and  any  of  the  methods  in  common 
use  on  potable  waters  are  applicable.  It  should  be  borne  in 
mind  that  such  substances  as  ozone  and  chlorine  harden  the 
stock,  and  therefore  only  the  minimum  amount  necessary  to 
accomplish  the  purpose  should  be  used,  and  any  unused  residue 
disposed  of  by  aeration  or  chemical  treatment  before  the  water 
is  ready  to  use  in  the  factory.  In  the  absence  of  organic  matter 
(which  may  consume  appreciable  quantities  of  oxidizing  agents) 
a  fraction  of  a  part  per  million,  if  left  in  contact  with  the  water 
for  a  sufficient  length  of  time,  will  produce  practically  complete 
sterilization. 

For  the  actual  boiling  of  the  glue,  distilled  water  is  always  to 
be  preferred.  Any  salts  in  the  water  will  be  concentrated  during 
evaporation  and  remain  in  the  finished  product  to  the  probable 
detriment  of  the  appearance.  If  the  liquors  are  drained  off  at 
say  5  per  cent  concentration,  that  means  that  there  will  be 
nineteen  times  as  much  mineral  matter  added  to  each  pound  of 
glue  as  there  is  in  each  pound  of  water. 

Air  is  another  important  secondary  raw  material.  As  about 
a  ton  of  air  is  generally  required  to  evaporate  the  water  from  the 
jelly  containing  a  single  pound  of  dry  glue,  the  quantities  handled 
are  enormous.  Other  than  situating  the  air  intake  so  as  to 
obtain  as  pure  air  as  possible,  nothing  is  done  in  the  way  of 
purifying  or  preparing  this  raw  material.  The  advantages  and 
possibilities  of  purification  will  be  discussed  in  the  treatment  of 
the  drying  process. 

Sulphur  dioxide  is  used  in  the  acid  treatment  and  for  bleaching. 
It  is  generally  produced  in  the  plant  by  burning  sulphur  in  cast- 
iron  furnaces  with  a  slight  excess  of  air.  This  excess  of  air  is 
necessary  to  minimize  the  sublimation  of  sulphur  under  the  heat 
produced  by  the  combustion.  The  gas  may  be  led  into  pottery 
lined  absorption  towers  for  the  production  of  a  dilute  solution 
which  may  be  used  direct  or  after  further  dilution.  This  liquor 
is  generally  stored  and  transported  in  wood.  Sulphur  dioxide 
gas  is  piped  direct  from  the  burners  to  the  storage  and  treating 
tanks  where  it  is  blown  into  the  liquors  in  the  purification  and 
bleaching  processes. 


MANUFACTURE  OF  GLUE  AND  GELATIN  277 

Muriatic  acid  is  used  for  the  acid  washing  of  the  stock  and  for 
acidulation  of  bone  in  the  production  of  ossein.  So  far  as  known 
it  is  never  produced  by  the  glue  maker,  but  is  often  regenerated 
from  the  leach  liquors. 

Lime  and  other  alkalies  are  used  in  the  hydrolysis  of  the  stock. 
The  lime  may  be  purchased  either  as  oxide  or  hydroxide  and  made 
into  a  thin  paste  or  milk  which  should  always  be  thoroughly  cool 
when  used.  It  is  also  used  in  the  formation  of  the  calcium  phos- 
phates which  are  a  by-product  of  the  ossein  industry. 


II.   MANUFACTURE 

1.  Hide  Glue. — Preparation  and  Preservation  of  the  Stock.1— 
Hide  stock  may  be  received  from  the  butcher  green,  or  fresh  and 
untreated.  If  it  is  not  to  be  used  immediately  it  must  be  pre 
served,  otherwise  putrefaction  will  soon  destroy  its  value. 
Salt,  lime  and  desiccation  are  commonly  used.  If  the  stock  is 
piled  alternately  with  layers  of  salt,  it  will  give  up  the  greater 
portion  of  its  water  to  the  salt  and  become  shrunken.  If  after 
thorough  salting  this  stock  is  partially  dried  and  stored  it  will 
keep  almost  indefinitely  with  no  change  other  than  a  slow  harden- 
ing which  disappears  on  careful  washing  and  liming. 

Partially  limed  stock  may  be  stored  by  piling  with  sufficient 
lime  paste  to  replace  that  used  up  in  the  slow  hydrolysis  which 
continues  even  in  the  absence  of  drainable  water,  but  storage 
must  not  be  continued  too  long  or  losses  of  a  serious  nature  will 
take  place. 

Complete  desiccation  of  unsalted  and  unlimed  stock  results  in  a 
hardness  which  is  but  slowly  removed.  Most  of  the  common 
so-called  chemical  preservatives  have  not  been  found  satisfactory 
if  used  alone  or  with  desiccation,  but  in  conjunction  with  salt 
or  lime  they  are  often  valuable. 

The  United  States  government  requires  that  all  hides  entering 
this  country  be  thoroughly  sterilized  to  prevent  the  spread  of 
disease  and  have  recently  published  specifications  for  this  pur- 
pose.2 The  Regulations  prescribe  either  treatment  with  hot 
water  or  milk  of  lime,  both  of  which  are  stages  of  the  glue  making 
process.  Heat,  if  applied  for  a  sufficient  length  of  time,  of  course, 

1  See  bibliography  at  end  of  chapter  for  all  references. 


278  GELATIN  AND  GLUE 

will  give  good  disinfection,  but  the  lime  treatment  is  of  very 
doubtful  value,  many  bacteria  thriving  in  lime  liquors,  as  will  be 
shown  in  the  discussion  of  the  liming  process. 

Soaking  and  Washing. — Whether  green  or  dry,  the  first 
treatment  given  the  stock  is  a  thorough  washing  with  water. 
With  green  stock  this  is  for  the  purpose  of  removing  all  possible 
blood  and  dirt  which  would  injure  the  color  of  the  finished 
product.  Dry  stock  is  soaked  until  thoroughly  softened  and  then 
washed  until  the  salt  is  completely  removed.  It  has  already  been 
shown  that  practically  all  substances  produce  some  changes  in 
the  physical  properties  of  gelatin.  The  same  is  true  of  the  glue 
stock.  In  general,  any  neutral  salt  prevents  the  fullest  swelling 
in  water,  and  hinders  it  very  markedly  in  the  liming  process 
which  follows,  resulting  in  a  slower  and  less  uniform  liming. 

The  washing  is  carried  on  in  machines  similar  in  principle  to  the 
ordinary  family  washing  machine,  and  of  almost  as  great  a 
variety  of  construction.  Those  more  commonly  used  are  as 
follows : 

The  cone  mill,  log  mill,  or  roller  mill  consists  of  a  large  tub 
twelve  or  sixteen  feet  in  dameter  containing  a  central  stationary 
upright  post.  To  this  is  fixed  the  apex  of  a  wooden  cone,  the 
length  of  which  is  approximately  the  radius  of  the  tub.  To  the 
conical  surface  are  fixed  tapered  strips  of  hardwood  about  four 
inches  square  at  the  large  end  and  equally  spaced,  producing 
corrugations  for  the  purpose  of  giving  a  kneading  effect.  The 
shaft  passing  through  the  cone  is  attached  by  a  drag  to  an  over- 
head drive  beam  which  causes  it  to  revolve  around  the  tank  with 
a  rolling  motion.  Water  is  fed  in  from  above  either  continuously 
or  intermittently  depending  upon  the  quantity  of  substance  to  be 
washed  out.  Perforated  strainers  or  screens  are  inserted  in  the 
sides  or  the  bottom  of  the  tub  and  the  flow  of  waste  water 
through  these  controlled  with  valves. 

The  tumbler  or  barrel  mill,  consists  of  a  barrel  mounted  hori- 
zontally on  its  axis,  with  baffles  on  the  periphery  to  keep  the 
stock  from  sliding  and  carrying  the  stock  part  way  around  the 
mill,  thus  causing  it  to  drop  through  the  wash  water  with  which 
the  mill  is  filled.  The  stock  and  water  are  introduced  through 
an  opening  in  the  cylindrical  side  of  the  mill,  which  is  closed 
during  rotation.  *' 

The  Hollander  or  beating  engine ,3  more  generally  known  in 
connection  with  the  paper  industry,  consists  of  a  shallow  ellip- 


MANUFACTURE  OF  GLUE  AND  GELATIN  279 

tical  tub  with  a  vertical  central  partition  reaching  perhaps  two- 
thirds  of  its  length.  Extending  from  this  partition  to  one  side 
is  a  horizontal  revolving  cylinder,  with  longitudinal  blades 
mounted  on  its  periphery.  The  cylinder  can  be  raised  or 
lowered  to  adjust  the  distance  between  the  revolving  blade's  and. a 
set  of  stationary  blades  mounted  on  a  raised  portion  of  the  tub 
bottom  under  the  cylinder.  The  rotation  of  the  cylinder  squeezes 
and  rubs  the  stock  through  between  the  two  sets  of  blades  and  at 
the  same  time  propels  the  water  around  the  tub.  On  the  oppo- 
site side  of  the  partition  a  revolving  cylindrical  screen  with 
central  drain  removes  the  wash  water. 

The  half-round  mill  may  most  easily  be  described  as  like  the 
paddle  wheel  of  a  stern  wheel  steamer  mounted  in  a  half  barrel 
lying  on  its  side.  A  perforated  false  bottom  controlled  with  a 
cock  is  provided  for  draining  off  the  wash  water. 

These  four  representative  types  may  to  a  certain  extent,  be 
used  interchangeably.  The  cone  mill,  which  produces  a  com- 
bination of  rubbing,  kneading  and  washing  can  be  used  on 
almost  any  kind  of  hide  stock.  In  the  tumbler  the  pounding 
and  kneading  predominate  and  it  is,  therefore,  more  suitable  for 
hard  or  thick  stock.  In  the  hollander  the  rubbing  predominates 
and  it  is  most  serviceable  for  loosening  foreign  materials  which 
adhere  rather  firmly  to  the  fibers,  but  its  construction  prohibits 
use  on  heavy  or  non-uniform  stock.  The  half-round  mill  gives 
a  simple  agitation  and  rapid  circulation  of  water  suitable  only  for 
finely  divided  and  light  stock. 

Liming.* — Skin  consists  of  several  distinct  layers.  First, 
the  outside  surface  consists  of  the  epidermis,  epithelium  or 
cuticle,  which  is  albuminous  in  its  nature  and  is  of  no  value  to  the 
glue  maker.  The  hair,  nails,  and  hoofs  are  epidermal  formations 
and  are  related  to  the  epidermis  in  composition.  Under  the 
epidermis  is  the  hyaline  layer,  a  glossy  structure  which  produces 
the  grain  surface.  The  corium,  derma  or  cutis  lies  beneath  this. 
This  is  the  true  skin  and  consists  of  bundles  of  interwoven  fibers 
cemented  together  by  a  somewhat  more  soluble  substance. 
The  corium  consists  principally  of  collagen,  the  glue  forming 
substance,  along  with  proteins  of  a  mucinous  nature.  The  skin 
is  attached  to  the  animal  by  the  panniculus  adiposus,  a  network 
of  connective  tissue  and  fat  cells.  This  is  the  major  constituent 
of  fleshings. 

The  purpose  of  the  liming  process  is  to  dissolve  out  the  albu- 


280  GELATIN  AND  GLUE 

minous  and  mucinous  constituents.  By  this  means  the  hair  and 
insoluble  parts  of  the  epidermis  are  loosened  and  a  portion  of  the 
fat  (probably  principally  that  in  broken  cells)  saponified.  Both 
albumin  and  mucin  are  soluble  in  alkali,  forming  alkaline  albu- 
minates  and  other  hydrolysis  products.  Alkaline  albuminates 
can  be  precipitated  by  acids,  but  will  also  redissolve  in  acids. 
The  compounds  formed  from  mucins,  if  precipitated  by  acids 
become  insoluble  and,  therefore,  must  be  completely  removed 
before  the  subsequent  acid  treatment  if  a  bright  glue  is  to  be 
obtained  without  excessive  clarification.  Dilute  alkali  also 
dissolves  collagen,  but  to  a  lesser  extent.  The  principal  action 
here  is  to  produce  a  swelling  by  absorption  of  water. 

Successful  liming,  therefore,  consists  in  a  careful  control  of 
alkaline  hydrolysis  so  that  the  albuminous  and  mucinous  mate- 
rials are  practically  completely  dissolved  without  allowing  the 
hydrolysis  of  the  collagen  to  progress  far  enough  to  cause  appre- 
ciable solution.  Saturated  lime  water  has  a  very  suitable 
alkalinity  for  the  purpose  (0.67  grams  of  hydroxyl  per  liter). 
As  alkali  is  used  up  in  the  reactions  involved,  the  use  of  clear 
lime  water  would  require  repeated  changing  to  maintain  the 
desired  concentration.  Commercially,  therefore,  a  suspension 
is  used  instead,  so  that  as  rapidly  as  the  hydrate  is  used  up  it 
will  be  replaced  by  solution  of  the  excess  in  suspension,  and  the 
alkalinity  maintained  at  practically  a  constant  value.  Also 
lime  is  the  cheapest  alkali  and  in  the  concentration  handled  the 
Baume  hydrometer  readings  approximate  the  percentage  concen- 
tration. Calcium  hydrate  is  more  soluble  in  cold  water  than  in 
hot  water,  so  that  there  is  a  partial  compensation  for  increased 
hydrolysis  with  warmer  weather  in  this  automatic  decrease  in 
concentration. 

This  ease  of  controlling  hydrolysis  is  partially  offset  by  the 
fact  that  some  bacteria  present  in  the  stock  are  not  killed  by  the 
alkali,  but  continue  to  multiply  in  the  stock  and  finally  find  their 
way  into  the  lime  solution  itself.  Counts  of  several  million 
bacteria  per  c.c.  of  lime  liquor  have  often  been  made.  The 
bacteria  are  apparently  acid  formers  and  use  up  a  part  of  the 
lime  to  form  salts,  which  in  turn  inhibit  to  a  certain  extent 
the  alkaline  swelling  of  the  stock.  Bacterial  growth  can  be  mini- 
mized by  frequent  turning  over  of  the  stock,  and  better  yet  by 
also  replacing  the  liquor  with  fresh  milk  of  lime.  Occasionally 
glue  made  from  stock  which  is  infected  will  show  a  higher  test 


MANUFACTURE  OF  GLUE  AND  GELATIN  281 

than  that  made  from  stock  limed  under  more  carefully  controlled 
conditions,  but  the  yield  is  always  low,  indicating  that  the 
bacteria  have  developed  at  the  expense  of  the  more  highly 
hydrolysed  portions  of  the  glue  stock. 

Bacterial  decomposition  can  be  overcome  by  the  use  of  other 
alkalies  at  concentrations  which  will  produce  sterilization. 
However,  increased  alkalinity  decreases  the  swelling,  and,  there- 
fore, the  penetration  of  the  alkali  to  the  interior  of  the  stock  is 
somewhat  slower  than  would  be  expected.  At  the  same  time 
hydrolysis  of  the  exterior  is  more  rapid  because  of  the  increased 
concentration.  With  certain  kinds  of  stock,  however,  if  fre- 
quently turned  and  carefully  watched,  very  satisfactory  results 
can  be  obtained. 

The  stock  is  often  run  through  shredding  or  cutting  machines 
to  give  the  pieces  a  more  uniform  size  and  therefore  produce  more 
uniform  liming  and  extraction.  It  is  then  thrown  into  wooden 
or  concrete  vats  containing  the  alkaline  liquors  and  turned  as 
often  as  is  necessary  to  insure  even  distribution  of  alkali  through- 
out the  stock.  Several  complete  changes  of  alkali  are  used. 
The  total  amount  of  lime  used  is  something  like  10  per  cent  of 
the  weight  of  the  stock.  If  caustic  soda  is  used  much  less  is 
required.  The  total  time  in  soak  is  dependent  principally  upon 
the  thickness  and  kind  of  stock  and  varies  from  30  to  60  days. 
With  the  use  of  soda  the  time  is  very  materially  shortened. 

Bleaching  agents  and  preservatives  are  sometimes  used  at 
some  stage  of  the  liming  process  with  beneficial  results,  but  they 
introduce  disturbances  which  will  be  discussed  with  a  later 
process.  Phenolic  compounds,  bleaching  powder,  and  sodium 
peroxide  are  among  the  favorites. 

Along  with  the  other  hydrolysis  products  formed  during  liming 
there  is  given  off  a  small  quantity  of  ammonia.  However,  the 
concentration  occurring  in  the  lime  liquor  at  any  one  time  is 
always  less  than  one  tenth  of  one  per  cent. 

Washing  and  Deliming.5 — After  the  liming  has  progressed 
until  the  inner  portions  have  been  practical-ly  cleared  of  their 
mucins  and  the  stock  shows  a  fairly  uniform  swelling  and  trans- 
parency, it  is  considered  thoroughly  limed.  The  alkali  and  its 
hydrolysis  products  must  now  be  completely  removed.  This  is 
done  by  returning  the  stock  to  the  wash  mills  and  washing  with 
clear  water.6  At  this  stage  the  stock  swells  still  further  in 
consequence  of  the  removal  of  the  salts,  but  to  accomplish  com- 


282  GELATIN  AND  GLUE 

plete  removal  would  require  a  prohibitive  length  of  time.  As 
soon  as  the  wash  waters  run  clear,  dilute  acid  is  added  with  the 
water  to  further  increase  the  swelling  and  hasten  the  removal  of 
the  remaining  salts. 

The  maximum  swelling  of  gelatin  occurs  in  a  solution  contain- 
ing about  0.0025  grams  of  hydrogen  ion  per  liter.  At  this  point 
gelatin  will  absorb  something  like  fifty  times  its  weight  of  cold 
water.  To  obtain  such  a  low  concentration  uniformly  through- 
out a  thick  piece  of  glue  stock  by  use  of  such  very  dilute  acids- 
would  be  practically  impossible,  so  concentrations  probably  a 
hundred  times  greater  are  used  at  the  start  and  the  stock  allowed 
to  remain  in  contact  with  this  acid  until  the  last  parts  to  be 
reached  have  attained  a  concentration  somewhat  above  the 
desired  0.0025  normal  and  then  the  washing  continued  with  clear 
water  until  the  average  concentration  is  about  0.0025  normal. 
Of  course,  this  means  that  the  outer  layers  are  below  this  value 
and  the  inner  layers  above  it,  but  by  this  time  the  salts  present 
and  formed  by  neutralization  have  been  pretty  well  dialysed 
out  and  the  stock  appears  to  be  uniformly  swollen.  It  is  still, 
however,  far  short  of  reaching  the  theoretical  maximum  swelling 
of  gelatin. 

The  most  commonly  used  acids  are  sulphurous  and  hydro- 
chloric. Sulphurous  acid,  besides  producing  a  better  swelling, 
has  both  bleaching  and  antiseptic  action,  and  would  perhaps  be 
considered  preferable  from  the  manufacturer's  standpoint,  but 
it  has  the  disadvantage  that  any  appreciable  amount  of  sulphites 
are  prohibitive  in  a  food  gelatin  and  will  have  to  be  removed 
later  in  the  process. 

Hydrochloric  acid  contains  iron  and  often  also  arsenic.  The 
iron  is  objectionable  on  account  of  its  effect  on  the  color  and 
arsenic  is,  of  course,  objectionable  in  a  food. 

The  Boiling  Process. — The  term  boiling  as  here  used  does 
not  necessarily  imply  actual  ebullition,  but  rather  a  gentle 
cooking  at  any  desired  temperature.  The  primitive  method  of 
boiling  glue  was  to  heat  the  stock  over  an  open  fire  with  a  large 
quantity  of  water  until  practically  all  had  gone  into  solution. 
The  time  necessary  to  hydrolyse  the  last  part  of  the  stock  was  so 
great  that  much  of  the  glue  first  dissolved  had  become  hydrolysed 
into  cleavage  products  of  little  or  no  adhesive  value. 

As  the  boiling  is  the  most  important  single  step  in  the  process 
an  attempt  will  be  made  to  analyze  the  factors  entering  into  it. 


MANUFACTURE  OF  GLUE  AND  GELATIN  283 

It  is  realized  that  the  desirable  and  undesirable  ones  are  so  inter- 
woven that  a  clear  exposition  of  their  control  will  be  difficult. 

Since  extraction  is  simply  hydrolysis  plus  solution  and  the 
deterioration  of  the  glue  liquor  is  also  simple  hydrolysis,  it  follows 
that  both  are  time-temperature  concentration  reactions,  and 
unseparable  except  that  the  latter  may  be  considered  a  later 
stage  of  a  progressive  hydrolysis.  Briefly  enumerated,  the 
following  conditions  are  to  be  desired: 

(1)  Rapid  extraction.     Cutting  down  the  total  time  of  boiling 
reduces  the  total  time  the  dissolved  glue  is  in  solution  and  allows 
proportionately  smaller  secondary  hydrolysis. 

(2)  Extraction    at    as    low    a    temperature    as    possible.     The 
swollen  stock  contracts  very  noticeably  with  heat,  and  as  the 
water  is  thus  given  up,  rate  of  solution — all  other  things  being 
equal — must  of  necessity  suffer,  for  not  only  will  the  concentra- 
tion of  the  dissolved  glue  in  the  stock  be  greater,  but  also  the 
rate  of  removal  through  the  stock  will  be  less,  due  to  the  stock 
being  so  much  denser. 

(3)  High    concentration    of   resulting    liquor.     Hydrolysis    in 
solution  is  proportional  to  the  amount  of  water  present  to  cause 
the  hydrolysis,  so  that  high  concentration  of  the  liquor  is  desir- 
able even  when  the  liquor  is  merely  standing  or  being  handled  in 
process.     No  matter  how  rapidly  or  how  carefully  evaporation 
in  the  vacuum  pan  is  conducted,  deterioration  is  bound  to  occur 
during  the  operation.     Also  the  volume  of  liquor  handled  is  in 
reality  the  limiting  factor  of  the  capacity  of  the  plant,  and 
increased  concentration  at  this  point  generally  means  increased 
plant  capacity. 

(4)  Continued  presence  of  fresh  water  or  dilute  liquor  in  con- 
tact with  the  unhydrolysed  stock,  so  that  the  maximum  rate  of 
solution  may  be  obtained.     As  pointed  out  in  (3),  solution  is 
more  rapid  if  the  solvent  is  dilute.     Agitation  of  the  stock,  and 
circulation,  or  percolation  of  the  water  are  desirable  means  of 
insuring  this  condition. 

(5)  Immediate   and   complete   removal   of  dissolved   glue  from 
the  zone  of  highest  temperature.     As  heat  must  be  continuously 
applied  to  the  extraction  vessels,  the  liquor  carrying  the  heat  to 
the  stock  will  always  be  slightly  higher  in  temperature  than  the 
stock  itself,  and  if  this  liquor  contains  glue  in  solution,  injurious 
hydrolysis  must  of  necessity  occur.     Immediate  removal  of  the 
dissolved  glue  to  some  other  container  will  prevent  its  being 


284  GELATIN  AND  GLUE 

used  as  a  means  of  heat  transference  and  at  the  same  time 
allow  a  certain  amount  of  desirable  cooling  to  take  place.  All 
this  argues  that  a  continuous  counter-current  scheme  of  boiling 
would  be  very  desirable. 

(6)  Little  or  no  pressure  or  load  on  the  stock  being  boiled,  for 
pressure,  even  if  only  momentary,  squeezes  out  the  liquor  and  by 
so    doing   slows    down    extraction.     It    has    been   proven  that 
squeezing  of  any  nature  is  detrimental  to  rapid  extraction,  due 
principally  to  the  fact  that  when  water  is  squeezed  out,  it  is  sub- 
sequently replaced  but  slowly. 

(7)  The  production  of  a  clear  liquor,  so  that  drastic  mechanical 
and  chemical  treatments  will  not  be  necessary  in  the  subseqeunt 
clarification. 

It  will  readily  be  seen  that  in  order  to  obtain  any  of  these 
conditions,  some  of  the  others  enumerated  must  be  sacrificed. 
For  example,  it  is  difficult  to  understand  how  a  rapid  extracting 
and  a  low  temperature  may  be  maintained  at  the  same  time. 
If  a  high  concentration  of  the  liquor  is  to  be  obtained,  then  the 
provisions  requiring  the  continued  presence  of  fresh  water  and  the 
immediate  removal  of  the  dissolved  glue  will  probably  have  to  be 
sacrificed.  To  reduce  the  pressure  on  the  stock  being  boiled,  the 
latter  may  be  distributed  over  a  considerable  area,  but  in  that 
case  the  heat  losses  are  increased.  To  obtain  a  clear  liquor, 
motion  during  boiling  should  be  avoided,  but  this  is  difficult  if 
fresh  water  is  to  be  kept  in  contact  with  the  unhydrolyzed  stock 
or  if  the  dissolved  glue  is  to  be  removed  as  soon  as  formed. 

Generally  the  boiling  of  hide  stock  is  conducted  in  vats  perhaps 
six  or  eight  feet  in  diameter  and  somewhat  less  in  depth.  Heat- 
ing coils,  preferably  for  closed  steam  are  placed  on  the  bottom, 
and  a  perforated  false  bottom  used  to  cover  them  and  prevent 
actual  contact  with  the  stock.  This  bottom  is  often  covered 
with  a  layer  of  excelsior  or  other  coarse  filtering  material.  An 
opening  with  well  gasketed  cover  may  be  provided  in  the  bottom 
of  the  vat  for  easy  discharge  of  the  residue  remaining  after  boiling. 

The  stock  is  dumped  into  the  vat  until  almost  full,  then  covered 
with  hot  water.  Heat  is  then  applied  and  the  temperature  main- 
tained at  perhaps  140°F.,  for  several  hours  or  until  the  liquor  has 
dissolved  about  5  per  cent  of  its  weight  of  glue.  The  liquor  is 
then  drawn  off  through  a  valve  in  the  bottom  and  fresh  water 
added.  This  time  and  heat  is  maintained  10  or  15°  higher  and 
after  about  the  same  time  this  liquor  is  drawn  off  as  before. 


MANUFACTURE  OF  GLUE  AND  GELATIN 


285 


Four  or  more  such  "runs"  are  made,  the  last  one  often  being  at  a 
temperature  of  212°F. 

A  study  of  the  properties  of  the  different  glues  resulting  from  a 
very  large  number  of  water  changes  has  shown  that  the  first  glue 
to  be  dissolved  is  that  which  was  so  highly  hydrolysed  in  liming 
that  raising  the  temperature  is  all  that  is  needed  to  bring  it  into 
solution.  Time  for  hydrolysis  is  not  required.  Due  to  over- 
liming  of  certain  portions  of  the  stock,  this  may  be  comparatively 
low  in  both  jelly  and  viscosity  tests,  and  it  has  a  low  melting 
point.  In  subsequent  runs  the  test  rises  at  first,  the  jelly  test 
reaching  a  maximum  almost  immediately  with  the  viscosity 
following  closely  behind.  Then  the  tests  drop  gradually  until 
the  glue  is  all  extracted.  If  quantities  of  water  added  and 


"0      10      20      30      40      50      60     70      80    90     100 

Per  Cent 
FIG.  48. — Average  tests  of  glues  made  by  repeated  additions  of  water. 

removed  are  so  frequent  that  in  effect  we  have  removal  of  the 
glue  as  soon  as  dissolved,  we  are  practically  removing  the  glue 
at  its  melting  point  and  as  the  heat  is  continuously  increased, 
the  melting  point  and  viscosity  continue  to  increase  even  though 
the  jelly  test  is  dropping.  The  only  explanation  for  such  a 
phenomenon  is  that  in  the  ordinary  progressive  boiling,  the 
hydrolysis  of  the  glue  already  in  solution,  during  the  time  the 
liquor  is  allowed  to  be  in  contact  with  the  stock  for  further 
concentration,  is  responsible  for  the  gradual  decrease  of  viscosity 
regularly  observed. 

Figure  48  illustrates  this  temporary  rise  in  jelly  test  and  the 
continuous  rise  in  viscosity. 

Although  far  from  satisfactory,  the  use  of  from  four  to  eight 
changes  only  of  water  has  been  considered  to  meet  many  of  the 
above  stated  requirements  to  a  much  greater  extent  than  the 


286  GELATIN  AND  GLUE 

old  single  boiling  method.  Many  manufacturers  have  intro- 
duced other  changes  tending  to  increase  the  rate  of  extraction 
by  attempts  to  improve  the  circulation  in  the  boiling  vat.  For 
example,  a  wide  perforated  vertical  tube  is  placed  in  the  center 
of  the  vat  before  the  stock  is  dumped  into  it.  This  unquestion- 
ably increases  the  rate  of  circulation,  but  the  liquor  coming  in 
contact  with  the  heating  coils  is  the  concentrated  liquor  gravita- 
ting from  the  stock  before  it  has  a  chance  to  mix  with  the  more 
dilute  main  volume  of  liquor. 

Agitation  by  the  introduction  of  a  slow  current  of  air  has  been 
found  to  be  of  considerable  help.  It  has  the  disadvantage, 
however,  of  increasing  markedly  the  rate  of  evaporation  of  the 
liquor,  thus  necessitating  a  greater  steam  consumption  and  a 
higher  temperature  differential  at  the  coil  surfaces  with  its  con- 
sequent deteriorating  effect.  Simple  hand  stirring  by  means  of 
wooden  paddles  has  proved  beneficial. 

Considerable  ingenuity  is  displayed  in  the  patent  literature  in 
the  design  of  widely  varying  mechanical  containers  for  keeping 
the  stock  or  liquor  in  motion  during  extraction.  In  many  of 
these  steam  is  used  in  place  of  hot  water  and  the  process  is  made 
continuous. 

Cormack7  packs  his  stock  in  a  centifuge  and  subjects  it  to  con- 
densing steam.  The  squeezing  effect  of  centrifugal  force  must 
certainly  have  a  hindering  effect,  but  the  glue  would  be  removed 
as  soon  as  dissolved,  thus  minimizing  deterioration. 

Thiele8  has  patented  what  is  in  effect  an  enclosed  steam  jack- 
eted percolator,  spraying  very  hot  water  over  the  top  and  allowing 
it  to  drain  off  immediately.  No  provision  is  made  for  insuring 
thorough  percolation  through  the  more  densely  packed  portions 
of  the  mass. 

Mauerhofer9  distributes  the  stock  in  thin  layers  over  per- 
forated plates  and  blows  steam  upon  it  from  a  central  perforated 
pipe.  The  addition  of  a  water  spray  at  the  top  would  overcome 
to  a  certain  extent  the  lack  of  heat  conduction  from  steam  to  the 
covered  portion  of  the  stock,  which  has  always  been  the  drawback 
in  such  means  of  extraction. 

Lehman's  patent10  for  the  use  of  an  Archimedes'  screw  for 
keeping  the  stock  in  motion  in  the  liquor  neglects  a  good  oppor- 
tunity for  making  a  true  counter-current  extraction.  He  simply 
immersed  the  screw  in  the  liquor. 

Upton11  uses  a  rotating  horizontal  cylinder  containing  per- 


MANUFACTURE  OF  GLUE  AND  GELATIN  287 

forations  on  a  part  of  its  periphery  and  provided  with  a  shutter 
for  prevention  of  the  escape  of  steam.  Steam  is  admitted 
through  a  central  perforated  pipe  and  the  liquor  formed  from  the 
condensed  water  allowed  to  drain  out  through  the  perforations  as 
soon  as  produced.  The  author  has  used  an  apparatus  having 
some  elements  in  common  with  this  extractor  and  has  succeeded 
in  obtaining  remarkable  increases  both  in  test  and  completeness 
of  extraction. 

These  methods  all  have  what  was  formerly  considered  a  very 
serious  objection,  namely,  the  agitation  causes  suspension  in 
the  glue  liquor  of  materials  which  would  otherwise  remain  in  the 
residue,  thus  diminishing  the  clearness  of  the  liquor  as  drawn 
from  the  extractor.  The  careful  and  effective  filtration  or 
clarification  necessitated  have  been  worked  out,  thus  making 
commercially  acceptable  those  processes  which  give  material 
increases  in  value  over  vat  process  liquors. 

Fine  shredding  of  the  stock,  by  increasing  the  surface  exposed 
to  the  water  hastens  the  extraction  materially.  As  the  swollen 
stock  ready  for  boiling  is  difficult  to  shread  without  squeezing 
out  much  of  its  water,  it  has  been  found  more  satisfactory  to  do 
the  cutting  earlier  in  the  process,  even  though  the  washing  losses 
are  materially  increased  thereby. 

Clarification12  and  Filtration.13 — The  suspended  materials 
contained  in  the  liquors  as  received  from  the  boiling  floor 
consist  of  undissolved  organic  matter,  albumins  and  mucins, 
lime  soap  and  grease  as  well  as  some  hair  and  mineral  particles, 
such  as  lime  or  bone  fragments. 

A  considerable  portion  of  this  foreign  material  can  be  removed 
by  screens  or  coarse  filters. 

The  use  of  the  centrifuge  has  found  favor  for  the  removal  of 
some  of  the  heavier  substances,  but  unless  the  liquor  is  extremely 
dilute  the  volume  handled  per  machine  is  very  limited.  Also  if 
the  liquor  is  practically  neutral,  lime  soaps  and  mucins  seem  to 
be  churned  in  rather  than  separated,  and  some  liquors  foam 
very  badly. 

Formerly  filter  presses  were  very  popular,  but  they  are  largely 
giving  way  to  gravity  or  mild  pressure  filters.  The  retention  of 
the  fine  particles  on  the  filtering  medium  seems  to  be  a  species  of 
adsorption  and  requires  an  enormous  surface  of  exposure  rather 
than  a  dense  fine  aperature  medium.  Both  the  mucinous  sub- 
stances and  lime  soaps  are  exceedingly  sticky,  and,  especially 


288  GELATIN  AND  GLUE 

if  under  the  pounding  influence  of  a  pump  stroke,  they  soon 
completely  fill  the  pores  of  any  dense  substance  and  cause 
blocking  and  breaking  through. 

The  mucinous  impurities  carry  nearly  the  same  electrical 
charge  on  their  colloid  particles  that  gelatin  particles  carry. 
On  this  account  the  protective  action  of  the  gelatin  narrows  very 
greatly  the  limits  within  which  good  filtration  can  be  accom- 
plished. The  substances  to  be  filtered  out  appear  all  to  be 
negative,  and  the  use  of  fullers  earth  which  is  strongly  negative 
does  not  appreciably  improve  the  appearance  of  the  glue.  On 
the  other  hand,  if  alumina,  a  strongly  positive  substance,  is  used, 
the  liquor  will  run  clear  but  the  filter  will  block  almost  immedi- 
ately. Charcoal  is  almost  neutral  and  will  produce  successful 
filtration  for  a  considerable  time.  However,  the  filtering  sub- 
stance preeminent  for  this  purpose  is  cellulose.  Being  very 
slightly  negative  it  holds  the  particles  without  holding  the  glue, 
and  although  the  actual  surface  per  unit  of  volume  is  probably 
much  less  than  with  charcoal,  the  filters  are  so  constituted  that 
they  will  hold  a  maximum  amount  of  precipitate  before  blocking. 
A  convenient  form  of  cellulose  is  a  good  grade  of  cotton  paper 
pulp.  Many  different  forms  of  filters  for  holding  such  pulp  are 
obtainable,  but  those  allowing  for  a  loose  packing  of  the  pulp 
on  the  intake  surface  will  provide  a  matte  of  considerably  longer 
life  than  if  the  filter  matte  is  required  to  be  densely  packed 
throughout. 

Although  certain  liquors  can  be  filtered  perfectly  bright, 
many  of  them  will  cloud  on  cooling  and  practically  all  of  them 
cloud  on  concentrating.  To  remove  this  precipitated  or  precipi- 
table  material  and  thus  obtain  a  brilliant  gelatin  it  is  necessary 
either  to  produce  a  colloidal  coagulation  or  to  cause  the  forma- 
tion of  a  precipitate  or  aggregate  which  will  collect  and  hold  by 
forces  allied  to  adsorption  the  objectionable  substances. 

The  oldest  and  best  known  substance  used  for  this  purpose  is 
egg  albumin.  The  albumin  in  water  solution  is  added  to  the 
comparatively  cool  liquor  and  the  temperature  gradually  raised 
until  coagulation  takes  place.  The  whole  is  then  allowed  to 
stand  for  the  separation  of  the  curd  or  coagulum,  which  often 
requires  hours.  The  clear  liquor  is  then  siphoned  off  and  filtered. 
The  effect  of  the  prolonged  exposure  to  heat  is  a  very  serious 
objection  and  good  egg  albumin  is  expensive.  Blood  albumin  can 
be  used  in  a  similar  manner  if  the  solution  is  kept  sufficiently  acid. 


MANUFACTURE  OF  GLUE  AND  GELATIN 


289 


Among  the  more  common  of  the  inorganic  precipitants  used 
are:  alum  and  lime  to  form  alumina  and  calcium  sulphate,  or 
such  acids  as  sulphurous  or  phosphoric  which  form  insoluble  salts 
with  the  alkaline  earths.  Recently  silver  salts  have  come  into 
use  for  the  removal  of  proteins  and  salts  which  are  objectionable 
in  photographic  gelatins.  The  requirement  in  any  of  these  pre- 
cipitations is  that  the  substances  formed  shall  all  be  insoluble,  or 
if  not,  either  volatile  or  incapable  of  crystallizing  out  in  the 
dried  glue. 


567 
Ti'me,  Hours 

FIG.  49. — Specific  conductivities  of  glue  solution  duiing  clarification, 
to  5  per  cent  concentration.) 


(Reduced 


There  are  only  two  really  effective  methods  of  controlling  the 
reactions  involved,  namely  by  observation  of  the  hydrogen  ion 
concentration,  or  of  the  conductivity  of  the  solution  during  the 
formation  of  the  precipitate.  The  first  of  these  has  been  the 
more  common,  using  litmus  as  the  indicator,  but  indicator  color 
changes  are  valuable  only  in  enabling  one  to  repeat  certain  con- 
ditions previously  observed,  and  make  no  allowance  for  changes 
in  apparent  acidity  caused  by  amphoteric  substances  which  may 
or  may  not  be  present.  A  low  ash  product  is  always  to  be  desired 
and  what  the  glue  maker  really  wishes  to  know  is  at  what  stage 
of  the  precipitation  the  soluble  salts  reach  a  minimum.  This 
is  more  easily  determined  by  measuring  the  conductivity  than 
by  any  other  means.  Ash  determination  and  analysis  tell  the 
quantity  and  substances  present  which  may  form  ash,  but  it  does 
not  tell  whether  they  are  combined  as  precipitates  or  as  soluble 
salts.  The  conductivity  method  was  found  very  satisfactory 
for  answering  this  question  and  a  series  of  observations  on  a 

19 


290  GELATIN  AND  GLUE 

normal  precipitation  in  the  factory  is  shown  in  Fig.  49  to  illustrate 
the  distinctness  with  which  such  hidden  facts  stand  forth. 

Bleaching  of  the  liquor  with  sulphur  dioxide14  is  quite  common. 
Other  substances  used  are:  sodium  hydrosulphite,  the  basic 
zinc  salt  of  formaldehyde,  sulphoxalate,  amalgams,  and  other 
strong  reducing  agents.  A  part  of  the  bleaching  effect  seems  to 
be  due  to  reduction  of  organic  coloring  matter,  while  in  the 
inorganic  substances  the  change  from  the  ferric  to  the  ferrous 
state  of  traces  of  iron  is  often  responsible  for  a  marked  improve- 
ment in  color. 

Evaporation.15 — The  liquors  (with  the  possible  exception  of 
those  intended  for  thin  cut  gelatins)  are  now  sent  to  the  vacuum 
pans.  The  types  of  evaporators  used  and  suitable  are  so  varied 
and  so  well  known  that  detailed  description  is  unnecessary.  The 
double-effect  is  most  commonly  used.  It  is  advisable  to  evapo- 
rate the  dilute  liquor  in  the  low  vacuum  high  temperature  pan, 
and  finish  in  the  high  vacuum  low  temperature  side,  thus  using 
the  gentler  heat  on  the  concentrated  liquors  and  minimizing 
total  hydrolysis. 

Vacuum  evaporation  is  more  economical  than  alley  drying, 
but  it  is  also  apt  to  be  more  harmful  to  the  test.  Exhaust 
steam,  or  at  least  very  low  pressure  steam,  should  be  used  and 
the  vacuum  maintained  at  the  highest  possible  point,  so  that 
the  temperature  may  be  maintained  at  the  minimum  consistent 
with  rapid  evaporation. 

Considered  as  evaporative  apparatus  only,  pans  are  all  practi- 
cally the  same,  that  is,  one  pound  of  steam  will  evaporate  practi- 
cally the  same  weight  of  water  in  any  make  of  double-effect, 
other  conditions  being  equal.  However,  the  injury  to  the  glue 
will  be  less  in  those  which  produce  a  high  rate  of  evaporation 
while  requiring  the  presence  of  only  a  small  volume  of  liquor  at 
any  one  time.  This  means  a  high  evaporative  surface-volume 
ratio,  and  adequate  facilities  for  taking  care  of  foam  or  priming 
and  entrainment.  Considerable  space  is  generally  allowed  for 
the  breaking  up  of  the  foam.  Alkaline  glues  often  foam  badly 
on  account  of  the  presence  of  soaps.  The  presence  of  any  insolu- 
ble substance  tends  to  increase  foam.  The  addition  of  substances 
such  as  grease  which  alter  the  surface  tension  will  decrease  very 
markedly  the  tendency  to  foam. 

Various  types  of  baffles  and  catchalls  are  introduced  into  the 
vapor  space  and  lines  to  catch  and  return  the  entrainment,  but 


MANUFACTURE  OF  GLUE  AND  GELATIN  291 

when  one  stops  to  consider  that  the  vapor  velocities  are  generally 
measurable  in  miles  per  minute,  it  is  apparent  that  many  of  them 
are  practically  valueless.  The  Parmelee  catchall  is  designed  to 
use  this  very  high  velocity  in  the  separation  and  removal  of  the 
entrainment  by  centrifugal  force  and  is  by  far  the  most  efficient 
design  yet  produced.  This  is  shown  in  Fig.  50. 

If  dry  glue  in  thin  sheets  is  to  be  produced  evaporation  may  be 
omitted  or  carried  to  a  glue  concentration  of  only  about  5  per 
cent.  To  chill  and  handle  a  jelly  of  such  low  concentration 
requires  that  it  be  of  the  highest  test.  For  lower  grades  the 
concentration  may  be  pushed  as  high  as  50  per  cent. 

Up  to  this  stage  in  the  process  time  has  literally  been  money 
to  the  glue  maker,  for  hydrolysis  into  products  of  less  value  has 
been  going  on  continuously.  The  liquor  can  now  be  cooled  and 
with  the  liquor  at  a  low  temperature  and  high  concentration 
natural  hydrolysis  becomes  much  less  marked.  The  lower  tem- 
perature, however,  is  favorable  to  the  growth  of  bacteria  and 
preservatives  are  sometimes  added  at  this  time.  The  choice  of  a 
preservative  is  always  a  difficult  matter.  Of  course  they  are 
only  used  in  the  lower  grades  of  glues  or  in  glues  for  special 
purposes.  Practically  all  antiseptics  cause  either  coagulation 
or  peptization,  for  it  is  apparently  from  the  hardening  or  dis- 
solving action  on  the  colloidal  cell*membrane  that  the  destruction 
of  the  bacterial  cell  results.  Such  harsh  substances  as  mercury 
bichloride  and  formaldehyde  cause  the  glue  to  become  insoluble 
on  drying.  Phenolic  compounds  cause  a  marked  drop  in  test 
and  even  the  mild  borax  or  boric  acid  affect  the  jelly  test.  Beta- 
naphthol,  cresylic  acid,  soaps,  and  sulphates  of  some  of  the  heavy 
metals  are  often  used,  but  none  of  them  are  without  effect  on 
the  glue  itself. 

If  sulphites  are  to  be  removed,  some  mild  oxidizing  agent  such 
as  hydrogen  peroxide  may  be  added  at  this  time. 

White  glues  are  made  by  the  addition  of  pigments  before 
drying.  To  get  a  clean  uniform  color  the  pigment  must  be 
highly  opaque  and  have  what  the  painter  might  describe  as 
tinctorial  power.  It  must  be  fine  and  in  perfect  suspension. 
This  is  best  insured  by  grinding  in  a  colloidal  vehicle. 

Other  substances  are  sometimes  added  at  this  stage  in  the 
process  for  the  production  of  glues  having  special  properties. 
For  example,  glycerin,  sugar  or  tar  oil  impart  greater  flexibility. 
Calcium  chloride,  sodium  naphthalene  sulphonate,  acids,  and 


292 


GELATIN  AND  GLUE 


chloral    hydrate    will   prevent  jellying  and    produce    a    liquid 
glue. 

Chilling  and  Spreading.™ — A  method  for  chilling  the  glue 
liquor  from  the  evaporator,  still  used  to  a  considerable  extent, 
consists  in  running  the  liquor  into  cooling  pans  of  wood  or  metal 


FIG.  50.— The  Parmelee  catchall. 


and  exposing  to  refrigeration  until  thoroughly  set.  The  filling 
of  these  pans  is  a  sloppy  procedure,  the  chill  room  is  seldom  uni- 
formly cooled,  and  condensed  evaporation  from  the  warm  liquor 
keeps  the  room  in  a  drippy  foggy  condition  and  often  makes  it  a 
veritable  incubator  for  moulds  and  bacteria.  In  fact  it  is 
impossible  to  keep  things  sterile  and  an  occasional  epidemic  of 
liquefaction  or  low  test  is  not  unknown  to  the  chill  room  of  any 
factory.  By  means  of  proper  circulation  and  careful  conditioning 


MANUFACTURE  OF  GLUE  AND  GELATIN  293 

of  the  air  the  temperature  and  humidity  may  be  so  controlled 
as  to  at  least  prevent  the  spread  of  any  infection  accidentally 
introduced.  Often  running  water  is  used  for  the  preliminary 
chilling  and  it  is  common  practice  to  have  the  rooms  arranged 
so  that  they  can  be  thrown  open  to  the  outdoors  in  winter 
weather. 

The  jelly  is  removed  from  the  pans  or  trays  by  cutting,  by 
immersing  in  hot  water  for  a  short  time,  or  by  exposing  to  steam. 
The  pans  are  then  returned  to  the  chill  room  for  refilling.  The 
cakes  are  cut  into  sheets  by  machinery.  The  older  type  of 
slicing  machines  were  similar  in  principle  to  the  butcher's  ham 
slicer  and  are  still  used  for  cutting  very  stiff  jellies.  Banks  of 
thin  knives  operating  in  closed  boxes  (to  prevent  deformation  of 
the  jelly)  were  also  used.  By  far  the  greater  portion  of  the  slic- 
ing, however,  is  done  with  tightly  stretched  piano  wire,  operating 
en  bane,  generally  on  some  form  of  endless  belt.  The  glue  slices 
are  laid  out  by  hand,  or  " spread"  upon  frames  perhaps  three  by 
six  feet  and  covered  with  netting  of  cotton,  zinc,  or  heavily 
galvanized  wire.  The  scrap  from  these  operations  is  gathered 
up,  melted,  the  dirt  settled  out,  and  the  clear  liquor  returned  to 
the  chill  room. 

As  the  chilling,  cutting  and  spreading  require  a  great  amount 
of  space  and  are  messy  and  necessitate  considerable  manual  labor, 
many  devices  have  been  designed  for*  chilling  the  liquor  on  belts 
and  transferring  immediately  to  the  frames.  The  Kind-Landes- 
mann  machine17  has  received  rather  wide  acceptance  in  the  last 
few  years  and  is  proving  very  satisfactory  for  all  grades  of  glues 
and  gelatins.  It  consists  of  a  perforated  pipe  for  distributing 
the  liquor  from  the  storage  tank  upon  an  endless  rubber  belt. 
This  belt  passes  through  a  chilling  tunnel  kept  cool  by  fan  circu- 
lation of  air  from  brine  coils  in  a  contiguous  chamber.  When 
the  belt  and  glue  appear  at  the  opposite  end  of  the  tunnel  the 
glue,  now  in  a  jelly  form,  is  scraped  off  by  means  of  a  knife  and 
falls  upon  a  short  transfer  belt  which  in  turn  drops  the  sheet 
upon  a  frame  which  passes  along  under  the  transfer  belt  at  the 
proper  speed  to  receive  the  sheet.  A  knife  is  also  provided  for 
cutting  the  endless  sheet  into  lengths  to  fit  the  frames.  All  the 
labor  required  is  to  put  the  empty  frames  into  the  machine  and 
remove  the  full  ones  to  the  trucks  for  placing  in  the  alleys.  Not 
over  fifteen  minutes  are  required  from  the  time  the  liquor  is 
put  on  the  belt  until  the  jelly  is  on  the  truck.  There  is  no  scrap 


294  GELATIN  AND  GLUE 

to  be  returned,  and  as  the  jelly  is  not  touched  by  hand  it  can 
receive  no  bacterial  contamination  from  that  source. 

Drying.18 — The  glue  receives  its  final  drying  in  long  tunnels 
or  alleys  so  constructed  as  to  receive  trucks  stacked  with  perhaps 
twenty  of  the  frames.  A  track  running  the  length  of  the  alley 
guides  the  trucks  through  and  the  most  approved  practice  is  to 
have  each  alley  long  enough  to  produce  complete  drying  and  to 
have  the  air  blown  through  counter- current  to  the  direction  of 
the  progress  of  the  trucks.  This  exposes  the  nearly  dry  glue  to 
the  dryest  air  and  utilizes  the  almost  saturated  and  much  cooler 
air  for  the  evaporation  of  moisture  from  the  fresh  jelly. 

Steam  coils  are  provided  in  some  part  of  the  intake  duct  for 
the  purpose  of  heating  the  air  and  thus  increasing  its  moisture 
carrying  capacity.  Low  concentration  jellies  require  shorter 
alleys  and  less  time  for  drying  than  do  those  of  higher  concen- 
trations. This  is  due  to  the  fact  that  besides  the  simple  evapora- 
tion of  the  water  we  must  consider  also  the  rate  of  conduction  of 
moisture  through  the  sheet  of  partially  dried  glue.  As  the  high 
concentration  jellies  produce  a  thicker  sheet  when  dry  the  last 
traces  of  moisture  are  driven  off  much  more  slowly.  If  the  air 
is  exceedingly  dry  a  hard  skin  is  formed  (called  case-hardening 
for  want  of  a  better  term)  which  admits  of  only  slow  moisture 
conductance,  and  if  progress  through  the  alley  is  too  rapid, 
the  central  portions  of  the  sheet  may  actually  melt  and  cause 
the  sheet  to  warp  out  of  shape.  Such  phenomena  are  common  in 
the  drying  of  many  kinds  of  material.  In  lumber  drying  this 
case-hardening  is  the  principal  cause  of  checking. 

As  an  aid  to  the  study  of  conditions  in  the  drying  alley  The 
Psychrometric  Tables  of  the  U.  S.  Weather  Bureau  (Bulletin 
235)  are  invaluable.  Figure  51  is  a  chart  plotted  from  these 
tables.  Air  at  any  given  temperature  is  capable  of  holding- a 
definite  amount  of  moisture.  If  it  is  exposed  to  an  excess  of 
water  it  absorbs  moisture  until  this  amount  is  reached,  when  it  is 
said  to  be  saturated.  These  amounts  are  shown  in  grams  per 
cubic  foot  on  the  upper  curve  marked  100  per  cent  saturated. 
The  natural  moisture  content  of  the  air  is  generally  expressed  in 
percentage  saturation,  sometimes  called  relative  humidity. 
This  expression  signifies  that  the  air  contains  only  the  given 
percentage  of  the  total  possible  moisture  weight  at  that  tem- 
perature. If  two  thermometers  are  placed  in  a  strong  current 
of  air,  one  of  them  having  the  bulb  surrounded  by  a  wick  satu- 


MANUFACTURE  OF  GLUE  AND  GELATIN 


295 


rated  with  water,  they  will  often  register  widely  different  tem- 
peratures.    The  dry  bulb  records  the  actual  temperature  of  the 


LA 


\/ 


^M 


\ 


v 


A 


^y 


CJ 

5 

0 

"  •* 

A, 

>s, 

0) 

E 

0 

k 


X^\A 


.i*,^? 


'  O 


air,  while  the  wet  bulb  records  the  temperature  to  which  the  air 
will  drop  in  saturating  itself  with  moisture. 

For  the  purpose  of  illustration,  consider  air  at  92°F.  with  a  wet 
bulb  temperature  of  70°.  This  is  32  per  cent  saturated  and 
contains  5  grains  of  moisture  per  cubic  foot.  In  traveling 


296 


GELATIN  AND  GLUE 


through  the  alley  it  will  take  up  moisture,  giving  up  the  heat 
necessary  to  evaporate  the  water  from  the  glue  and  conse- 
quently cooling  itself  to  a  lower  temperature.  If  the  alley  were 
infinitely  long  and  thoroughly  insulated  different  points  might 
record  conditions  as  follows : 


Dry  bulb  temp., 

Wet  bulb  temp., 

Percentage 

Grains  of  mois- 

op 

°F. 

saturation 

ture  per  cubic  foot 

92 

70 

32 

5.0 

85 

70 

47 

6.0 

80 

70 

61 

6.7 

75 

70 

78 

7.3 

70 

70 

100 

8.0 

The  conditions  have  followed  the  oblique  line  ending  at  70°F. 
on  the  saturation  curve.  This  oblique  line  or  cooling  curve 
covers  all  conditions  which  will  show  a  wet  bulb  temperature 
of  70°.  The  wet  bulb  temperature  can  be  changed  only  by  a 
change  in  the  absolute  heat  content  of  the  air.  If  the  alley 
were  short,  say  100  feet  and  the  last  truck  of  glue  freshly  added, 
the  temperature  of  75°  might  be  that  of  the  exit  air  in  place  of  70° 
as  in  the  infinitely  long  alley.  However,  the  fresh  glue  would 
have  a  temperature  of  70°  irrespective  of  the  length  of  the  alley. 
In  other  words,  provided  there  is  no  skin  on  the  glue,  it  will 
assume  the  wet  bulb  temperature  of  the  air,  due  to  the  cooling  effect 
of  evaporation,  no  matter  what  the  actual  temperature  of  the  air. 

Since  the  alley  walls  conduct  a  certain  amount  of  heat,  as  soon 
as  the  air  drops  below  outside  temperature  it  begins  to  take  up 
heat  from  the  surrounding  walls.  The  addition  of  heat  is  repre- 
sented on  the  chart  by  a  horizontal  movement  to  the  right  from 
any  given  point.  This  addition  of  heat  from  the  alley  walls 
causes  a  rise  in  both  the  wet  and  dry  bulb  temperature,  which 
would  show  on  the  chart  as  an  upward  deflection  from  the  normal 
cooling  curve,  never  amounting  to  more  than  a  few  degrees, 
however.  Under  the  trying  conditions  of  high  summer  humidity, 
even  two  degrees  rise  may  cause  considerable  damage.  This  is 
the  explanation  of  the  apparently  anomalous  observation  that  in 
hot  weather  the  glue  will  melt  down  first  in  the  coldest  of  two 
alleys  receiving  their  air  from  the  same  source.  It  is  simply 
a  case  of  the  alleys  being  loaded  to  the  point  that  the  wall  con- 


MANUFACTURE  OF  GLUE  AND  GELATIN  297 

duction  becomes  appreciable  and  raises  the  wet  bulb  temperature 
above  the  melting  point  of  the  glue. 

It  is  impossible  to  make  a  general  statement  as  to  what  is  the 
most  economical  length  of  drying  alley.  The  correct  construc- 
tion equipment  and  operation  for  any  specific  conditions  can 
be  easily  worked  out  from  the  psychrometric  tables,  however. 
In  the  winter  time  when  the  air  is  dry,  short  alleys  are  all  that  are 
required  for  the  actual  drying,  but  as  the  long  alley  produces  a 
slow  humidity  gradient,  it  offers  the  advantage  of  more  uniform 
drying  and  lack  of  case-hardening  and  warping.  Much  shorter 
alleys  are  required  for  low  gravity  jellies  (thin  cut),  but  on 
account  of  the  low  melting  point  of  dilute  jellies,  it  is  difficult  and 
often  impossible  to  handle  them  in  the  summer  time.  Also,  on 
account  of  the  much  greater  space  required  for  drying  a  given 
weight  of  thin  cut  material,  it  is  desirable  to  make  this  grade  of 
goods  in  the  winter  when  the  alley  capacity  is  at  a  maximum. 

Thick  jellies  always  require  a  long  alley  and  only  a  moderate 
air  velocity  for  the  minimizing  of  case-hardening,  and  even  then 
it  is  often  advisable  to  allow  the  very  thick  cut  glues  to  stand  on 
the  trucks  in  an  open  room  for  the  final  drying. 

It  is  evident  that  economy  in  operation  consists  in  evaporating 
the  maximum  weight  of  water  with  a  given  quantity  of  air,  and 
yet  the  unpardonable  sin  is  to  put  fresh  glue  into  an  alley  which 
is  already  delivering  saturated  air.  In  its  simplest  terms  the 
problem  consists  in  putting  all  the  heat  possible  into  the  air 
without  danger  of  the  wet  bulb  temperature  reaching  the  melting 
point  of  the  fresh  jelly,  also  the  addition  of  new  trucks  and 
withdrawal  of  old  ones  so  gradually  and  continuously  that  the 
exit  humidity  is  only  slightly  below  saturation,  and  the  regulation 
of  air  velocity  to  alley  length  such  that  smooth  sheets  of  fairly 
uniform  moisture  content  are  produced. 

If  the  alleys  are  equipped  with  individually  controlled  fans  and 
heating  coils,  and  hourly  determinations  of  the  wet  bulb  tempera- 
ture of  the  changing  outside  air  are  made,  all  that  is  necessary 
for  complete  control  is  to  note  from  the  chart  the  exact  amount 
of  heat  that  may  be  safely  added  to  an  alley  containing  glue  of 
any  given  melting  point.  The  given  runs  and  concentration  of 
solutions  in  any  well  operated  factory  are  consistent  enough  that 
the  melting  point  of  any  given  grade  of  product  is  easily  learned. 

As  the  weight  of  water  evaporated  by  a  given  weight  of  air 
sometimes  drops  as  low  as  a  quarter  of  one  per  cent  (of  the  air 


298  GELATIN  AND  GLUE 

handled) ,  and  as  all  air  contains  an  appreciable  quantity  of  dust, 
smoke,  etc.,  the  necessity  for  cool  pure  air  is  almost  as  important 
as  that  for  cool  pure  water.  The  ideal  location  for  a  factory  to 
make  a  thin  clear  product  would  be  at  a  high  altitude  far  from  the 
dirt  of  a  city.  Tyndall19  over  fifty  years  ago  found  that  50  per 
cent  of  the  total  dust  of  the  atmosphere  is  contained  in  the  first 
100  feet  above  the  earth's  surface,  due  largely  to  the  moisture 
condensing  effect  of  dust,  and  in  turn  the  dust-holding  properties 
of  moisture. 

This  indicates  clearly  that  the  air  intake  should  be  situated  at 
as  high  a  point  as  possible  and  where  the  prevailing  winds  will  not 
bring  smoke  from  nearby  stacks. 

Even  under  the  best  conditions  the  amount  of  dust  and  accom- 
panying bacteria  deposited  on  the  glue  by  the  enormous  volume 
of  air  is  considerable  and  it  is  almost  certain  that  at  some  time 
the  trade  will  demand  edible  gelatin  which  has  been  dried  with 
artificially  purified  air.  The  washing  and  conditioning  of  air  for 
public  buildings  of  all  sorts  is  becoming  quite  common.  As  high 
as  98  per  cent  of  the  dust  can  be  washed  from  the  air  in  efficiently 
operated  scrubbers. 

The  removal  of  moisture  from  the  air  used  in  iron  blast  furnaces 
has  been  practiced  for  years.  The  Gayley  dry  blast  process,  the 
first  and  best  known  air  drying  process,  is  dependent  upon 
refrigeration  for  removal  of  the  moisture.  How  complete  this 
can  be  is  shown  by  the  small  saturation  values  at  low  temperatures 
on  the  psychrometric  chart.  Other  processes  are  based  upon  the 
use  of  some  dehydrating  agent  which  is  regenerated  by  heat  in 
another  stage  of  the  process.  For  moderate  moisture  removal 
this  class  is  generally  more  economical  than  refrigeration. 
Figure  52  illustrates  the  moisture  absorbing  properties  of  solu- 
tions of  calcium  chloride  as  an  example  of  the  possibilities  of  this 
class  of  moisture  removal  methods. 

After  the  trucks  are  removed  from  the  alleys;  the  sheet  glue  is 
stripped  from  the  nets  by  hand  and  in  most  cases  broken  up  by 
means  of  crushers,  generally  of  the  hammer  mill  or  disintegrator 
type.  This  produces  the  flake  glue  of  commerce.  It  may  then 
be  ground  in  a  rotary  fine  crusher  to  a  coarse  powder.  For  many 
uses,  such  as  wall  finishes,  it  is  again  ground  to  an  almost  impal- 
pable powder.  Special  forms  such  as  sheet,  strip,  and  noodle  glue 
are  made  by  cutting  the  jelly  to  such  dimensions  that  the  pieces 
when  dried  will  attain  the  desired  shape  and  size. 


MANUFACTURE  OF  GLUE  AND  GELATIN 


299 


Many  methods  of  drying  glue  have  been  tried  as  substitutes  for 
this  laborious  process  of  chilling,  spreading,  and  alley  drying. 
Vacuum  dryers  which  spread  the  liquor  in  a  film  over  an  inter- 


nally  heated  rotary  drum  are  used  for  many  products  but  have 
met  with  little  favor  in  the  glue  and  gelatin  industry.  However » 
steam-heated  rolls  evaporating  directly  into  the  atmosphere  are 
finding  limited  use  on  low  grade  glues.  Atomization  of  the 


300  GELATIN  AND  GLUE 

liquor  into  a  rapidly  moving  current  of  very  hot  air20  is  a  success- 
ful method  for  drying  such  products  as  milk,  but  the  method  has 
been  applied  to  glues  and  gelatins  to  only  a  limited  extent. 

Market  glue  contains  from  8  to  16  per  cent  moisture,  according 
to  the  quality.  Other  conditions  being  equal,  the  higher  grade 
glues  contain  the  higher  percentage  of  moisture,  being  more 
hygroscopic  by  nature.  The  physical  properties  of  the  glue 
are  apparently  dependent,  in  certain  characteristics,  upon  this 
hygroscopic  moisture.  If  it  is  driven  off,  the  solubility  and  test 
suffer  in  proportion.  Completely  dehydrated  glue  is  practically 
insoluble  and  in  its  action  resembles  untreated  glue  stock  more 
than  finished  glue.  The  tensile  strength  of  the  dry  glue  is  also 
closely  allied  to  the  moisture  content.  The  low  grade  glues, 
which  have  low  tensile  strength  when  dry,  are  exceedingly  strong 
at  certain  stages  of  the  drying.  This  is  easily  shown  in  the 
making  of  a  frost-crystal  surface  on  glass.  The  glass  is  ground 
and  coated  with  a  heavy  solution  of  a  low  grade  glue,  then  dried 
in  an  oven.  During  drying  the  glue  film  shrinks  and  curls  away 
from  the  glass,  pulling  with  it  a  part  of  the  glass,  and  producing  in 
this  manner  the  appearance  of  crystals  often  several  inches  long. 
This  glue,  which  when  partially  dry  was  strong  enough  to  strip 
large  areas  from  the  glass,  may  fall  apart  of  its  own  accord  when 
allowed  to  dry  slowly  in  the  air. 

2.  Bone  Glue.21 — Since  in  many  respects  the  making  of  bone 
glue  is  similar  to  that  of  hide  glue,  for  the  sake  of  brevity  only 
those  processes  where  there  is  a  marked  difference  in  procedure 
will  be  discussed.  Green  bone,  immediately  upon  receipt  is 
passed  through  a  rotary  crusher,  then  over  a  screen  to  separate 
and  save  the  fine  particles  which  might  otherwise  be  lost  in 
process.  It  is  then  thoroughly  washed  in  a  barrel  mill  until  all 
blood,  etc.,  is  removed  and  finally  washed  in  dilute  sulphurous 
acid.  The  purpose  of  this  preliminary  treatment  is  not  only  to 
remove  all  color  left  by  the  blood  but  also  to  keep  the  stock  sweet 
and  minimize  development  of  rancidity  in  the  fat. 

Degreasing. — In  this  condition  the  bones  contain  roughly  50 
per  cent  moisture  and  5  to  20  per  cent  grease,  the  composition, 
of  course,  varying  according  to  the  kind  of  bone.  If  this  bone  is 
stored  in  the  raw  condition  the  fats  will  slowly  oxidize  in  the 
air  thus  reducing  the  value  of  the  stock.  On  this  account  it  is 
customary  to  remove  as  much  as  possible  of  the  grease  and  mois- 
ture before  storage.  This  may  be  accomplished  by  giving  the 


MANUFACTURE  OF  GLUE  AND  GELATIN 


301 


bones  a  short  preliminary  boiling  which  removes  all  but  about  1 
per  cent  of  the  grease,  after  which  they  are  spread  out  and  dried, 
yielding  a  product  similar  to  the  commercial  " packer  bone." 

Shin  bones,  after  such  treatment  are  dried  and  sold  as  fancy 
bones  for  button  and  novelty  making.  The  button  scrap,  known 
as  "dentelles,"  is  bought  back  and  returned  to  process  where  it 
left  off.  With  cattle  feet,  after  a  short  boiling  or  steaming,  the 
hoofs  are  separated  by  a  machine  which  squeezes  the  hoof  and 
causes  it  to  pop  off  the  toe.  With  pigs'  feet  the  hoofs  are  allowed 
to  remain  and  are  separated  from  the  residue  after  boiling. 


SEPARATOR 


j  J 


Water 


EX    T   R  A    C   T  0  R  8 


FIG.  53. — A  three-extractor  unit  for  degreasing  bones. 

Horns  and  hoofs  have  no  value  to  the  glue  maker  and  are  generally 
used  by  cutting  to  certain  shapes,  steaming  and  moulding  in  a 
press  while  hot,  or  they  are  ground  and  the  pulverized  horn  made 
into  a  mixture  which  can  be  moulded  like  any  other  plastic. 
Often  the  less  desirable  hoofs  and  horns  are  ground  and  mixed 
into  fertilizer. 

To  put  the  bone  into  the  best  possible  condition  for  glue 
making,  practically  all  the  grease  can  be  removed  by  solvent 
extraction.  The  solvent  used  may  be  benzine  or  gasoline,  ben- 
zol, carbon  tetrachloride,  or  carbon  disulphide.  This  last  has 
the  disadvantage  that  it  is  extremely  inflammable,  has  a  noxious 
odor  and  is  poisonous.  Carbon  tetrachloride  is  non-inflammable, 


302  GELATIN  AND  GLUE 

and  the  cost  is  the  only  objection.  Benzine  is  perhaps  the  most 
commonly  used.  To  obtain  good  extraction  the  bones  must  be 
dry — 10  per  cent  moisture  or  less — otherwise  extraction  is 
hindered  by  the  water  and  a  certain  amount  of  glue  is  extracted. 

The  accompanying  sketch,  Fig.  53,  furnished  through  the 
courtesy  of  D.  Obernethy  of  the  Wilson-Martin  Co.  shows  a 
three-extractor  unit. 

The  crushed  unscreened  bones  are  filled  into  the  extractors 
and  solvent  sufficient  to  cover  the  closed  steam  coils  is  run  in  from 
the  storage  tank.  Steam  is  then  admitted  to  the  coils  and  ex- 
traction proceeds  through  the  condensation  of  the  solvent  on  the 
bone  and  percolation  back  to  the  main  body  of  the  liquor  on  the 
coils.  After  a  short  time  the  outlet  valve  at  the  top  is  slightly 
opened  and  the  gas  (or  solvent)  and  water  distilled  into  the 
condenser.  The  liquor  from  the  condenser  passes  into  a  sepa- 
rator which  allows  the  solvent  to  return  to  the  storage  tank  and 
the  water  to  pass  from  the  lower  outlet. 

When  the  solvent  has  all  been  distilled  a  second  supply  of 
solvent  is  admitted  and  the  valve  closed,  the  process  being 
repeated  as  before.  After  a  second  extraction  and  distillation 
the  grease  emulsion  is  drawn  into  the  still.  The  whole  operation 
is  again  repeated  until  the  condenser  liquor  shows  very  little 
water,  when  extraction  is  assumed  to  be  complete.  After  the 
grease  emulsion  is  emptied  into  the  still  line,  steam  is  blown 
through  the  extractor  to  the  limit  of  the  condenser  to  completely 
expel  all  solvent  from  the  bone.  This  is  continued  until  the 
condensate  is  all  water,  when  all  valves  are  closed  and  the  top 
and  bottom  covers  opened  to  admit  of  a  good  circulation  of  air 
for  drying  the  bones  while  still  hot. 

While  one  extractor  is  being  emptied  and  refilled,  one  of  the 
stills  is  operated  on  the  condenser  which  would  otherwise  be 
idle.  When  all  water  and  gas  are  removed  the  grease  is  drawn 
to  the  refinery  where  it  is  washed  with  water  and  acid  as  described 
later. 

Extraction  may  be  conducted  under  either  pressure  or  vacuum, 
the  first  having  the  advantage  of  ease  of  operation,  the  latter  of 
low  temperature  and  consequent  minimum  injury  to  the  glue. 
The  finished  bone  contains  Jfo  Per  cent  of  grease  or  less  as 
compared  to  about  1  per  cent  in  bones  degreased  by  cooking. 
Also  the  space  required  for  storage  has  been  decreased  about  20 
per  cent,  and  the  condition  of  the  bones  is  such  that  they  will 


MANUFACTURE  OF  GLUE  AND  GELATIN  303 

permit  almost  indefinite  storage  without  deterioration,  producing 
glue  of  better  grade  and  color. 

Green  bone  may  be  boiled  with  no  preliminary  treatment 
other  than  a  thorough  washing  and  a  change  or  two  of  weak  acid. 
If  boiled  in  open  boxes  in  the  same  manner  as  hide  stock,  the 
time  of  boiling  is  increased  to  four  or  more  hours  per  run  and  the 
temperatures  are  higher,  most  of  the  runs  being  actually  boiled. 

If  autoclaves  or  pressure  tanks  are  used  for  boiling,  the  time 
is  materially  shortened  because  of  the  higher  temperature  ob- 
tained, the  liquor  is  more  easily  clarified,  and  the  bone  residue 
is  softer  and  more  completely  extracted.  In  some  instances  this 
addition  of  water  and  removal  of  liquor  from  the  pressure  tanks 
is  made  continuous,  principally  for  the  purpose  of  getting  a 
liquor  of  higher  concentration  and  a  more  complete  extraction. 
It  is  claimed  that  the  residue  contains  less  than  half  the  amount 
of  nitrogen  left  in  the  residue  from  open  boiling.  Several  patents 
have  been  taken  out  for  the  alternate  application  of  pressure  and 
vacuum  in  the  autoclaves,  for  producing  better  penetration  of 
the  water  and  thus  giving  a  more  concentrated  liquor. 

The  liquors  from  bones  are  treated  similarly  to  hide  glues 
throughout  the  remainder  of  the  process,  with  the  exception  that, 
since  they  are  of  a  lower  test  they  are  evaporated  to  a  higher  con- 
centration  before  being  chilled  and  cut. 

If  sulphurous  acid  is  passed  over  bones  they  Avill  absorb  from 
11  to  12  per  cent  of  their  weight  of  the  gas,  forming  insoluble 
dicalcium  phosphate  and  calcium  sulphite.  In  this  condition 
the  bones  are  readily  disintegrated  by  hot  water  and  the  gelatin 
rapidly  extracted.  This  is  the  Grillo-Schroeder  process.22  The 
mud  residue  is  oxidized  or  exposed  to  the  air  to  convert  the  sul- 
phite to  sulphate  and  then  used  for  fertilizer. 
^3.  Ossein. — Bone,  exclusive  of  the  marrow,  blood  vessels, 
nerves  and  the  like,  consists  of  a  cell  substance  and  matrix.  The 
cell  substance  is  very  resistant  to  both  acids  and  alkalies  and 
yields  no  gelatin  on  boiling  with  water.  The  organic  portions  of 
the  matrix  yield  gelatin,  and  the  problem  of  the  manufacturer  is 
to  so  prepare  the  bone  that  the  hydrolysis  to  gelatin  may  be 
conducted  with  the  least  possible  deterioration  of  the  product. 

Prolonged  treatment  with  dilute  acid  seems  to  be  the  most 
efficient  method  of  preparation,  but  at'  the  same  time  it  is  the 
most  troublesome  and  expensive.  By  such  treatment  almost  all 
the  inorganic  substances  are  removed.  The  cell  substance 


304  GELATIN  AND  GLUE 

is  partly  lost,  the  little  which  remains  being  the  truly  insoluble 
portion,  and  there  is  left  only  the  periosteum  and  the  organic 
portion  of  the  matrix. 

This  ossein  can  be  limed  and  handled  in  a  manner  similar  to 
hide  or  sinew  stock,  and  if  the  proper  care  is  used  throughout,  the 
process  yields  a  bright  strong  gelatin  of  the  highest  quality. 

For  the  making  of  ossein,  selected  degreased  stock  only  is  used, 
hornpiths,  dentelles,  and  jaw,  knuckle,  and  rib  bones  being  among 
the  favorites. 

The  number  of  acids  suitable  for  the  production  of  ossein  are 
rather  limited.  Although  the  calcium  salts  of  several  acids  are 
soluble  in  water,  the  acid  radicle  has  an  injurious  effect  on  the  glue- 
forming  material.  Nitric  and  acetic  acids  are  examples  of  this 
class.  Hydrochloric  acid  is  by  far  the  most  commonly  used. 
The  reactions  are  as  follows: 

Ca3(P04)2  +  4HC1  ->  2CaCl2  +  Ca(H2PO4)2. 

If  the  liquor  is  allowed  to  stand  in  contact  with  the  bone  indefi- 
nitely, the  acid  calcium  phosphate  reacts  further  as  follows: 
Ca(H2PO4)2  +  Ca3(PO4)2  -»  4CaHPO4  which  is  practically 
insoluble,  leaving  nothing  but  CaCl2  in  solution.  This  can  be 
regenerated  by  sulphuric  acid : 

CaCl2  +  H2S04  ->  CaS04  +  2HC1, 
or  if  the  liquor  contains  monocalcium  phosphate : 

Ca(H2PO4)2  +  H2SO4  -»  CaSO4  +  2H3PO4. 

Both  of  these  acids  can  then  be  used  for  a  second  leaching. 
As  far  as  convenience  is  concerned  hydrochloric  acid  is  much  to 
be  preferred,  for  although  4  parts  of  acid  dissolve  3  parts  of 
lime  in  both  cases: 

4H3P04  +  Ca3(P04)2  ~»  3Ca(H2P04)2, 

the  weight  of  phosphoric  acid  required  is  almost  three  times  as 
great,  and  the  reaction  is  somewhat  slower.  On  the  other  hand, 
commercial  muriatic  acid  always  contains  iron,  and  iron  is  one  of 
the  principal  sources  of  color  in  glue.  If  the  acidity  of  the  liquor 
is  allowed  to  drop  very  low  during  any  stage  of  the  leaching 
process  iron  is  deposited  as  ferric  hydrate  which  it  is  almost 


MANUFACTURE  OF  GLUE  AND  GELATIN 


305 


impossible  to  remove  completely  by  increase  in  acidity.  If 
sulphurous  and  phosphoric  acids  are  used  in  the  final  stages  of  the 
leaching,  the  sulphurous  acid  reduces  the  iron  to  the  ferrous  state, 
which  then  forms  ferrous  phosphate  which  is  colorless.  Appar- 
ently the  difficulty  in  removing  this  color  is  not  altogether  due  to 


.5  1.0  0.5 

Molecular  Concentration 

FIG.  54. — Acidity  changes  in  ossein  leach  liquor. 


the  slight  solubility  of  the  iron  salts,  but  to  the  formation  of 
organic  combinations  or  adsorption  compounds  which  do  not 
respond  to  the  same  treatment  as  pure  iron  salts. 

The  strength  of  the  hydrochloric  acid  used  generally  varies 
from  2  to  5  per  cent.  The  bones  are  placed  in  wooden  vats  and 
covered  with  the  dilute  acid.  Great  care  is  necessary  that  the 
stock  is  not  injured  by  a  rise  in  temperature.  When  the  acid  is 
exhausted  it  is  drawn  off  and  replaced  by  fresh  acid.  This  is 
repeated,  changing  the  concentration  as  required,  until  the  bones 
are  practically  free  from  lime,  when  they  are  ready  to  be  removed 
and  washed  and  perhaps  dried  for  storage.  One  pound  of  bone 
requires  roughly  one  pound  of  22°  acid  for  complete  extraction. 
Figure  54  is  illustrative  of  the  change  in  acidity  of  a  leach  liquor 
20 


306  GELATIN  AND  GLUE 

used  on  bones  perhaps  two  thirds  exhausted.  This  is  merely  a 
graphical  representation  of  the  distribution  and  fate  of  the  total 
ionizable  hydrogen  supplied  in  the  2  normal  hydrochloric  acid 
used  for  this  change. 

In  the  Bergmann  process23  sulphur  dioxide  is  circulated  through 
closed  tanks  containing  the  bones  and  continuously  enriched  with 
sulphur  dioxide.  The  acidity  is  maintained  high  enough  to 
retain  the  phosphate  in  solution.  The  sulphur  dioxide  is  then 
recovered  from  the  solution  by  treatment  with  steam  in  a  lead 
lined  digester  and  the  calcium  sulphite  then  decomposed  by  the 
addition  of  the  required  quantity  of  hydrochloric  acid.  The 
reactions  may  be  represented  as  follows : 

Leaching:  Ca3PO4  +  4H20  +  4SO2  -> 

j  2Ca(HS03)2  +  Ca(H2PO4)2; 
Steam  regeneration:  Ca(H2POI)2  +  2Ca(HSO3)2  -> 

CaHPO,  +  CaS03  +  3SO3  +  3H2O; 
Acid  regeneration:  CaSO8  +  2HC1  ->  CaCl2  +  SO2  +  H,0. 

The  precipitated  dicalcium  phosphate  is  washed  free  from  the 
soluble  chloride  and  dried  for  market. 

4.  Gelatin. — The  making  of  gelatin  differs  from  that  of  glue 
only  in  the  selection  of  the  raw  material  and  the  care  and  extreme 
cleanliness  in  operation.  The  raw  materials  for  gelatin  manu- 
facture may  be  hide  stock,  green  bone,  or  ossein.  If  hide  stock, 
it  is  preferably  that  of  young  animals.  Perhaps  50  per  cent  of 
the  gelatin  produced  in  this  country  is  made  from  calf  stock. 
If  green  bone  is  used,  selected  parts  of  the  animal  such  as  jaws, 
feet,  and  knuckles  are  chosen  from  the  killing  houses  and  worked 
up  immediately  so  as  to  admit  of  no  development  of  color  and 
rancidity.  Ossein  must  be  firm  and  of  good  colorv< 

In  making  a  product  for  consumption  as  a  food,  especially  when 
the  material  is  as  subject  to  bacterial  decomposition  as  glue  stock, 
eternal  vigilance  is  the  price  of  success.  If  the  liming  is  very 
carefully  watched,  bacterial  formation  of  sulphides  can  be  pre- 
vented. The  whole  plant  must  be  so  constructed  that  the 
equipment  may  be  kept  immaculately  clean  without  excessive 
labor.  The  use  of  wooden  containers  for  liquors  is  inadvisable 
because  of  the  difficulty  in  keeping  them  sterile.  As  the  liquors 
generally  are  kept  slightly  acid,  such  metals  as  copper  and  zinc 


MANUFACTURE  OF  GLUE  AND  GELATIN  307 

offer  danger  of  metallic  contamination.  Aluminum  seems  there- 
fore to  be  the  preeminently  desirable  constructional  material. 
The  acid  and  other  secondary  raw  materials  must  be  free  from 
arsenic  and  other  prohibitive  impurities  which  may  appear  in  the 
finished  gelatin.  Sulphites  can  be  changed  to  sulphates  by  the 
use  of  hydrogen  peroxide  and  removed  in  the  clarification.  To 
have  the  highest  market  value  the  color  must  be  at  a  minimum 
and  the  product  bright  and  sparkling  both  in  the  dry  state  and 
in  solution. 

However,  from  the  standpoint  of  health,  the  objectionable 
substances  which  may  be  present  are  of  little  consequence  com- 
pared to  the  possibility  of  excessive  bacterial  contamination. 
Occasionally  objectionable  stock  has  been  used  in  making 
"edible"  gelatin,  or  the  method  of  handling  has  resulted  in  a 
partially  decomposed  or  otherwise  offensive  product.  It  is  not 
always  a  simple  matter  for  such  abuse  to  be  detected  in  the 
laboratory,  and  the  most  effective  means  for  preventing  imposi- 
tions of  such  a  type  from  being  continued  seems  to  lie  in  a  system 
of  rigid  inspection  of  all  establishments  purporting  to  manu- 
facture an  edible  gelatin.  Wilful  violation  of  the  public  confi- 
dence should  be  regarded  as  a  serious  crime  and  dealt  with 
accordingly* 


III.  BY-PRODUCTS 

The  most  important  by  product,  in  quanitty  at  least,  is  the 
residue  from  boiling  bones.  When  it  is  discharged  from  the 
boiling  vat  or  pressure  tank  it  is  soft  and  easily  crushed.  This 
is  spread  on  gunny  bags  steaming  hot  in  layers  of  perhaps  six 
inches  in  thickness,  completely  surrounded  by  the  bag  and  stacked 
alternately  with  lattices  made  of  wood  into  an  hydraulic  press. 
Here  the  last  of  the  glue  liquor  and  grease  are  pressed  out  and 
returned  to  process.  The  press-cake  is  broken  up,  and  spread 
out  to  cool  and  dry,  or  dried  in  a  rotary  direct  heat  or  steam 
dryer.  This  is  then  analyzed  and  mixed  with  other  constituents 
to  form  a  fertilizer  of  any  desired  analysis,  ground  and  sacked 
for  market. 

Hair  tankage,  which  is  the  residue  left  from  the  boiling  of  hide 
stock  and  fleshings,  etc.,  contains  a  great  deal  of  lime  soap  as 
well  as  hair  and  other  organic  matter.  To  recover  the  grease 
the  tankage  is  boiled  with  dilute  sulphuric  acid  and  pressed  the 


308  GELATIN  AND  GLUE 

same  as  the  bone  tankage.  The  remaining  hair  may  be  used  as 
such  or  treated  with  hydrofluoric  and  sulphuric  acids  and  con- 
verted to  ammonium  sulphate  for  fertilizer. 

Grease  is  an  important  by-product.  From  the  hock  joint  of 
cattle  feet  neatsfoot  oil  is  obtained  and  on  account  of  its  value 
the  hock  joints  are  always  boiled  separately.  Neatsfoot  oil  is 
considered  the  best  basis  of  leather  dressings  on  account  of  its 
property  of  working  into  moist  leather  and  keeping  it  soft  and 
pliable  under  all  conditions  of  moisture  and  temperature.  The 
clear  oil  from  sweet  fresh  bones  gives  a  tallow  of  good  color  and 
test.  Bone  tallows  are  always  lower  melting  than  the  tallows 
obtained  from  the  other  parts  of  the  animal. 

The  oil  from  pigs'  feet  has  the  lowest  melting  point  of  any  of  the 
glue  works,  oils  and  that  from  sheep  stock  naturally  has  the 
highest,  as  mutton  tallow  is  very  high  melting. 

Hide  stock  produces  very  little  oil.  Fleshings  often  produce  a 
weight  of  grease  equal  to  that  of  the  glue.  The  part  of  this  that 
comes  off  clear  in  the  early  boilings  is  of  medium  grade  and  has  a 
good  demand  from  the  soap  makers. 

The  grease  from  the  later  boilings  contains  quite  a  bit  of  lime 
soaps,  as  does  also  the  press-grease  from  the  hair  tankage.  This 
is  most  easily  decomposed  by  boiling  with  acid,  after  which  it  is 
well  washed  with  water  and  dried  by  heating.  Unless  the  lime 
and  glue  are  completely  washed  out,  bacterial  decomposition 
soon  sets  in,  resulting  in  great  loss  of  value.  All  grades  of  grease 
must  be  thoroughly  washed  with  water  and  dried  to  assure  good 
keeping  qualities. 

Mention  was  made  in  discussing  green  bones,  of  the  necessity 
of  keeping  the  stock  sweet  if  rancidity  of  the  fat  is  to  be  pre- 
vented. Although  sulphurous  acid,  added  immediately  after 
washing,  is  the  most  common  substance  used  for  this  purpose, 
many  other  bactericidal  preservatives  may  be  used  if  they  fulfill 
the  two  requirements  of  having  no  injurious  effect  on  the  glue, 
and  of  preventing  oxidation,  for  the  development  of  rancidity 
is  an  oxidation  phenomenon. 

Although  the  acidity  of  the  best  of  the  fats  is  exceedingly  low, 
there  are  many  uses,  especially  for  edible  products,  which  require 
that  the  fat  be  neutral.  A  neutral  fat  is  produced  by  alkaline 
refining.  Briefly  the  operation  consists  of  the  addition  of  a 
calculated  amount  of  caustic  sufficient  to  a  little  more  than 
satisfy  the  free  fatty  acid.  This  is  conducted  under  carefully 


MANUFACTURE  OF  GLUE  AND  GELATIN  309 

controlled  conditions  of  temperature  and  dilution  so  that  a  good 
curd  of  soap  is  produced.  Sodium  silicate  is  sometimes  added 
to  assist  in  the  curd  formation.  The  fat  is  then  drawn  off  from 
the  lower  layer,  washed  and  dried.  This  lower  layer  consists  of 
soap,  water,  and  some  neutral  fat,  and  can  be  acidulated  to 
recover  the  fatty  acid  or  used  directly  for  soap  making. 

In  the  manufacture  of  ossein,  the  large  volume  of  leach  liquors 
containing  acid  calcium  phosphate  constitute  a  valuable  by- 
product.24 They  are  commonly  precipitated  with  milk  of  lime 
to  produce  dicalcium  phosphate,  CaHPO-i,  washed  well  with 
water  to  free  from  calcium  chloride,  if  hydrochloric  acid  has  been 
used  in  the  leaching,  then  dried  by  steam  or  in  a  direct  heat 
dryer  and  sold  for  fertilizer. 

This  so  called  " precipitated  bone"  contains  35  to  40  per  cent 
total  P2O5,  most  of  it  in  the  citrate  soluble  form.  It  is  an  attempt 
to  make  a  pure  CaHP04.2H20  which  would  analyze  a  little  over 
41  per  cent  P2O5. 

The  dicalcium  phosphate,  if  carefully  made,  may  be  calcined 
to  rid  it  of  all  organic  and  volatile  constituents  and  used  in  the 
manufacture  of  phosphate  baking  powders.  The  almost  total 
absence  of  fluorides  and  iron  in  bone  phosphate  makes  it  the  only 
really  desirable  source  of  phosphate  for  this  purpose. 

The  formation  commercially  of  a  calcium  phosphate^of  definite 
composition  is  not  a  simple  problem  by  any  means.  Being 
salts  which  easily  hydrolyze  they  cannot  even  be  washed  with 
water  and  maintain  a  definite  composition.  As  an  illustration: 
If  chemically  pure  dicalcium  phosphate  (CaHPO4.2H20)  be 
washed  repeatedly  with  water  which  is  allowed  to  stand  until  it 
has  become  saturated,  analysis  will  show  the  wash  waters  to  be 
relatively  high  in  P205  and  the  insoluble  salt  to  have  become 
correspondingly  richer  in  CaO. 

Also  if  chemically  pure  tricalcium  phosphate  is  allowed  to 
stand  with  water,  samples  from  some  sources  may  give  alkaline 
liquors  while  others  may  give  acid  liquors.  Atterton  Seidell  and 
others  have  made  a  study  of  these  equilibria  and  the  general 
facts  of  their  observations  are  shown  in  Figs.  55  and  56,  Starting 
at  the  left  of  Fig.  55:  Mixtures  Ca(OH)2  and  Ca3P04  give 
liquors  of  very  rapidly  decreasing  CaO  content  until  a  min- 
imum is  reached  at  CaO  =  0.01  and  P2O5  =  0.02  gram  per 
liter.  From  this  point  the  solid  phase  is  Ca3PO4,  or  more  cor- 
rectly, a  solid  solution  of  CaO  in  CaHP04.2H20,  and  when  CaO 


310 


GELATIN  AND  GLUE 


U.3 
f\Q 

uo 

fi7 

06 

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^ 

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v.c 

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tf& 

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A 

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x^" 

-goW 

0       0.1        0.2      0.3       0.4       0.5       0.6      0.7       0.8       0.9        1.0        1.         1.2 
Grams  P205  per  Lifer 
FIG    55  —  Composition   of  saturated   solutions  of  calcium  phosphate.  I.     (A 
SeidelL) 

ou 
TO 
GO 

IBO 
1 

0  4-0 

5 

tO 

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10 
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5        50       100      150      200     250     300      350     400     4bU      bUU     bb 
Orams  P^^S  Per  Liter 

5(3 — Composition   of  saturated  solutions  of  calcium  phosphate.  II.     (A 

Seidell.) 


MANUFACTURE  OF  GLUE  AND  GELATIN  311 

=  0.03  and  P2O5  —  0.07  the  solution  changes  from  alkaline  to 
acid.  The  general  direction  of  the  curve  continues  until  some- 
where in  the  region  of  CaO  =  0.4  and  P2O5  =  1.1  when  the  slight 
upward  trend  turns  and  becomes  downward  from  the  former 
general  direction.  This  is  probably  the  point  of  disappearance 
of  CaHPO4.2H2O  and  its  replacement  by  CaHPO4  in  the  solid 
phase.  Finally,  at  CaO  =  77  and  P2O5  =  317,  in  Fig.  56,  the 
small  scale  graph,  the  curve  takes  a  new  and  sharply  downward 
direction  indicating  the  final  solid  phase  of  Ca(H2P04)2.H2O 
which  probably  persists  as  long  as  any  solid  is  present. 

The  precipitation  of  calcium  phosphate  of  any  definite  analysis 
may  be  accomplished  by  addition  of  lime  until  the  analysis  of 
the  liquor  corresponds  to  that  of  the  liquor  in  equilibrium  with 
solid  of  the  desired  composition  on  the  graph.  This  is  desirable 
because  a  routine  gravimetric  analysis  of  the  solid  phase  requires 
considerable  time  while  a  titration  of  the  liquor  may  be  obtained 
in  about  10  minutes.  Also  if  the  calculated  amount  of  lime  is 
added  and  by  accident  the  desired  point  is  passed,  the  addition 
of  liquor  according  to  a  new  calculation  for  correction  does  not 
produce  a  precipitate  as  citrate-soluble  as  that  resulting  when 
no  over-reaching  has  occurred. 

The  preferable  procedure  is  to  slowly  add  milk  of  lime  while 
stirring  thoroughly  until  90  or  95  per  cent  of  the  desired  amount 
is  added,  then  to  allow  the  mixture  to  stand  over  night  to  come 
to  equilibrium.  The  following  morning  the  liquor  is  analyzed 
and  the  amount  of  lime  necessary  for  the  completion  of  the 
reaction  added  as  before.  After  standing  again  the  results  are 
checked  up  by  a  third  analysis  of  the  liquor,  and  if  found  correct, 
the  liquor  is  drawn  off  and  the  precipitate  washed  and  dried. 
Instead  of  using  the  titration  method  described  in  Chap.  IX, 
conductivity  determinations  may  be  substituted  but  they  take 
somewhat  longer  and  variations  in  the  amount  of  other  soluble 
salts  present  introduce  an  unknown  error. 

If  a  distinctly  acid  phosphate  is  being  produced,  the  phosphate 
remaining  in  the  liquor  drawn  off  may  be  precipitated  by  lirne 
and  the  precipitate  produced  added  to  the  liquor  for  the  next 
batch  without  any  washing  or  further  treatment. 

As  before  stated,  washing  increases  the  alkalinity  of  the 
precipitate  and,  therefore,  only  the  minimum  amount  of  wash 
water  necessary  to  remove  the  undesirable  soluble  salts  should  be 
used. 


312 


GELATIN  AND  GLUE 


The  leach  liquors  may  be   converted  into   phosphoric  acid 
according  to  the  reaction  : 


Ca(H2P04) 


CaS04  +  2H3P04 


by  the  addition  of  sulphuric  acid.  If  a  high  percentage  of  hydro- 
chloric acid  is  present,  it  may  be  distilled  off  after  concentration, 
or  even  distilled  off  before  the  addition  of  the  sulphuric  acid. 
It  is  difficult  to  find  material  satisfactory  for  holding  solutions 


5/f/V? 

wrings 

Sinews 
Tendons 

Rawhide 

Tannery 
Scrap 

(artilcye 

Bones 

Vornptfk 

Hoofs 

I  LIME    ATS- 


—  £  \DEGREAS 

-----  ' 


I  WASH  MILLS  (Water+Dilute/Jc/d+Wafer 


I  LING  VA  TS 


VA  C  UUM  PANS 


I  DRYING   ALLEYS    \ 


STORAGE  OR 
MARHET 


Route  common  fo  -fwo  or  more  materials 


I  Pulverizer 


which  however  may  be  kept  separate.  '^Special'  ~! 
•Route  of  single  grade  or  material 


LARGE  Letters  -  Required  or  common  operations 
Small  Letters-  Optional  operations  for  certain  grades 
Ending  ing  used  on  operations  which  a  re  practically  hand  worn 


FIG.  57. — Course  of  glue  stock  through  the  plant. 

containing  a  high  percentage  of  phosphoric  acid,  especially 
if  high  temperatures  are  to  be  used.  Phosphoric  acid,  especially 
in  the  presence  of  some  of  its  salts,  attacks  silica  very  readily, 
and  therefore  has  a  marked  action  on  glass.  However,  there  is 
on  the  market  at  least  one  make  of  glass-lined  steel  tank  which 
stands  up  very  well  at  moderate  temperatures,  and  at  very  high 
temperatures  duriron  may  be  used.  The  purification  of  phos- 
phoric acid  is  a  tedious  process,  consisting  of  precipitation  of  the 


MANUFACTURE  OF  GLUE  AND  GELATIN  313 

individual  impurities,  and  is  beyond  the  scope  of  this  book  to 
discuss. 

It  ought  to  be  possible  to  produce  phosphoric  acid  and  any 
desired  calcium  phosphate  from  the  leach  liquors  by  electrolysis 
but  there  are  a  number  of  unsolved  difficulties  at  present  to  be 
overcome.  In  the  first  place  a  tripartite  cell  would  be  required 
so  that  the  liquor  could  be  introduced  into  the  central  or  neutral 
compartment  and  the  acid  and  precipitate  produced  in  their 
appropriate  compartments.  The  diaphragm  difficulties  of  the 
soda  electrolysis  are  small  as  compared  with  the  difficulties  here. 
The  disintegrating  action  of  phosphoric  acid  is  to  be  reckoned 
with  in  the  acid  diaphragm,  and  precipitation  troubles  have  to  be 
taken  care  of  in  the  alkaline  diaphragm.  An  investigation  of 
this  problem  is  recommended  to  any  chemist  who  desires  an 
extremely  interesting  subject  for  research-. 

An  outline  of  the  distribution  and  course  of  the  glue  stock 
throughout  the  entire  manufacturing  process  is  sketched  in 
Fig.  57. 

IV.  A  SELECT  BIBLIOGRAPHY  ON  THE  MANUFACTURE  OF  GLUE 

AND  GELATIN 

1.  ABT,  G.,  The  role  played  by  bacteria  in  the  production  of  salt  stains  in 
hides  and  skins,  Collegium,  6  (1913),  204;  Chem.  Abstracts,  7  (1913),  4093. 

ROEHM,  O.,  Process  of  disinfecting  hides  and  skins  in  the  lanyard. 
Ger.  Pat.  254,131  (1911),  J.  Soc.  Chem.  Ind.,  32  (1913),  483. 

2.  MOHLER,  J.  R.,  Disinfection  regulations,  J.  Am.  Leather  Chem.  Assn., 
13  (1918),  47;  Chem.  Abstracts,  12  (1918),  1133. 

3.  HAUSSNEB,  ALF.,  Theory  of  the  beating  engine,  Papier  Fabrikant 
(1913),  46. 

4.  CHING,  W.  C.,  Soaking  in  reference  to  dissolved  hide  substance,  Suppl. 
Rep.  Tanners'  Inst.  (1913),  20;  Chem.  Abstracts,  8  (1914),  1521. 

GRASSEB,  G.,  Chemical  control  of  liming,  Technikum  (1912),  65;  Chem. 
Abstracts,  7  (1913),  3854. 

MOORE,  R.  L.,  Change  in  composition  of  limes  during  the  process  of 
unhairing,  Suppl.  Rep.  Tanners'  Inst.  (1913),  15;  Chem.  Abstracts,  8  (1914), 
1521. 

PICKLES,  J.  E.,  Depilation  of  hides  and  skins,  /.  Soc.  Chem.  Ind.,  36 
(1916),  456;  Chem.  Abstracts,  10  (1.916),  1801. 

ROEHM,  OTTO,  A  new  unhairing  process,  Collegium  (1913),  374. 

VENULETH  and  ELLENBEBGEB,  Verarbeitung  der  leimwasser  aus 
abdeckerein,  Swiss  Pat.,  58,  217  (1912);  Chem.  Ztg.,  37  (1913),  228. 

5.  BRACKET,  A.,  Relation  between  the  conductivity  (and  molecular  weight) 
of  acids  and  their  absorption  by  hides,  Compt.  rend.,  155  (1912),  1614;  J. 
Soc.  Chem.  Ind.,  32  (1913),  152. 


314  GELATIN  AND  GLUE 

FISCHEL,  M.,  Treatment  of  shreads  or  cuttings  of  hides  for  the  manu- 
facture of  gelatin,  glue,  and  the  like,  Eng.  Pat.  12,165  (1912);  J.  Soc.  Chem. 
Ind.,  32  (1913),  229. 

HILDEBRAND,  J.,  Comparative  valuation  of  deliming  material,  Gerber, 
38,  129;  143;  Chem.  Abstracts,  7  (1913),  1635. 

PROCTER,  H.  R.,  The  acid  deliming  process,  Tanners'  Year  Book 
(1913),  75;  Chem.  Abstracts,  7  (1913),  4094. 

6.  ANON,  The  cost  of  hypochlorite  disinfection,  Eng.  Record,  67,  No.  1-16; 
J.  Ind.  Eng.  Chem.,  5  (1913),  255. 

BOOTH,  W.  M.,  Water  problems,  J.  Ind.  Eng.  Chem.,  2  (1910),  503. 

BUTTON,  M.  S.,  Sterilization  by  liquid  chlorine  and  hypochlorite  of 
lime,  J.  Am.  Water  Works  Assn.,  4  (1917),  228. 

PHELPS,  E.  B.,  The  chemical  disinfection  of  water,  U.  S.  Public  Health 
Repts.,  Reprint  225  (1914). 

7.  CORMACK,  Glue  extraction  apparatus,  U.  S.  Pat.  728,205  (1903). 

8.  THIELE,  L.,  Apparatus  for  glue  making,  U.  S.  Pat.  989,826  (1911). 

9.  MANERHOFER,    L.,    Autoclave    tray  for   cooking   glue,    U.    S.   Pat. 
1,337,146  (1919). 

10.  LEHMANN,    J.,  Extraction  of  glue-containing  substances,  U.  S.  Pat. 
964,980  (1910). 

11.  UPTON,  G.,  Glue  and  gelatin  manufacture,  U.  S.  Pat.  1,063,229  (1913). 

12.  BANCROFT,    W.    D.,    The    coagulation  of  albumin  by  electrolytes, 
Trans.  Am.  Electrochem.  Soc.,  27  (1915),  195;  Chem.  Abstracts,  9  (1915),  1339. 

HANCK,  E.,  Clarifying  glue,  Ger.  Pat.  234,859  (1909). 

MICHAELIS,  L.  and  DAVIDSON,  H.,  The  influence  of  H+  concentration  on 
mixtures  of  colloids,  Biochem.  Z.,  54  (1913),  323;  Chem.  Abstracts,  8  (1914). 
3802. 

MUTSCHELLER,  A.,  Colloidal  adsorption,  J.  Am.  Chem.  Soc.,  42  (1920). 
2142. 

NEIL,  T.  M.,  Glue  manufacture,  Chem.  Abstracts,  6  (1912),  3208. 

PROCTER,  H.  R.,  The  action  of  dilute  acids  and  salt  solutions  on  gelatin. 
Kolloidchem.  Beihefte,  2  (1910-11),  243;  J.  Chem.  Soc.,  100  (1911),  I,  342. 

SCARPA,  O.,  Reversible  transformation  of  emulsoid  solutions  of  gum 
Arabic  and  gelatin  into  suspensoid  condition  and  the  properties  of  such 
systems,  Kolloid-Z.,  15  (1914),  8. 

SNELL,  J.  F.,  The  analysis  of  maple  products,  I.  An  electrical  conduc- 
tivity test  for  purity  of  maple  syrup,  /.  Ind.  Eng.  Chem.,  5  (1913),  740. 

TIEBACKX,  F.  W.,  The  coagulum  from  gelatin-gum  sols  and  its  analogy 
with  casein,  Z.  Chem.  Ind.  Kolloide,  8  (1911),  238. 

TTEBACKX,  F.  W.,  Simultaneous  coagulation  of  two  colloids,  Z.  Chem. 
Ind.  Kolloide,  8  (1911),  198. 

WILSON,  J.  A.  and  W.,  Colloidal  phenomena  and  the  adsorption  formula, 
J.  Am.  Chem.  Soc.,  40  (1918),  886. 

13.  ALEXANDER,  J.,  Colloid  chemistry  and  some  of  its  technical  aspects, 
J.  Soc.  Chem.  Ind.,  28  (1909),  280. 

BANCROFT,  W.  D.,  Neutralization  of  adsorbed  ions,  Trans.  Am.  Electro- 
chem. Soc.,  27  (1915),  175;  Chem.  Abstracts,  9  (1915),  1265. 

14.  BADISCHE,  A.  and  S.  F.,  Process  of  bleaching  glue,  D.  R.  P.  187,261 
(1907). 


MANUFACTURE  OF  GLUE  AND  GELATIN  315 

DEVOS,  A.  D.,  Purification  and  decolorization  of  solutions  of  glue  and 
sugar  saps,  French  Pat.  446,549  (1912). 

MEINFORD,  R.  W.,  Bleaching  glue,  etc.,  Brit.  Pat.  134,011  (1918). 
Chem.  Abstracts,  14  (1920),  806. 

PICKERING,  S.  N.,  The  color  intensity  of  iron  and  copper  compounds. 
J.  Chem.  Soc.,  105  (1914),  464;  Chem.  Abstracts,  8  (1914),  3737. 

15.  HUSBRAND,  E.,  Verdampfen,  kondensiren,  und  kuhlen;  erklarungent 
formeln  und  tabellen  fur  den  praktischen  gebrausch,  Ed.  5  (1912).     Eng. 
Translation  by  A.  C.  Wright.     Scott,  Greenwood  &  Co.,  London,  Ed.  2, 
(1903). 

KERR,  E.  W.,  Tests  upon  transmission  of  heat  in  vacuum  evaporators, 
J.  Am.  Soc.  Mech.  Eng.,  35  (1913),  1525;  Chem.  Met.  Eng.,  11  (1913),  611. 

MERRELL,  L.  C.,  et.  al.,  Separating  the  moisture  from  the  constituents 
of  solids,  or  liquids,  U.  S.  Pat.  860,929  (1907). 

MOORE,  H.  K.,  Some  general  aspects  of  evaporation  and  drying,  Chem. 
Met.  Eng.,  18  (1913),  128;  186. 

SADTLER,  P.  B.,  Vacuum  evaporation,  J.  Ind.  Eng.  Chem.,  1  (1909), 
644;  Chem.  Abstracts,  4  (1910),  648;  Evaporators  and  Vacuum  pans,  Chem. 
Met.  Eng.,  9  (1911),  206;  250;  305;  349. 

16.  ANDERSON,  J.  W.,  Vapor  compression  refrigerating  machines,  Engi- 
neering, 94,  754;  792;  821. 

ARROWOOD,  M.  W.,  Refrigeration;  a  practical  treatise  on  the  production 
of  low  temperatures  as  applied  to  the  manufacture  of  ice  and  to  the  design 
and  operation  of  cold  storage  plants,  Am.  Sch.  of  Corresp.  (1913). 

CAVALIER,  P.,  Application  of  cold  in  glue  and  gelatin  industries,  Chem. 
Ztg.,  35  (1911),  17;  J.  Soc.  Chem.  Ind.,  30  (1911),  143. 

GRINDLEY,  J.  H.,  A  contribution  to  the  theory  of  refrigerating  machines, 
Engineering,  94,  789;  Chem.  Abstracts,  7  (1913),  1083. 

LUCKE,  C.  E.,  Mechanical  refrigeration  in  manufacture,  /.  Ind.  Eng. 
Chem.,  7  (1915),  462. 

MACINTIRE,  H.  J.,  Chemical  and  mechanical  applications  of  mechanical 
refrigeration,  Chem.  Met.  Eng.,  7  (1914),  447. 

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Nat.  Elec.  Light  Assoc.,  To  calculate  the  amount  of  refrigeration  required, 
Refrigeration  World,  46  (1913),  47. 

SIEBEL,  J.  E.,  Compend  of  mechanical  refrigeration  and  engineering. 
Ed.  8  (1911). 

17.  KIND,  M.,  Glue  making  machines,  U.  S.  Pat.  1,046,307  (1912). 
LOWENSTEIN,  A.,  Some  machinery  employed  in  the  manufacture  of 

glue,  J.  Ind.  Eng.  Chem.,  9  (1917),  710. 

18.  BAXTER,  J.  B.  and  STARKWEATHER,  H.  W.,  The  efficiency  of  CaCl2, 
NaOH,    and    KOH   as    drying   agents,  ./.    Am.    Chem.    Soc.,    38    (1916), 
2038. 

BERT,  E.,  Drying  of  air  for  blast  furnaces,  Chem.  Met.  Eng.,  10  (1912), 
511. 

BORN,  S.  and  CARTHANS,  W.  F.,  Purification  and  sterilization  of  air, 
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(1917),  199;  Chem.  Abstracts,  11  (1917),  2655. 


316  GELATIN  AND  GLUE 

CAMPBELL,  C.  H.,  Process  of  drying  glue  and  gelatin,  U.  S.  Pat.  1,047,165 
(1912). 

CARRIER,  W.  H.,  The  theory  of  atmospheric  evaporation  with  special 
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CARRIER,  W.  H.  and  STACY,  A.  E.,  The  compartment  dryer,  J.  Ind. 
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DAUBINE,  F.  A.  and  ROY,  E.  V.,  Desiccation  of  air  by  calcium  chloride, 
Chem.  Met.  Eng.,  9  (1911),  343. 

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(1911). 

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moist  air  on  sulphuric  acid,  U.  S.  Pat.  1,069,241  (1913). 

FLEMING,  R.  S.,  The  spray  process  of  drying,  J.  Ind.  Eng.  Chem.,  13 
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FORRESTER,  H.  C.,  Apparatus  for  evaporating  milk,  blood,  and  other 
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52. 

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(1913),  71. 

GROSVENOR,  W.  M.,  Calculations  for  dryer  designs,  Trans.  Am.  Met. 
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(1914),  757. 

LEWIS,  W.  K.,  The  rate  of  drying  of  solid  materials,  /.  Ind.  Eng.  Chem., 
13  (1921),  427. 

LEWIS,  W.  K.,  On  the  efficiency  of  air  dryers,  J.  Ind.  Eng.  Chem.,  8 
(1916),  570. 

MARLOW,  T.  G.,  Drying  machinery  and  practice,  London  (1910). 
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materials,  Farben-Ztg.,  IB,  2806. 

MERRILL,  I.  S.  and  O.  E.,  Desiccating  milk  or  other  liquid  by  spraying 
into  a  whirling  current  of  air,  U.  S.  Pat.  1,102,601  (1914). 

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ing in  gelatin  and  glue  manufacture,  Farhen-Ztg.,  26  (1920),  2189. 

ROZENS,  P.,  Theorie  et  pratique  du  sechage  industriel,  Paris  (1909). 
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humidity  regulated  and  recirculating  dry  kiln,  U.  S.   Dept.   Agr.    Bull. 
509  (1917). 

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19.  TYNDALL,  Experiments  on  absorption  and  radiation  of  moist  air,  show- 
ing that  water  vapor  radiates  readily,  Phil.  Mag.,  26  (1863),  30. 

20.  MERRELL,  I.  S.,  Desiccating  apparatus,  U.  S.  Pats.  954,451  (1910); 
985,747  (1911);  1,082,468  (1913);  1,088,436  (1914). 

21.  BUNZEL,    H.,    Pre treating   bones   for  gelatin  manufacture,  Ger.  Pat. 
267,630  (1912);  Chem.  Abstracts,  8  (1914),  1034. 


MANUFACTURE  OF  GLUE  AND  GELATIN  317 

DOBNEB,  L.,  Obtaining  glue  and  fat  from  bones,  Ger.  Pat.  218,487  (1908). 
GIBSENWALD,  C.  V.,  Glue  and  gelatin  manufacture  with  steam,  Brit. 
Pat.  12,566  (1911);  Chem.  Ztg.,  36  (1912),  62. 

LAMBEBT,  T.,  Bone  products  and  manures,  London  (1901). 

Low,  A.  and  FISCHEB,  E.,  Making  glue,  U.  S.  Pat.  1,086,149  (1914). 

22.  GBILLO  and  SCHBOEDEB,  Ossein  by  the  use  of  sulphur  dioxide,  Brit. 
Pat.  2175  (1894);  /.  Soc.  Chem.  Ind.,  13  (1894),  408. 

23.  COMBON,  V.,   Manufacture  of  ossein  by  the  sulphuric  acid  process, 
Mat.  grasses,  4,  2117;  Chem.  Abstracts,  5  (1911),  2757. 

24.  DUNHAM,  H.  V.,  Enriched  superphosphates,  Brit.  Pat.  13,350  (1912); 
Chem.  Abstracts,  7  (1913),  3811. 

GANTLIEB,  A.,  The  manufacture  of  calcium  phosphate  and  the  extrac- 
tion of  ossein,  Chemist,  3,  46;  Chem.  Abstracts,  6  (1912),  2809. 

JENKINS,  E.  and  STBEET,  J.  P.,  Precipitated  bones,  37th  Rept.  Conn. 
Agr.  Exp.  Sta.  (1913),  117. 

KOCHETKOV,  V.  P.,  Preparation  of  the  phosphate  extracted  by  sulphur- 
ous acid  from  Viatka  phosphate,  Chem.  Abstracts,  9  (1915),  2746;  2743. 


CHAPTER  VII 

WATER-RESISTANT    GLUES    AND    GLUES    OF   MARINE 

ORIGIN 

The  art  of   making  glue  consists  in  know- 
ing  what   to  do   and   how  to   do  it. 

Fernbach     (1906). 

PAGE 

I.  Water-Resistant  Glues 318 

1.  Casein  Glue 319 

The  Manufacture  of  Casein 320 

Government  Specifications  for  Casein 325 

Influence  of  Method  of  Manufacture  upon  the  Use  of  Casein  in 

Glue 325 

Methods  of  Testing  and  Analysis  of  Casein .  327 

The  Preparation  of  Casein  Glue 333 

The  Application  of  Casein  Glue 337 

The  Strength  and  Water  Resistance  of  Casein  Glue 339 

Bibliography  of  United  States  Patents  on  Casein  Adhesives .  .  .  340 

2.  Blood  Albumin  Glue 344 

Preparation  of  the  Glue 345 

The  Application  of  Blood  Albumin  Glue.  .  . 345 

Dry  Glue  Process  for  Thin  Veneer 347 

II.  Glues  of  Marine  Origin 349 

1.  Isinglass 350 

The  Manufacture  of  Isinglass 350 

The  Composition,  Properties,  and  Uses  of  Isinglass 354 

2.  Liquid  Fish  Glue 356 

Source  of  Raw  Materials 357 

Methods  of  Manufacture 358 

Practical  Tests  to  Determine  the  Quality  of  Fish  Glue 359 

Composition 361 

Properties 364 

Uses 365 

A  Select  Bibliography  on  Fish  Glues 365 

III.  Comparison  of  Various  Glues ... 362 


I.  WATER-RESISTANT  GLUES 

Many  attempts  have  been  made  to  produce  an  insoluble  and 
water-resistant  glue  from  ordinary  animal  glue  by  the  addition 
to  it  of  some  " hardening"  reagent.  Formaldehyde  and  potas- 
sium bichromate  have  been  used  for  this  purpose,  but  although 

318 


WATER-RESISTANT  GLUES  319 

they  do  produce  a  product  that  is  nearly  insoluble,  and  that 
swells  to  a  much  smaller  extent  than  the  untreated  glue,  yet 
joints  made  by  them  fail  to  retain  their  strength  when  subjected 
to  a  drastic  treatment  with  either  cold  or  hot  water.  The  glue 
in  the  joint  is  still  capable  of  imbibing  water  from  a  humid 
atmosphere  or  from  rain,  for  example,  to  a  degree  that  makes  it 
unsafe  for  general  waterproof  service.  To  be  satisfactory  as  a 
water  resistant  material  the  glue  must  not  only  be  insoluble  but 
must  also  retain  a  fair  degree  of  its  original  strength  upon  sub- 
jection to  severe  water  treatment.  A  slight  swelling  of  the  glue 
in  the  joint  is  not  permissible  as  a  dangerous  weakening  inevitably 
results  therefrom. 

The  only  glues  that  have  proved  satisfactory  for  water 
resistant  purposes,  and  that  are  strong  enough  for  the  best  joint 
and  veneer  work,  are  the  casein  and  blood  albumin  glues 
described  below. 

1.  Casein  Glue. — The  employment  of  casein  as  an  adhesive 
had,  prior  to  1918,  been  confined  almost  entirely  to  a  few  trades 
and  districts  in  Europe  where  it  had  found  a  limited  application 
in  bookbinding  and  cabinet  work,  but  large  scale  production  was 
unknown.  Casein  had  however  been  used  in  quantity  in  the 
sizing  and  coating  of  paper  for  many  years  prior  to  the  war. 
The  material  was  mostly  imported  from  South  America  in  the 
dry  state  as  natural  casein,  and  brought  into  a  thin  creamy 
solution  before  use  by  dissolving  in  a  weak  solution  of  ammonia, 
soda-ash,  or  borax.  Reed  furniture  is  often  sized  with  casein 
glue  to  give  it  a  light  creamy,  rather  than  amber,  tone. 

In  July,  1918,  a  committee  of  technical  men  appointed  by  the 
United  States  Shipping  Board  seriously  considered  the  possi- 
bilities of  casein  as  a  basis  for  waterproof  glue  for  the  first  time. 
The  need  for  such  a  product  came  primarily  from  the  Bureau  of 
Aircraft  Production.  Ordinary  types  of  animal  glue,  or  of  other 
'types  that  were  in  common  service,  as  marine  and  vegetable 
glues,  were  sufficiently  strong  for  all  purposes  under  ordinary 
conditions  of  weather,  but  when  exposed  to  water,  or  even  to  a 
highly  humid  atmosphere  for  some  time  they  became  weak  due 
to  the  absorption  of  water,  and  the  consequent  swelling  which 
they  underwent.  This  objectionable  feature  of  animal  glues 
could  be  largely  overcome  by  the  application  of  a  waterproof 
varnish  or  lacquer  over  the  surface  exposed,  but  such  a  practice 
seemed  not  to  be  practicable  in  all  cases.  Propeller  blades  were 


320  GELATIN  AND  GLUE 

treated  in  that  manner  with  success,  but  the  plywood  entering 
into  the  construction  of  the  craft  could  not  be  treated  satis- 
factorily, and  a  weakening  and  separation  of  the  layers  of  wood 
occurred. 

The  need  for  a  waterproof  glue  therefore  became  very  urgent. 
Representatives  of  the  Army  and  Navy  estimated  an  annual 
requirement  of  3,000,000  pounds,1  and  there  was  at  the  time 
only  one  plant  in  the  country  manufacturing  a  casein  glue.  To 
augment  the  difficulty  further  it  was  known  that  different  makes 
of  casein  were  very  dissimilar  in  their  adaptability  to  glue  manu- 
facture. In  short,  a  practically  new  industry  was  to  be  initiated 
for  an  immediate  large  scale  production,  and  a  uniformity  of 
high  grade  product,  previously  unattained  even  in  small  scale 
production,  had  to  be  developed. 

The  Dairy  Division  of  the  Bureau  of  Animal  Industry  and  the 
Forest  Products  Laboratory  of  the  Department  of  Agriculture 
were  assigned  the  problem,  and  the  literature  upon  the  subject 
is  essentially  confined  to  these  two  stations. 

The  Manufacture  of  Casein. — The  only  source  of  casein  is 
milk.  It  occurs  as  the  principal  protein  of  the  milk  in  the  form 
of  the  calcium  salt,  and  in  this  form  is  suspended  as  a  colloid. 
Approximately  3  per  cent  of  cow's  milk  is  casein.  Whenever 
any  acid  is  introduced  into  the  milk  the  calcium  caseinate  is 
decomposed  in  an  entirely  analogous  manner  to  the  metal 
gelatinates  that  have  already  been  described2  forming  the  ions 
of  the  inorganic  calcium  salt,  and  uncombined  casein.  This 
uncombined  casein  is  insoluble,  and  separates  from  the  remaining 
liquid  as  a  curdy  precipitate. 

Lactic  Acid  or  Natural  Sour  Method.* — In  the  natural  souring 
of  milk  the  bacterium  lactis  acidi  attacks  the  lactose  or  milk 
sugar  producing  lactic  acid,  and  this  acid  functions  as  above 
bringing  about  the  coagulation  of  the  casein.  Whole  milk  is 
never  used  since  the  separation  of  the  fat  and  its  utilization  for 
the  manufacture  of  butter  is  a  necessary  economic  part  of  the 
milk  industry.  Buttermilk  is  sometimes  used  but  skim  milk 
is  best.  The  milk  is  allowed  to  stand  at  room  temperature  until 
the  lactic  acid  fermentation  has  proceeded  to  the  point  where  the 
curd  begins  to  separate  from  the  whey,  and  then  is  warmed  to 

1  Cf.  W.  M.  CLARK,  J.  Ind.  Eng.  Chem.,  12  (1920),  1162. 

2  Cf.  Chapter  V. 

3  S.  BUTERMAN,  J.  Ind.  Eng.  Chem.,  12  (1920),  141. 


WATER-RESISTANT  GLUES  321 

130°F.,  when  the  separation  becomes  complete.  After  draining 
off  the  whey,  the  curd  may  or  may  not  be  washed  with  cold 
water.  The  curd  is  squeezed  in  cloth  bags  to  remove  as  much 
of  the  liquid  as  possible  and  dried  by  spreading  on  trays  in  a 
drying  alley  through  which  air  at  a  temperature  of  about  130°F. 
is  circulating.  It  is  then  ground  and  is  ready  for  use. 

Fresh  skim  milk  is  sometimes  soured  rapidly  by  mixing  a 
stream  of  it,  as  it  falls  into  a  tank,  with  a  second  stream  of  the 
whey  that  has  been  run  off  from  a  batch  previously  treated. 

Dahlberg1  has  reported  an  " ejector"  method  developed  by  the 
Bureau  of  Animal  Industry.  The  milk  is  allowed  to  sour  until 
its  acidity,  expressed  as  lactic  acid,  and  using  phenolphthalein 
as  an  indicator,  is  0.8  to  0.9  per  cent.  It  is  then  allowed  to  run 
out  of  the  tank  through  an  ejector  where  it  is  heated  rapidly  by 
introducing  steam,  and  falls  into  a  second  tank.  The  curd 
collects  on  the  top  and,  after  draining  off  the  whey,  is  washed, 
pressed,  and  dried  as  above. 

Acid  Coagulation  Method. — Much  time  may  be  saved  by  adding 
an  acid  directly  to  the  milk  rather  than  to  allow  the  bacteria  to 
produce  the  required  acidity.  The  greater  part  of  the  casein 
manufactured  in  this  country  has  been  made  by  this  process, 
although  the  grain-curd  method  described  later  offers  such  great 
advantages  that  it  will  probably  soon  be  the  leading  procedure. 

The  fresh  skim  milk  is  heated  with  steam  to  a  temperature  of 
120°F.,  and  sulphuric  acid  added  to  the  extent  of  one  pint  of 
acid  (sp.  gr.  1.84)  mixed  with  a  gallon  of  water  to  each  1,000 
pounds  of  milk.  The  mixture  is  stirred  until  the  curd  separates, 
when  the  whey  is  drained  off,  and  the  curd  washed,  pressed,  and 
dried. 

Instead  of  finishing  in  the  above  manner,  the  curd  is  sometimes 
covered  with  water  and  brought  to  170  or  175°F.  This  causes 
the  curd  to  collect  into  a  semi-fluid,  plastic,  tough  mass.  The 
water  is  drained  off,  and  the  soft  curd  barreled  and  shipped  in 
that  form.  Hydrochloric  acid  is  substituted  for  the  sulphuric 
acid  in  some  creameries  where  the  whey  is  subsequently  treated 
for  the  recovery  of  lactose,  as  the  latter  acid  introduces  mechan- 
ical difficulties  in  that  separation. 

Rennet  Coagulation  Method. — The  casein  of  milk  may  also  be 
coagulated  by  treatment  with  rennet.     This  method  does 
appear  to  be  in  favor  among  manufacturers,  chiefly,  perhaps,  on 

1  A.  O.  DAHLBERG,  U.  S.  Dept.  Agr.  Bull  661. 
21 


322  GELATIN  AND  GLUE 

account  of  the  very  high  ash  content  of  the  resulting  casein,  and, 
as  will  be  shown  later,  the  ash  content  has  much  to  do  with  the 
properties  of  the  product  in  glue  manufacture. 

The  Grain-curd  Method. — When  it  became  imperative  to  manu- 
facture large  amounts  of  casein  of  a  high  quality  for  waterproof 
glues  the  several  methods  in  use  were  examined  and  none  of 
these  seemed  properly  adapted  to  the  purpose.  The  natural-sour 
method  was  eliminated  on  account  of  its  slow  action.  Sulphuric 
acid  precipitation  was  undesirable  because  of  the  precipitation  of 
calcium  sulphate  which  interfered  with  the  lactose  separation. 
High  temperature  methods  were  rejected  on  account  of  other 
complications  which  it  made  difficult  to  control.  Precipitation 
with  hydrochloric  acid  remained,  but  this  method  had  pre- 
viously yielded  very  poor  casein.  Van  Slyke  and  Baker1  had 
improved  this  method  but  the  product  was  very  finely  divided 
and  could  not  be  handled  properly  in  large  quantities.  The 
hydrochloric  acid  precipitation  method  was  accordingly  studied 
by  the  Dairy  Division  of  the  Bureau  of  Animal  Industry,  and  a 
new  procedure,  designated  as  the  grain-curd  method  has  been 
reported  by  Clark,  Zoller,  Dahlberg,  and  Weimar.2 

The  new  method  makes  use  of  the  fact  that  casein  is  an  ampho- 
teric  electrolyte,  having  its  isoelectric  point  at  pH  4.6.  A^  this 
hydrogen  ion  concentration  the  casein  is  not  only  more  insoluble 
than  at  any  other  point,  but  it  is  also  uncombined  with  any 
cation  or  anion  in  the  solution.  Thus,  in  milk,  casein  is  in  the 
form  of  calcium  caseinate,  and  at  any  pH  greater  than  about  4.6 
some  of  the  calcium  must  remain  in  combination  with  the  casein, 
and  no  amount  of  washing  could  liberate  it.  The  amount  of  the 
calcium  so  combined  is  proportional  to  the  extent  the  pH  diverges 
from  4.6,  and  at  the  latter  value  it  is  theoretically  zero,  so  that  by 
proper  washing  at  that  pH,  practically  all  of  the  calcium  may  be 
eliminated,  and  a  product  of  very  low  ash  be  obtained.  The 
significance  of  this  will  be  explained  later. 

The  authors  of  the  process  found  that  methyl  red  could  be 
used  in  plant  practice  with  remarkable  success  for  the  control  of 
the  acidity  of  the  milk.  Methyl  red  does  not  indicate  correctly 
in  the  presence  of  whey,  but  it  was  observed  that  an  apparent  pH 
of  4.6,  which  was  by  electrometric  methods  actually  only  4.1, 

1  VAN  SLYKE  and  BAKER,  /.  Biol.  Chem.,  35  (1918),  127. 

2  W.  M.  CLARK,  et.  al,  J.  Ind.  Eng.  Chem.,  12  (1920),  1163;  H.  F.  ZOLLER, 
ibid.,  13  (1921),  510;  Y.  OKUDA  and  H.  F.  ZOLLER,  idem,  515. 


WATER-RESISTANT  GLUES  323 

produced  a  casein  curd  of  the  particular  resiliency  and  con- 
sistency most  favorable  for  washing  and  handling. 

If  this  curd  were  then  washed,  after  draining  off  the  acid,  with 
water,  especially  if  any  alkaline  salts  were  present,  there  would  be 
a  tendency  to  redispersion  of  the  casein  and  loss  of  resiliency  and 
porosity  of  the  mass  upon  which  efficient  commercial  washing 
depends.  For  this  reason  the  wash  water  must  also  be  acidified 
to  the  same  pH  as  the  casein,  and  hydrochloric  acid  is  used  for  the 
purpose,  methyl  red  again  serving  as  the  indicator. 

Buffer  solutions  are  recommended  for  the  standard  in  compari- 
sons, the  phthalate-sodium  hydroxide  buffer  solutions  of  Clark 
and  Lubs1  being  most  convenient.  A  color  comparator2  may  be 
used  if  desired. 

Instructions  for  the  Preparation  of  Technical  Grain-curd  Casein.3 — The 
skim  milk  should  be  as  free  from  fat  as  possible  and  sweet  as  possible.  It 
should  be  delivered  to  the  casein  precipitating  vat  at  a  temperature  of 
90°F.  or,  preferably,  lower. 

The  hydrochloric  (muriatic)  acid  used  in  the  precipitation  of  the  casein 
should  be  diluted  with  water  in  the  ratio  of  1  Ib.  of  acid  (20°Be.)  to  8  Ibs. 
of  water. 

The  wash  water  used  in  washing  the  casein  should  be  made  up  as  follows : 
To  a  tank  of  water  add  hydrochloric  acid  until  an  acidity  of  4.8  is  shown  by 
use  of  the  methyl  red  indicator  solution.  Stir  the  acid  thoroughly  into  the 
water. 

The  indicator  solution  may  be  prepared  by  dissolving  0.4  gram  of  the 
pure  crystals  of  methyl  red  in  1,000  c.c.  of  95  per  cent  grain  alcohol.  This 
gives  about  0.04  per  cent  solution. 

The  indicator  standards  should  be  prepared  by  measuring,  with  a  10  c.c. 
pipet,  10  c.c.  of  the  standard  solution  mixtures  marked  "4.6-4.8,"  etc., 
into  test  tubes  of  uniform  diameter,  and  adding  to  each  10  c.c.  portion  5 
drops  of  the  methyl  red  indicator.  Close  each  test  tube  with  a  clean  cork 
stopper  and  seal  with  paraffin.  If  it  is  found  necessary  to  use  10  drops  of 
the  indicator  solution  in  the  unknown  solution  in  order  to  make  easier 
readings,  this  may  be  done.  In  any  case,  the  same  number  of  drops  should 
be  used  in  the  "unknown  solution"  as  are  used  in  the  indicator  standard. 

Procedure  in  the  Precipitation  of  Casein. — (1)  Heat  the  skim  milk,  if 
reasonably  fresh,  to  94°F.  If  the  milk  is  very  sour,  a  temperature  of  93°F. 
is  more  satisfactory.  Under  no  circumstance  should  the  temperature  be 
above  96°F. 

2.  Add  the  diluted  hydrochloric  acid  to  the  milk  in  fine  spray  and  stir 
rapidly  until  the  milk  "breaks."  Just  before  the  milk  breaks  the  acid  should 
be  added  more  slowly,  but  the  stirring  should  be  rapid.  It  is  good  economy 

1  Cf.  Appendix,  page  603. 

2  See  Appendix,  page  606. 

3  As  issued  to  manufacturers  by  W.  M.  CLARK,  et.  al.,  loc.  cit. 


324  GELATIN  AND  GLUE 

to  add  no  more  acid  at  this  stage  than  is  required  for  the  complete  separation 
of  the  casein  from  the  whey,  but  it  is  essential  that  the  casein,  after  having 
had  most  of  the  whey  drained  away,  should  be  acidified  as  in  Paragraph  4. 

3.  Allow  the  curd  to  settle  in  the  vat;  pull  it  away  from  the  gate  valve  and 
drain  off  through  the  drain  cloth  at  least  one  half  of  the  whey  as  indicated 
by  a  measuring  stick. 

4.  Stir  the  curd  thoroughly  but  not  rapidly  with  the  remaining  whey  in 
order  to  break  up  any  lumps,  so  that  the  washing  may  be  done  more  thor- 
oughly.    Now  add  more  acid  in  small  quantities,  mixing  it  with  the  curd. 
Test  10  c.c.  portions  of  the  whey  by  means  of  the  indicator  solution  until 
an  apparent  acidity  of  pH  4.8  to  4.6  is  obtained.     It  is  well  to  avoid  breaking 
the  curd  into  fine  particles  at  this  stage,  so  that  the  drain  cloth  will  not 
become  clogged.     The  curd  should  be  firm  but  not  slippery  when  the 
correct  quantity  of  acid  has  been  added.     The  temperature  has  a  marked 
influence  on  the  "feel"  of  the  curd  at  this  stage. 

5.  Now  drain  off  the  remainder  of  the  whey  through  the  drain  cloth, 
retaining  as  much  of  the  curd  in  the  vat  as  practicable. 

6.  Pour  a  little  wash  water1  over  the  casein  which  has  run  upon  the  drain 
cloth.     Also  wash  the  casein  in  the  vat  by  completely  covering  it  with  wash 
water  and  mixing  the  curd  carefully  with  it. 

7.  Now  drain  all  the  curd  upon  the  drain  cloth  and  wash  again  by 
pouring  a  liberal  quantity  of  wash  water  over  it.     Allow  the  casein  to 
drain  as  well  as  practicable  before  placing  it  in  the  press. 

8.  Dump  the  casein  and  fill  it  into  the  casein  press  in  thin  layers  so  that 
the  pressed  cake  is  about  2.5  inches  in  thickness.     At  first  very  little  pressure 
should  be  brought  to  bear  upon  the  casein  in  the  press.     The  pressure  may 
be  increased  slowly  as  the  liquid  drains  from  the  cake.     Uniform  pressure 
should  be  maintained  over  the  entire  cake. 

9.  The  pressed  casein  should  now  be  ground  through  a  curd  mill  and 
spread  in  layers  about  0.25  inch  thick  on  trays  in  the  drying  tunnel. 

10.  The  temperature  of  the  drying  tunnel  should  be  about  125°F.,  using 
a  strong  blast  of  air. 

The  above  directions  must  be  modified  slightly,  however,  if 
the  milk  used  has  previously  been  pasteurized.  Zoller2  has 
shown  that  the  curd  precipitated  by  the  above  method  from  milk 
that  has  been  heated  is  soggy  and  not  easily  handled.  He  finds, 
however,  that  by  raising  the  temperature  of  precipitation  from 
94  to  about  112°F.  good  results  may  be  obtained.  The  longer 
the  time  period  of  the  original  pasteurization,  and  the  higher  the 
temperature  of  the  pasteurization,  the  higher  also  must  be  the 
temperature  of  precipitation  in  order  to  obtain  a  firm  and 
workable  curd. 

1  It  must  be  remembered  that  this  wash  water  is  the  very  dilute  hydro- 
chloric acid  solution  described  above. 

2  H.  F.  ZOLLER,  J.  Ind.  Eng.  Chem.,  13  (1921),  510. 


WATER-RESISTANT  GLUES 


325 


Government  Specifications  for  Casein. — The  government  speci- 
fications1 require  that  all  casein  used  for  making  water-resist- 
ant glue  for  use  in  airplanes  should  pass  the  following  tests: 

Color :  White  or  light  cream. 

Odor:  Nearly  odorless,  with  not  more  than  a  trace  of  sourness. 

Moisture:  Not  more  than  10.0  per  cent. 

Fat:  Not  more  than  1.0  per  cent. 

Ash:  Not  more  than  4.0  per  cent. 

Nitrogen:  Not  less  than  14.25  per  cent. 

Acidity:  Not  more  than  10.5  c.c.  N/10  alkali  per  gram. 

Influence  of  Method  of  Manufacture  upon  the  Use  of  Casein 
in  Glue. — The  caseins  that  are  commercially  available  range  in 
quality  from  almost  pure  white,  sweet  material,  low  in  ash  and 
other  impurities,  to  a  dark  brown  substance,  which  may  be  very 
sour  and  give  off  an  offensive  odor.  A  peculiar  anomaly  among 
manufacturers  of  casein  glue  lies  in  the  preferences  that  some 
have  to  a  casein  made,  for  example,  by  the  natural-sour  process, 
while  others  can  obtain  satisfactory  results  only  by  the  use  of  a 
product  that  has  been  precipitated  by  mineral  acids. 

A  chemical  examination  of  the  caseins  produced  by  the  several 
methods  of  manufacture  show  (1)  fairly  uniform  results  for  a 
product  made  in  any  given  way,  but  (2)  widely  different  results 
between  products  made  by  different  methods.  The  following 
table  illustrates  this.  The  analysis  under  each  method  is 
entirely  representative  of  that  particular  procedure. 

TABLE  44. — ANALYSES  OF  CASEIN  MADE  BY  DIFFERENT  METHODS2 


Method 

Moisture, 
per  cent 

Fat  (mois- 
ture-free 
basis),  per 
cent 

Ash  (mois- 
ture-free 
basis),    per 
cent 

•  Nitrogen 
(moisture- 
fat-  ash-free 
basis),     per 
cent 

Acidity, 
c.c. 

Buttermilk 

6  97 

9  56 

1  36 

14.77 

9.2 

9  48 

0  33 

1  65 

14.84 

9.9 

7  87 

0  27 

2.16 

14.84 

8.7 

Sulphuric  acid 

7  81 

0  35 

4.05 

14.46 

7.6 

Sulphuric  acid   cooked 

8  89 

0.12 

4.25 

15.04 

5.9 

Hydrochloric  acid  cooked  
Hydrochloric  acid  

9.44 
7.10 

0.18 
0.16 

4.71 

5.74 

15.03 
14.32 

5.2 
6.7 

Rennet  

8.29 

0.63 

7.79 

14.41 

7.9 

Acidity  is  expressed  as  c.c.  of  N/10  sodium  hydroxide  solution  required  to  dissolve  1  gram 
of  moisture-  fat-  and  ash-free  casein  and  give  a  solution  neutral  to  phenolphthalein. 

1U.  S.  Department  of  the  Navy,  Bureau  of  Construction  and  Repair, 
Aeronautical  Specification,  85,  Jan.  15,  1919. 

2  S.  BUTTERMAN,  J.  Ind.  Eng.  Chem.,  12  (1920),  141. 


326 


GELATIN  AND  GLUE 


One  of  the  most  apparent  differences  observed  between  caseins 
manufactured  by  the  three  commercial  methods — lactic  acid, 
mineral  acid,  and  rennet — was  in  the  different  amounts  of  water 
required  to  produce  glues  prepared  from  them  of  the  same  visco- 
sity. Butterman1  concludes  that  "in  general,  caseins  of  the 
same  type  require  a  quantity  of  water  which  varies  within  a 
definite  range  and  is  somewhat  sharply  differentiated  from  the 
quantity  required  by  other  types.  Indeed,  in  most  cases  (except 
in  caseins  made  by  the  method  of  Sammis  or  by  the  grain-curd 
method)  it  is  possible  to  name  the  method  of  manufacture  by  a 
mere  observation  of  the  relative  amount  of  water  required  by 
the  casein  under  investigation."  By  an  exhaustive  comparison 
Butterman  established  the  reason  for  this.  He  found  that  the 
higher  the  ash  content  of  a  casein,  the  greater  the  dilution  at 
which  it  must  be  mixed  to  give  a  standard  viscosity,  and  also  the 
shorter  will  be  the  "life"  of  the  product.  By  "life"  is  meant  the 
period  of  time  between  the  preparation  of  the  glue  and  the  point 
where  it  becomes  too  thick  to  spread  properly.  These  points 
are  illustrated  in  the  following  table  of  average  values : 


TABLE  45. — EFFECT  OF  ASH  CONTENT  ON  PROPERTIES  OF  CASEIN  GLUE 


Type  of  casein 

Ash,  per  cent 

Water-casein 
ratio 

Life,  hours 

Grain-curd  _.  .  *^  

1.8 

2.3 

12 

Lactic  acidJ^(W%j5^1*^V 

2.5 

2.4 

10 

Mineral  acid  

4.0 

2.8 

7 

Rennet  

8.6 

3.9 

5 

By  plotting  the  ash  content  against  the  water-casein  ratio 
Butterman  obtained  a  curve  from  which  he  derived  an  equation 
of  the  general  type: 

y  =  mx  +  c. 

Hence  if  A  is  the  ash  content  of  the  casein,  and  W  the  water- 
casein  ratio  required  to  give  a  glue  of  medium  viscosity,  the  glue 
formula  4-A  gives  the  equation:2 

1  S.  BUTTERMAN,  loc.  cit. 

2  Vide  page  335.     The  equation  would  vary  slightly  with  any  variation  in 
the  formula  used. 


WATER-RESISTANT  GLUES  327 

W  =  0.24A  +  1.85. 

It  is  therefore  possible,  by  making  a  determination  of  the  ash 
content  of  any  casein,  and  applying  the  results  to  the  equation, 
to  tell  at  once  the  proper  proportion  of  the  ingredients  required 
to  mix  it  into  a  satisfactory  glue,  regardless  of  the  method  by 
which  the  casein  has  been  made,  and  by  that  means  to  be  certain 
of  uniform  results. 

The  control  of  the  ash  content  of  the  casein  lies  in  two  factors : 
the  hydrogen  ion  concentration  to  which  the  milk  is  brought 
during  the  precipitation,  as  already  described,  and  the  washing 
which  the  curd  receives.  If  the  proper  pH  is  maintained,  the 
ash  content  decreases  directly  as  the  amount  of  washing  it  re- 
ceives. It  is  easily  possible,  in  the  grain-curd  process,  to  bring 
the  ash  down  to  about  1.7  per  cent  by  only  two  washings,  and  in 
plant  practice  it  should  not  rise  above  2.5  per  cent. 

Clark1  has  shown  further  that  all  caseins  change  weight  very 
rapidly  by  absorption  or  loss  of  water  when  subjected  to  atmos- 
pheres only  slightly  different  from  the  normal.  In  cool  dry  air 
they  lose  weight  rapidly,  while  in  warm  moist  air  they  quickly 
increase  in  weight.  At  humidities  between  85  and  95  per  cent 
the  absorption  of  moisture  is  so  rapid  as  to  become  "dangerous." 
The  grain-curd  caseins  are  however  shown  to  be  superior  to  any 
of  the  other  types  in  that  they  respond  more  slowly  to  changes 
in  atmospheric  conditions,  and  tend  to  retain  a  smaller  quantity 
of  moisture.  Casein  that  has  been  subjected  to  a  high  tempera- 
ture treatment  is  most  sensitive  to  increases  in  humidity,  and 
also  tends  to  retain  the  moisture  more  firmly  than  the  other  types. 
The  ash  content  is  not  held  responsible  for  these  changes  in 
water-absorptive  capacity,  but  rather  the  effect  of  heat  upon  the 
casein  molecule. 

Methods  of  Testing  and  Analysis  of  Casein. — A  few  scattered 
tests  upon  casein  have  been  suggested  by  earlier  workers,  but 
these  are  for  the  most  part  of  doubtful  significance.  A  "  borax 
solubility  test"2  was  one  of  the  earliest  of  these,  and  an  " adhe- 
sive" test3  has  been  employed.  Reuter4  examined  casein  by 
testing  for  iron,  acidity,  metals,  sulphates,  and  chlorides,  in 

1  W.  M.  CLARK,  et.  al.,  loc.  cit. 

2  A.  DAHLBERG,  U.  S.  Dept.  Agr.  Bull.  661. 

3  A.  DAHLBERG,  idem. 

4  REUTER,  Papier-Ztg.  (2),  32  (1907),  3374. 


328  GELATIN  AND  GLUE 

addition  to  using  the  borax  solubility  test.  Hopfner  and  Bur- 
meister1  and  also  Burr2  have  suggested  the  determination  of 
moisture,  fat,  ash,  nitrogen,  and  free  acidity. 

These  determinations  were  adopted  with  several  modifications 
by  the  Forest  Products  Laboratory3  and  used  as  the  official 
methods  of  examination  by  the  Office  of  Aircraft  Production  in 
their  specifications  for  casein  to  be  used  in  making  glue.  A  care- 
ful study  of  these  methods  was  made  later  by  the  Dairy  Division 
of  the  Bureau  of  Animal  Industry,4  and  it  was  found  that,  in 
some  instances,  wide  discrepancies  arose  among  the  results  of 
different  analysts,  and  that  a  further  alteration  in  the  methods 
was  indicated. 

In  the  following  description  of  methods  the  official  procedure 
will  be  given  in  all  cases,  followed  by  the  modifications  suggested 
by  the  Dairy  Division.5 

Sampling  for  Analysis. — If  the  sample  is  gathered  from  bins  at  the 
creamery,  portions  should  be  taken  systematically  from  all  parts  of  the  bin. 
These  should  be  ground  together,  if  that  has  not  been  done  previously. 
After  the  powder  has  been  thoroughly  mixed,  the  final  sample  is  taken  out 
to  be  sent  to  the  laboratory.  A  100  gram  sample  will  be  found  to  be  suffi- 
cient for  the  determinations  described.  The  sample  received  at  the  labor- 
atory should  be  thoroughly  mixed,  50  grams  set  aside  for  the  determination 
of  fineness,  and  the  remainder  reduced  to  60  mesh  size. 

Color. — If  possible  the  color  of  the  casein  should  be  observed  at  the 
creamery  as  it  is  taken  from  the  driers.  Grinding  makes  the  casein  appear 
much  lighter  in  color.  Commercial  casein  may  be  obtained  which  is  almost 
pure  white,  and  the  color  need  never  be  more  than  a  pale  yellow  or  cream. 

Odor. — About  10  grams  of  the  casein  are  soaked  in  about  10  c.c.  of  water 
and  an  equal  volume  of  a  rather  thick  "milk  of  lime"  added  with  stirring. 
After  the  mixture  has  stood  a  few  moments  the  odor  is  noted.  Commercial 
casein  may  be  obtained  which  is  entirely  free  from  odor,  or,  at  most,  has  an 
odor  resembling  that  of  sweet  milk.  The  rancid  odor  frequently  associated 
with  casein  is  not  due  to  the  casein,  but  to  impurities  or  decomposition 
products  of  casein. 

Fineness. — A  50  gram  sample  is  placed  in  a  60  mesh  sieve,  the  sieve  is  held 
in  one  hand  and  moved  horizontally  back  and  forth  at  the  rate  of  about  120 
strokes  per  minute,  being  allowed  to  strike  at  the  end  of  each  stroke  against 

1  HOPFNER  and  BURMEISTER,  Chem.  Ztg.,  36  (1912),  1053. 

2  BURR,  Milch  Zentr.,  6  (1910),  385. 

3F.  L.  BROWNE,  J.  Ind.  Eng.  Chem.,  11  (1919),  1019. 

*R.  H.  SHAW,  ibid.,  12  (1920),  1168. 

5  The  official  methods  are  quoted  from  F.  L.  BROWN,  loc.  tit.,  and  the 
modifications  of  the  Dairy  Division  taken  from  the  papers  by  R.  H.  SHAW, 
loc.  cit.,  W.  M.  CLARK,  /.  Ind.  Eng.  Chem.,  12  (1920),  1170,  and  H.  F. 
ZOLLER,  idem.,  1171. 


WATER-RESISTANT  GLUES  329 

the  palm  of  the  other  hand  which  is  held  stationary.  The  portion  passing 
through  in  10  minutes  is  weighed,  and  reported  as  per  cent  passing  60  mesh. 
Moisture. — This  is  most  accurately  determined  by  weighing  out  a  3  gram 
sample  in  a  glass-stoppered  weighing  bottle,  heating  to  constant  weight  in  a 
vacuum  oven  at  70  to  80°C.,  cooling  in  a  desiccator,  and  weighing.  For 
most  purposes  it  is  more  convenient  and  sufficiently  accurate  to  use  a  porce- 
lain evaporating  dish  and  make  the  determination  .by  heating  in  a  Freas 
oven  at  98°C.,  and  atmospheric  pressure  for  5  hours. 

Shaw  has  pointed  out  that  five  different  analysts  reported 
moisture  determinations  of  identical  samples  of  casein  which 
differed  from  7.55  to  9.01  per  cent,  or  a  difference  of  1.46  per 
cent  between  the  highest  and  lowest  figures.  In  following  up 
the  reason  for  such  a  discrepancy  they  submitted  79  samples  to 
the  Bureau  of  Chemistry,  and  determinations  for  moisture  were 
made  by  both  the  open  dish — at  atmospheric  pressure  and  98°C., 
— method,  and  by  the  partial  vacuum  method.  The  average 
percentage  of  moisture  by  the  former  method  was  7.44  while  by 
the  latter  method  it  was  8.21,  showing  a  difference  of  0.77  per 
cent  in  favor  of  the  latter  method. 

Fat. — The  residue  from  the  moisture  determination  is  transferred  to  an 
extraction  thimble  and  extracted  for  16  hours  with  anhydrous  redistilled 
ethyl  ether  in  a  Cauldwell  or  Soxhlet  apparatus.  The  ether  is  evaporated 
from  the  extract,  and  the  residue,  corrected  for  the  moisture  content  of  the 
casein,  is  called  fat.  It  is  important  that  the  sample  for  the  fat  extraction 
be  finely  ground. 

Shaw  reports  that  a  modification  of  the  Roese-Gotlieb  method 
was  applied  to  casein  with  very  good  results.  He  gives  the 
following  procedure: 

Weigh  out  a  1  gram  charge  of  the  casein  into  the  Roehrig  tube,  and  add 
10  c.c.  of  water.  Shake  vigorously,  but  not  so  as  to  carry  particles  of  casein 
near  the  top  of  the  tube.  Let  soak  for  at  least  15  minutes;  a  longer  time  is 
advisable  if  the  sample  is  not  finely  ground.  Add  2  c.c.  of  strong  ammonia 
water,  and  shake  vigorously,  again  taking  care  not  to  carry  particles  of 
casein  near  the  top  of  the  tube.  Let  stand  10  minutes  with  occasional 
shaking.  Add  10  c.c.  of  95  per  cent  alcohol  and  shake  until  the  casein  is 
completely  dissolved.  From  this  point  procede  as  usual  with  the  Roese- 
Gotlieb  method. 

The  Roese-Gotlieb  procedure  is  as  follows:1 

Add  25  c.c.  of  washed  ether  and  shake  vigorously  for  30  seconds,  then 
25  c.c.  of  petroleum  ether  .(redistilled  slowly  at  a  temperature  below  60°C.) 

1  Assoc.  Official  Agr.  Chemists,  "Methods  of  Analysis"  (1920),  227. 


330  GELATIN  AND  GLUE 

and  shake  again  for  30  seconds.  Let  stand  20  minutes,  or  until  the  upper 
liquid  is  practically  clear.  Draw  off  as  much  as  possible  of  the  ether-fat 
solution  (usually  0.5  to  0.8  c.c.  will  be  left)  into  a  weighed  flask  through  a 
small  quick-acting  filter.  The  flask  should  always  be  weighed  with  a  similar 
one  as  a  counterpoise.  Re-extract  the  liquid  remaining  in  the  tube,  this 
time  with  only  15  c.c.  of  ether,  shake  vigorously  30  seconds  with  each  and 
allow  to  settle.  Draw  off  the  clear  solution  through  the  small  filter  into  the 
same  flask  as  before  and  wash  the  tip  of  spigot,  the  funnel  and  the  filter  with 
a  few  c.c.  of  a  mixture  of  the  two  ethers  in  equal  parts  free  from  suspended 
water.  For  absolutely  exact  results  the  re-extraction  must  be  repeated. 
The  third  extraction  yields  usually  not  more  than  about  1  mg.  of  fat  if  the 
previous  ether-fat  solutions  have  been  drawn  off  closely.  Evaporate  the 
ethers  slowly  on  a  steam  bath,  then  dry  the  fat  in  a  boiling  water  oven  to 
constant  weight. 

Confirm  the  purity  of  the  fat  by  dissolving  in  a  little  petroleum  ether. 
Should  a  residue  remain,  remove  the  fat  completely  with  petroleum  ether, 
dry  the  residue,  weigh  and  deduct  the  weight.  Finally  correct  this  weight 
by  a  blank  determination  on  the  reagents  used. 

The  Roese-Gotlieb  method  yields  results  that  are  considerably 
higher  than  those  obtained  by  the  ordinary  extraction  procedure. 
Shaw  believes  that  this  is  due  to  an  incomplete  extraction  in  the 
latter  case,  and  that  the  results  obtained  by  the  former  method 
are  the  more  nearly  correct  and  reliable.  This  may  be  due 
to  the  fact  that  the  grains  of  casein  are  hard  and  not  easily  pene- 
trated by  the  solvent,  while  by  the  Roese-Gotlieb  method  the 
casein  is  in  solution  and  it  is  impossible  for  any  fat  to  remain  out 
of  contact  with  the  solvent. 

Ash. — A  3  gram  sample  is  weighed  out  in  a  vitreosil  dish  and  carefully 
charred  over  a  low  flame  of  a  Bunsen  burner.  When  completely  carbonized, 
it  is  placed  in  an  electric  muffle  furnace  and  heated  at  a  dull  red  heat  (not 
over  600°C.)  until  the  ash  is  white,  or  at  least  light  gray,  and  the  weight  is 
constant.  A  small  amount  of  ammonium  nitrate  may  be  added  to  facilitate 
the  combustion  of  the  last  traces  of  carbon.  Care  should  be  taken  to  avoid 
fusion  of  the  ash  if  possible.  Results  are  reported  on  a  moisture-free 
basis. 

The  presence  of  phosphorus,  sulphur,  and  alkali  chlorides  in  the 
ash  make  the  determination  uncertain  unless  especial  precautions 
are  observed.  If  a  temperature  in  excess  of  a  dull  red  heat  is  used 
there  is  danger  of  volatilization  of  alkali  chlorides,  and  if  the 
casein  is  low  in  lime,  so  that  there  is  not  enough  present  to  com- 
bine with  all  of  the  organic  phosphorus  and  sulphur,  these  latter 
will  also  be  volatilized.  This  difficulty  may  be  overcome  by 
mixing  with  the  casein,  before  charring,  a  calcium  salt.1  Five 

1  U.  S.  Bureau  of  Chemistry,  Bull.  107  (1908),  21. 


WATER-RESISTANT  GLUES  331 

cubic  centimeters  of  a  solution  of  calcium  acetate  yielding  about 
0.1  gram  of  CaO  upon  ignition  may  be  added  to  the  3  gram 
sample  of  casein,  allowed  to  stand  until  the  solution  is  absorbed, 
then  dried  in  a  drying  oven,  carefully  charred  over  a  small 
flame,  and  finally  ignited  in  an  electric  muffle  furnace  at  a  low 
redness.  The  weight  of  CaO  added  is  subtracted  and  the  results 
reported  as  before.  This  procedure  is  quite  necessary  in  the  case 
of  low  ash  caseins,  but  if  the  ash  is  medium  or  high  it  is  not 
required. 

Nitrogen. — A  one  half  gram  sample  is  weighed  out  into  an  800  c.c.  Kjeldahl 
flask,  20  c.c.  of  concentrated  sulphuric  acid,  10  grams  of  crystallized  sodium 
sulphate,  and  a  small  crystal  of  copper  sulphate  are  added,  and  the  con- 
tents digested  until  a  clear  solution  is  obtained,  and  then  for  30  minutes 
longer.  300  c.c.  of  distilled  water,  50  c.c.  of  a  1 :1  solution  of  sodium  hydrox- 
ide, and  about  one  fourth  gram  of  granulated  zinc  are  added.  About  250  c.c. 
are  then  distilled  over  and  caught  in  standard  sulphuric  or  hydrochloric 
acid.  (30  c.c.  of  N/5  acid  will  be  sufficient.)  The  excess  acid  is  back- 
titrated  with  standard  sodium  hydroxide,  methyl  red  being  used  as  indicator. 
Since  the  nitrogen  determination  is  made  as  a  measure  of  the  impurities 
other  than  moisture,  fat  or  ash,  results  are  reported  on  a  moisture-,  fat-,  and 
ash-free  basis.1 

Acidity. — A  one  gram  sample  is  placed  in  a  flask  and  25  c.c.  of  N/10 
sodium  hydroxide  solution  run  in  from  a  pipet.  During  this  addition  the 
flask  is  gently  agitated.  The  flask  is  then  stoppered  and  the  agitation 
continued  until  the  solution  is  complete.  This  should  require  only  5  or 
10  minutes.  The  stopper  is  then  removed  and  the  portion  of  solution  wet- 
ting it  washed  into  the  flask  with  a  stream  of  water  from  a  wash-bottle. 
100  c.c.  of  distilled  water  (neutral  to  phenolphthalein)  are  added,  and  the 
solution  back-titrated  at  once  with  N/10  acid,  using  0.5  c.c.  of  alcoholic 
phenolphthalein  solution  (1  gram  per  100  c.c.)  as  indicator.  The  acid 
is  run  in  fairly  rapidly  with  vigorous  shaking  of  the  flask  so  as  to  prevent 
precipitation  of  the  casein  locally.  The  number  of  cubic  centimeters  of 
N/10  alkali  used  up  by  1  gram  of  moisture-,  fat-,  and  ash-free  casein  is 
called  the  "acidity"  of  the  sample. 

Browne  adds  that  if  concordant  results  are  to  be  obtained  by 
this  method  the  following  precautions  must  be  observed:  "(1) 
The  flask  should  be  kept  stoppered  except  when  making  addi- 
tions or  titrating;  (2)  the  amount  of  indicator  specified  must  be 
used  and  it  must  be  adjusted  with  alkali  so  that  one  drop  added 
to  distilled  water  does  not  change  its  reactions:  (3)  local  coagula- 
tion of  casein  during  titration  must  be  avoided;  (4)  the  total 

1  The  nitrogen  content  of  pure  casein  is  15.67  (Richmond,  "Dairy  Chem- 
istry"), so  the  conversion  factor  of  nitrogen  to  casein  becomes. 100/15. 67  = 
6.38. 


332  GELATIN  AND  GLUE 

time  during  which  the  casein  is  allowed  to  stand  in  contact  with 
alkali  must  not  exceed  30  minutes  at  room  temperature." 
Clark  points  out  that  this  procedure  is  quite  inadequate.  It 
consists  only  in  "  titrating  to  an  arbitrary  pH,  as  indicated  by 
phenolphthalein,  a  mixture  of  amphoteric  protein,  occluded 
salts,  and  the  products  of  alkali  hydrolysis  of  the  protein."  It 
was  therefore  only  to  be  expected  that  the  method  should  give 
no  consistent  results,  ''partly  because  of  the  difficulty  of  titrating 
to  an  arbitrary  and  insignificant  pH,  and  partly  because  of  the 
hydrolysis  of  the  casein."  By  making  use  of  the  hydrogen 
electrode,  and  extrapolating  back  to  zero  time  of  contact  between 
alkali  and  casein  more  satisfactory  results  were  obtained. 

The  Borax  Solubility  Test. — This  test  has  been  reported  as  follows:1 
To  50  grams  of  casein  (ground  to  pass  a  20  mesh  sieve)  are  added  300 
c.c.  of  water  containing  7.5  grams  of  borax.  This  mixture  is  stirred  thor- 
oughly and  is  immediately  set  in  a  water  bath  controlled  at  a  temperature  of 
6.5°C.  With  continuous  stirring  the  casein  should  be  completely  dissolved 
in  10  minutes. 

A  special  study  of  this  test  made  by  Zoller  brings  out  several 
points  of  importance.  The  chief  value  of  the  test  seems  to  be  to 
determine  whether  the  casein  under  examination  exhibits 
suitable  working  properties,  when  dissolved  by  certain  alkalies. 
Chick  and  Martin2  showed  that  the  viscosity  of  casein  in  sodium 
hydroxide  increased  rapidly  after  the  concentration  of  the  casein 
had  reached  10  per  cent.  Zoller  found  the  same  to  be  true  of 
casein  dissolved  in  borax  solution.  For  differentiating  between 
the  properties  of  several  caseins  a  concentration  of  about  15  per 
cent  was  found  most  suitable. 

The  viscosity3  of  a  solution  of  casein  dissolved  in  borax  was 
found  to  rise  enormously  (from  10  to  110  angular  degrees  as 
measured  by  the  MacMichael  viscosimeter)  upon  increasing  the 
pH  of  the  solution  from  the  isoelectric  point  (pH  4.6)  to  pH  8.15, 
but  upon  further  increases  the  viscosity  again  dropped  rapidly 
until  at  pH  9.0  it  had  become  practically  constant.  (About  21 
angular  degrees.)  In  making  the  test  it  is  obvious  that  a  pH 
should  be  attained  such  that  the  viscosity  would  fall  upon  the 
constant  part  of  the  curve.  When  dissolved  in  other  alkalies  the 

1  U.  S.  Dept.  Agr.  Bull.  661  (1918). 

2  CHICK  and  MARTIN,  Kolloid-Z.,  11  (1902),  102. 

3  Cf.  also  H.  F.  ZOLLER,  /.  Gen.  Physiol,  3  (1921),  635. 


WATER-RESISTANT  GLUES  333 

maximum  viscosity  is  reached  at  a  pH  of  9. 1  to  9.25.  It  is  highest 
in  ammonium  hydroxide. 

High  temperatures  were  found  to  alter  seriously  the  physical 
properties  of  the  casein,  and  a  temperature  of  30°  C.  was 
fixed  as  the  most  practicable. 

The  revised  method  as  given  by  Zoller  follows : 

The  casein  is  ground  to  pass  a  40  mesh  sieve;  15  grams  of  the  casein  are 
measured  into  a  250  c.c.  beaker;  100  c.c.  of  0.2  M  borax  at  30°C.  (76.32  grams 
of  Na2B4O7. 10  H2O  diluted  to  1  liter)  are  added  with  vigorous  stirring.  This 
is  allowed  to  stand  for  30  minutes,  with  thorough  stirring  at  intervals  of  5 
minutes.  During  the  first  5  minutes  the  mixture  should  be  stirred  rather 
frequently.  A  casein  of  known  purity  and  conduct,  should  be  used  as 
control  until  thorough  familiarity  with  the  method  is  gained.  Usually 
the  character  of  the  casein  shows  up  during  the  first  10  minutes,  but  30 
minutes  is  advised  for  safety.  Longer  periods  are  unsatisfactory  because 
of  difficulty  in  interpretation. 

The  principal  advantage  of  the  test  seems  to  lie  in  the  rigid 
differentiation  which  it  permits  between  high  and  low  tempera- 
ture caseins,  the  former,  without  fail,  tending  to  imbibe  water 
and  form  a  jelly  in  the  test,  while  the  latter  are  still  smooth  clear 
solutions  at  the  end  of  a  half  hour.  If  much  fat  is  present  the 
mix  will  tend  to  be  somewhat  gelatinous  during  the  first  10 
minutes,  but  after  a  half  hour,  with  frequent  stirring,  will, 
unless  it  has  also  been  heated,  become  redispersed  into  a  milky 
and  uncohesive  liquid. 

The  Preparation  of  Casein  Glue. — Casein  is  insoluble  in  pure 
water,  but  in  the  presence  of  any  alkali  wil]  pass  readily  into  a 
colloidal  solution.  Lime  is  most  commonly  used  for  this  purpose 
in  the  preparation  of  casein  glue.  Casein,  water  and  lime  will 
produce  a  glue  that  has  good  water-resistant  properties,  but  it 
sets  rather  rapidly  into  a  thick  paste  which  cannot  be  easily 
worked.  It  is  therefore  said  to  have  a  short  life.  Several  other 
ingredients  have  been  proposed  to  be  added  to  the  mixture  to 
increase  the  life  and  otherwise  improve  the  properties  of  the 
glue.  Formulas  have  been  patented  involving  the  addition  of 
sodium  silicate,  sodium  hydroxide,  and  sodium  fluoride.  Oils 
are  sometimes  added  to  prevent  dusting. 

Casein  glues  may  be  designated  as  the  dry-mix  and  the  wet- 
mix  types.  The  dry-mix  glues  are  prepared  by  the  glue  manu- 
facturer from  formulas  which  are  not  made  public,  and  shipped  in 
dry  form.  These  are  prepared  for  use  according  to  specific 


334  GELATIN  AND  GLUE 

directions  which  come  with  each  formula  glue.  The  National 
Advisory  Committee  for  Aeronautics1  defines  the  principal  points 
to  be  observed  in  the  mixing  of  prepared  casein  glues  as  follows: 

1.  A  thorough  mixing  of  the  dry  glue  from  each  or  all  containers  before 
adding  to  the  water.     This  is  advisable  on  account  of  the  segregation  of 
ingredients  of  different  specific  gravities  which  may  occur  during  shipment 
from  the  factory  to  the  consuming  plant.     Sifting  is  not  advisable,  as  it 
may  remove  from  the  glue  some  essential  component. 

2.  Proportions  of  glue  and  water  should  always  be  weighed,  not  measured. 

3.  The  glue  should  be  added  slowly  to  the  water  accompanied  by  vigorous 
agitation  in  order  to  avoid  a  lumpy  mixture. 

4.  After  the  glue  is  well  mixed  into  the  water,  the  stirring  should  con- 
tinue more  slowly  until  all  particles  are  thoroughly  dissolved  and  the  glue 
appears  of  a  smooth  creamy  consistency. 

5.  The  desired  consistency  of  the  glue  should  be  obtained  during  the 
mixing  and  no  attempt  should  be  made  to  thin  the  glue  should  it  become  too 
thick  in  use.     It  should  be  mixed  only  as  fast  as  it  is  being  used. 

The  proportions  of  dry  glue  and  water  should,  in  general,  be  as  directed 
by  the  manufacturer.  However,  the  exact  proportions  will  vary  with  (1) 
different  glues,  (2)  different  shipments  of  the  same  glue,  and  (3)  the  kind  of 
work  for  which  the  glue  is  to  be  used.  Only  average  proportions  can  be 
stipulated  by  the  manufacturer;  and  the  operator,  in  order  to  obtain  satis- 
factory consistencies,  may  find  it  necessary  at  times  to  vary  from  the  average 
proportions  specified. 

The  wet-mix  glues  are  made  up  by  the  consumer  from  the 
raw  casein,  lime,  and  other  ingredients. 

Prior  to  the  war  there  had  been  a  number  of  casein  cements 
suggested  which  were  being  employed  to  a  limited  extent  as 
water-resistant  glue.  Typical  of  these  may  be  mentioned  the 
following : 

One  part  of  gum  arabic  is  dissolved  in  5  parts  of  40  per  cent  water-glass 
and  evaporated  on  the  water-bath  until  sufficiently  dry  to  grind.  It  is 
then  ground  to  50  mesh,  and  mixed  with  150  mesh  calcium  hydroxide  and 
40  mesh  casein  in  the  proportions : 

Gum-silicate  mixture 20  parts 

Casein 40  parts 

Calcium  hydroxide 25  parts 

This  mixture  is  then  made  up  with  water  in  the  proportions  of  45  parts 
dry  mixture  to  100  parts  of  water. 

Another  typical  formula  is  as  follows: 

Casein 47. 0  parts 

Calcium  hydroxide 29 . 5  parts 

Sodium  silicate 15 . 5  parts 

Gum  arabic 8.0  parts 

1  National  Advisory  Committee  for  Aeronautics,  Report  66  (1920),  13. 


WATER-RESISTANT  GLUES  335 

A  formula  developed  at  the  Forest  Products  Laboratory1  using 
sodium  silicate  has  proved  especially  satisfactory.  It  is  specified 
as  follows: 

Formula — Glue   No.   4- A. 

100  parts  casein  }       ,    . 

10^  1    oo«  soak  15  minutes 

130  to  280  parts  water  ] 

15  to  22  parts  hydra  ted  powdered  lime  1     . 

90  parts  water 

70  parts  silicate  of  soda. 

This  formula  is  prepared  as  follows : 

The  proper  quantity  of  water  is  introduced  into  the  glue  pot,  and  the 
mixing  -blade  is  brought  into  action  at  a  speed  corresponding  to  about  50 
or  60  revolutions  per  minute.  The  stirring  is  allowed  to  continue  during  the 
addition  of  the  casein  to  the  water  and  for  a  few  minutes  thereafter  until  the 
mixture  becomes  mush-like  in  consistency  through  the  absorption  of 
the  free  water  by  the  casein.  The  blade  is  then  stopped  and  the  mixture 
allowed  to  soak. 

After  a  period  of  15  minutes  the  soaking  is  considered  complete  and  the 
mixing  blade  is  again  brought  into  action.  The  lime  water  mixture  is  now 
added  and  two  or  three  minutes  later  the  silicate  of  soda  is  introduced. 

The  mixing  is  allowed  to  continue  for  from  20  minutes  to  %  hour  after  the 
addition  of  the  silicate  of  soda,  whereupon  a  smooth,  freely  flowing  mixture, 
of  uniform  texture  and  free  from  lumps,  should  be  produced. 

No  precise  quantity  of  water  can  be  prescribed  because  of  the  variation 
of  the  water-absorbing  qualities  of  different  caseins.  The  criterion  of  whether 
or  not  the  proper  quantity  of  soaking  water  has  been  added  is  the  viscosity 
of  the  finished  (mixed)  glue.  If  its  consistency  is  too  thin,  an  excess  of 
water  beyond  that  required  has  been  used,  and  it  is  best  to  reject  the  batch 
and  try  again.  Similarly,  if  the  consistency  is  too  thick  and  heavy,  an 
insufficient  quantity  of  water  has  been  used.  The  water  required  for  various 
types  of  casein  lies  in  the  following  ranges : 

Lactic  acid  casein 130  to  170  parts  water 

Sulphuric  acid  casein         j 170  to  220  parte  water 

Hydrochloric  acid  casein  J 

Rennet  casein 280  parts  water 

The  silicates  of  soda  that  may  be  used  in  the  formula  are  the  ordinary 
liquid  water-glasses,  and  should  lie  within  the  following  analytical  limits : 

Specific  gravity 1 . 38  to    1 . 42 

Density  (Baume  scale) 40.31  to  42. 96 

Sodium  oxide,  per  cent 9 . 38  to    9 . 88 

Silica,  per  cent 31.41  to  32.38 

1  U.  S.  Patent  No.  1,291,369,  granted  to  S.  Butterman,  and  assigned  to  the 
United  States  Government. 


336  GELATIN  AND  GLUE 

The  Air  Board  specifications  employed  a  formula  differing 
from  the  above  by  substituting  sodium  hydroxide  for  a  part  of  the 
hydrated  lime,  omitting  sodium  silicate,  and  introducing  sodium 
fluoride  and  paraffin  oil.  It  is  made  up  as  follows: 

Casein 100 . 0  parts 

Freshly  slaked  lime 18.0  parts 

Commercial  sodium  hydroxide  (not  less  than  95 

per  cent  pure) 11.0  parts 

Sodium  fluoride 3.0  parts 

Paraffin  oil 1.5  parts 

The  above  are  mixed  and  are  prepared  for  use  by  mixing  with  200  to  250 
parts  of  water.1 

This  mixture  has  the  great  advantage  over  those  previously 
mentioned  in  that  it  acquires  a  very  smooth  and  limpid  consist- 
ency which  remains  in  a  workable  condition  for  much  longer 
periods  of  time  than  any  to  which  sodium  fluoride  is  not  added. 

The  utilization  of  sodium  fluoride  in  casein  glue  was  probably 
first  introduced  for  the  prevention  of  the  growth  of  molds  or 
bacteria,  and  its  exceptional  value  as  a  liquid  stabilizer  was 
discovered  only  by  accident.  That  it  not  only  possesses  such  a 
value,  but  is  also  quite  unique  in  this  respect  seems  indisputable. 
Intensive  research  has  seemed  to  indicate  that  the  influence  of  the 
fluoride  ion  is  specific.  It  is  probable  that  the  sodium  fluoride 
and  calcium  hydroxide  react  with  each  other  to  some  extent 
forming  sodium  hydroxide  and  an  insoluble  calcium  fluoride. 
There  seems,  however,  to  be  some  kind  of  a  solvent  action  on  the 
casein  that  is  peculiar  to  the  sodium  fluoride.  Of  course  almost 
any  sodium  salt  that  will  form  sodium  hydroxide  by  double 
decomposition  with  calcium  hydroxide  will  dissolve  casein,  but 
the  interaction  with  sodium  fluoride  seems  to  give  a  glue  of 
better  consistency,  higher  luster,  greater  water  resistance,  and 
especially  longer  life  than  any  other  salt.  We  understand  that 
there  is  at  least  one  American  manufacturer  who  is  producing  a 
casein  glue  in  the  liquid  form  which  will  remain  in  such  a 
condition  indefinitely. 

The  effectiveness  of  sodium  fluoride  as  a  preservative  is  not  as 

Approximately  the  same  composition  with  the  omission  of  the  sodium 
fluoride  is  given  in  U.  S.  Patent  No.  1,310,706  July  22,  1919,  to  Alfred  C. 
Lindaner,  assignor  to  U.  S.  A. 


WATER  RESISTANT  GLUES  337 

marked  as  might  be  expected.  The  employment  of  Beta- 
naphthol  is  finding  especial  favor  as  a  most  efficient  preservative. 
About  2  per  cent  is  used,  based  on  the  dry  casein  content  of  the 
glue.  Since  Beta-naphthol  is  insoluble  in  cold  water,  it  is 
necessary  when  using  it  to  heat  the  mixture  after  the  addition  of 
the  water. 

The  Type  of  Mixer. — Mixers  used  for  animal  and  vegetable 
glues  are  not  well  adapted  to  the  peculiar  needs  of  casein  glues. 
According  to  the  Forest  Products  Laboratory  the  essential 
requisites  for  a  casein  glue  mixer  are:  "(1)  Rapid  agitation 
and,  preferably,  different  speeds  of  the  paddle;  (2)  a  glue  pot 
that  can  be  readily  cleaned — preferably  one  that  can  be  detached 
from  the  machine  itself;  and  (3)  a  glue  pot  of  metal  that  will  not 
corrode  under  the  action  of  alkali.  The  mixing  pot  should  not 
be  of  brass,  copper,  or  aluminum,  as  the  alkali  usually;  present  in 
casein  glues  will  attack  these  metals.  No  provision  need  be  made 
for  heating,  as  casein  glues  must  not  be  heated." 

A  mixer  that  has  proved  satisfactory  at  the  Forest  Products 
Laboratory  is  a  power  cake-dough  mixer  of  the  type  used  by 
bakers.  It  is  provided  with  a  double-acting  paddle,  and  may  be 
operated  at  three  different  speeds.  A  sketch  of  this  mixer  is 
shown  in  the  accompanying  illustration,  Fig.  58. 

The  Application  of  Casein  Glue. — On  account  of  the  peculiar 
limited  working  life  of  casein  glues  they  must  be  handled  as  soon 
as  possible  after  being  made  up.  They  should,  however,  if 
properly  made,  remain  workable  for  at  least  four  hours,  and  some 
will  retain  their  proper  consistency  for  12  or  more  hours.  As 
long  as  the  proper  "flow"  of  the  liquid  is  maintained,  so  that  it 
can  be  evenly  and  uniformly  spread,  it  may  be  used  without 
danger,  but  when  once  too  thick  it  should  be  discarded. 

Casein  glues  work  well  on  the  ordinary  corrugated  roll  type  of 
machine  spreader.  The  glue  should  be  applied  rather  freely, 
and  the  excess  squeezed  off  under  rollers,  or  when  the  pressure 
is  applied.  The  time  allowed  between  the  spreading  of  the  glue 
and  the  application  of  the  pressure  should  not  be  more  than  a  few 
minutes,  and  in  no  case  should  the  glue  be  permitted  to  set 
before  finishing  the  joint.  The  pressure  usually  applied  varies 
from  75  to  100  pounds  per  square  inch,  but  both  higher  and  lower 
pressures  are  also  used.  The  pressure  should  remain  upon  the 
joint  for  at  least  an  hour,  and  preferably  for  a  much  longer  period, 
as  over  night.  After  removing  from  the  presses  the  joints  should 
22 


338  GELATIN  AND  GLUE 

be  permitted  to  " condition"  for  a  few  days  before  being  finished 
if  the  best  results  are  desired. 

The  time  relations  in  the  working  of  casein  glues  have  been 
studied  by  the  Forest  Products  Laboratory.1     They  report  that 


FIG.    58. — Mixer    for    casein    glues.     (Kindness    of  F.    L.    Browne,     Madison 

Wisconsin.) 

casein  glue  joints  in  spruce  proved  as  strong  as  the  wood  after 
4  hours,  and  in  hard  maple  after  6  hours.  "When  maximum 
speed  of  production  is  essential,  such  woods  may  be  machined  at 
the  ends  of  the  periods  stated,  without  sacrificing  the  strength  of 
the  joint.  In  some  kinds  of  work,  however,  machining  so  soon 
after  gluing  is  not  advisable,  because  of  the  danger  of  warping  or 
the  production  of  sunken  joints  as  the  moisture  content  of  the 
glued  wood  equalizes. 

"Another  important  fact  brought  out  by  the  tests  on  joint 
strength  is  that  joints  released  from  pressure  at  the  end  of  two 
hours  and  then  allowed  to  season  for  22  hours  proved  as  strong  as 
those  that  had  been  pressed  for  24  hours.  Joints  pressed  for 

1  Forest  Products  Laboratory,  Technical  Notes,  No.  142  (1921). 


WATER-RESISTANT  GLUES  339 

only  a  half  hour  and  seasoned,  although  of  good  strength,  on  the 
average,  were  somewhat  erratic  in  this  respect  and  probably 
would  not  be  dependable  where  maximum  strength  is  important." 

The  Strength  and  Water  Resistance  of  Casein  Glue. — The  strength 
of  casein  glues  when  properly  made  is  high.  While  inferior  to 
the  highest  grades  of  animal  glue,  they  are  nevertheless  as  strong 
as  the  wood  of  most  of  our  common  species.  Tests  made  at  the 
Forest  Products  Laboratory  showed  shearing  strength  ranging 
from  2,000  to  2,500  pounds  per  square  inch. 

The  exceptional  value  of  these  glues  is  due  to  the  great  water 
resistance  which  they  show.  The  government  specifications 
required  that  there  should  be  no  separation  of  plies  after  boiling 
in  water  for  8  hours,  or  soaking  in  cold  water  for  10  days.  The 
shearing  strength  in  plywood  was  required  to  be  at  least  150 
pounds  to  the  square  inch.  The  casein  glue  tests  averaged  much 
better  than  this.  After  soaking  for  several  days  casein  glues 
commonly  gave  from  20  to  40  per  cent  of  their  dry  plywood  shear 
strength.  Upon  redrying,  the  original  strength  is  largely 
recovered. 

But  although  casein  glues  are  highly  water-resistant,  they 
ultimately  decompose  when  exposed  to  a  damp  atmosphere  for  a 
long  time.1  The  Forest  Products  Laboratory  reaches  the  con- 
clusion that  the  decomposition  of  ordinary  alkaline  casein  glues 
is  not  due  to  the  action  of  bacteria  or  molds,  but  rather  to  a 
chemical  action  of  the  alkali  in  the  glue.  This  conclusion  is 
based  upon  the  following  observations: 

Increasing  the  amount  of  alkali  in'  the  glue  increases  the  rate  of  decom- 
position when  the  glue  is  kept  wet. 

Glues  containing  no  sodium  hydroxide,  although  deficient  in  some  impor- 
tant respects,  do  not  decompose  as  rapidly  as  similar  glues  containing  sodium 
hydroxide. 

Cultures  of  molds  and  bacteria  could  not  be  obtained  from  decomposed 
alkaline  glues. 

Some  chemicals  which  have  antiseptic  properties  are  found  to  improve 
casein  glue,  but  this  improvement  is  due  to  their  chemical  action  rather  than 
to  their  toxic  properties. 

Glues  can  be  completely  decomposed  in  a  short  time  at  temperatures 
above  that  at  which  bacteria  can  grow. 

It  has  been  found  that  copper  salts  added  to  casein  glues 
greatly  increase  their  resistance  to  moisture  and  also  make 
them  more  durable  when  exposed  to  the  action  of  molds  and 

1  Forest  Products  Laboratory,  Technical  Notes,  No.  138  (1921). 


340  GELATIN  AND  GLUE 

fungi.  Casein  glues  containing  copper  are  nearly  as  moisture 
resistant  as  blood  albumin  glues. 

In  the  preparation  of  copper-casein  glue  at  the  Forest  Prod- 
ucts Laboratory,1  2  to  3  parts  by  weight  of  copper  chloride  or 
copper  sulphate  are  dissolved  in  about  30  parts  of  water  and  are 
added  to  every  500  parts  of  the  ordinary  casein,  lime,  and  water- 
glass  glue.  The  copper  solution  is  poured  into  the  glue  in  a 
thin  stream.  The  violet-colored  lumps  formed  at  first  by  the 
coagulation  of  the  glue  by  the  copper  solution  are  reduced  by 
stirring  vigorously  for  about  15  minutes,  and  a  smooth  violet- 
colored  glue  results.  It  is  necessary  to  add  the  copper  salts 
after  the  other  ingredients  are  thoroughly  mixed,  in  order  to 
obtain  beneficial  results.  Copper  added  to  the  casein  before 
the  lime  and  water-glass  is  ineffective. 

Glues  containing  little  lime  are  especially  improved  by  the 
addition  of  copper.  A  low-lime  glue  with  copper  may  be  as 
resistant  to  moisture  as  a  glue  with  more  lime  in  it,  and  copper 
does  not  shorten  the  "life"  or  period  of  workability  of  the  glue 
so  much  as  would  more  lime. 

BIBLIOGRAPHY  OF  UNITED  STATES  PATENTS  ON 
CASEIN  ADHESIVES 

8,035  April  15,  1851.  Chas.  A.  Broquette,  St.  Martin,  France.  Im- 
provement in  material  for  transferring  color  in  calico  printing. 
86,398  February  2,  1869.  Joseph  Hirsh,  Chicago,  111.  Improved 
adhesive  compound  and  plaster. 

132,659  October  29,  1872.  William  Grune,  Berlin,  Germany.  Improve- 
ment in  transfer  paper. 

169,053  October  19,  1875.  Johann  G.  W.  Steffens,  Brooklyn,  N.  Y. 
Improvement  in  composition  for  ornaments. 

183,024  October  10,  1876.  John  H.  Ross  and  Charles  D.  Ross,  Albion, 
N.  Y.  Improvement  in  processes  of  preparing  glue. 

223,459  January  13,  1880.  George  Vining,  Stockbridge,  Mass.  As- 
signor of  one  fourth  to  E.  F.  Peck,  Great  Barrington,  Mass. 
Sizing  for  paper,  etc. 

241,897  May  24,  1881.  Emil  R.  Von  Portheim,  Prague,  Austria. 
Manufacture  of  glue. 

243.178  June  21,    1881.     Emil   R.   Von   Portheim,   Smichow,   Austria. 

Process  of  manufacturing  an  inspissating  and  sizing  paste 
from  animal  proteins. 

307.179  October  28,  1884.     Emery  E.  Childs,  Brooklyn,  N.  Y.     Prepa- 

ration of  casein  and  of  articles  made  therefrom. 

307,269  October  28,  1884.  Emery  E.  Childs,  Brooklyn,  N.  Y.  Prepa- 
ration of  casein  and  of  articles  made  therefrom. 

1  Forest  Products  Laboratory,  Technical  Note  170  (1922). 


WATER-RESISTANT  GLUES 


341 


321,109     June  30,    1885.     Karl  A.   Hohenstein,   Brooklyn,   N.   Y.     As- 
signor to  David  McMeekan  and  A.  S.  Wright,  both  of  same 

place.     Calcimine. 
377,283     January   31,    1888.     Isaac   B.   Abrahams,    New   York,    N.    Y. 

Manufacture  of  wall-covering. 
500,428     June  27,  1893.     Edward  Rauppach  and  Leopold  Bergel,  Gauchtl, 

Austria.     Process  of  making  glue. 
522,831     July  10,  1894.     Peter  Cooper  Hewitt,  New  York,  N.  Y.     Process 

of  purifying  glue. 
537,096     April   9,    1895.     Carl  Wittkowsky,  Berlin,  Germany.     Process 

of  uniting  veneers. 
586,854     July  20,  1897.     Wilhelm  Majert,  Griinau,  Germany.     Process 

of  making  ammoniacal  casein. 
607,281     July    12,    1898.     Percy    Gerald    Sanford,    London,    England. 

Mode  of  preserving  albuminous  matter. 
609,200     August  16,   1898.     William  A.  Hall,  Bellows  Falls,  Vermont. 

Water-proofing  compounds. 
615,446     December  6,   1898.     Byron  B.  Goldsmith,  New  York,  N.  Y. 

Finishing  fibrous  or  absorbent  surfaces. 
619,040     February   7,    1899.     Thomas   A.    Haynes,    New   York,    N.   Y. 

Process  of  making  casein  cement. 

623,541     April  25,  1899.     William  A.  Hall,  Bellows  Falls,  Vit.     Sizing 
632,195     August    29,    1899.     William    W.    McLaurin,    Millken    Park,. 

Scotland.     Assignor  to  the  Smith,  McLaurin  Co.,  of  the  same 

place.     Substitute  for  leather. 
633,834     September  26,    1899.     Gustav   J.    Gruendler,    St.   Louis,    Mo. 

Adhesive. 
650,003     May  22,  1900.     Hermann  Bremer,  Munich,  Germany.     Process 

of  dissolving  albumin. 
651,851     June  19,  1900.     William  A.  Hall,  Bellows  Falls,  Vt.     Calcimine 

compound. 
664,318     December    18,    1900.     William    A.    Hall,    Bellows    Falls,    Vt. 

Assignor  to  the  Casein  Co.,  of  America,  N.  Y.  City.     Process 

of  making  soluble  casein. 
670,372     March  19,  1901.     John  A.  Just,  Syracuse,  N.  Y.     Assignor  of 

three  fifths   to   William   D.    Carpenter  of   the  same  place. 

Process  of  producing  casein  products. 
681,436     August    27,    1901.     Charles    H.    Bellamy,    Philadelphia,    Pa. 

Assignor  to  M.  R.  Isaacs  of  the  same  place.     Manufacture  of 

casein  and  casein  glue. 
682,549     September  10,  1901.     John  A.  Just,  Syracuse,  N.  Y.     Casein 

powder. 
684,545     October  15,  1901.     William  A.  Hall,  Bellows  Falls,  Vt.     Casein 

glue. 
684,985     October  22,   1901.     Thomas  Alfred  Haynes,   Hoboken,   N.  J. 

Artificial  glue  or  size. 
692,450     February  4,   1902.     John  A.  Just,  Syracuse,  N.  Y.     Assignor 

of  one-half  to  D.  H.  Burrell  &  Co.,  of  Little  Falls,  N.  Y 

Adhesive  casein  and  process  of  making  same. 


342 


GELATIN  AND  GLUE 


695,926  March  25,  1902.  William  A.  Hall,  Bellows  Falls,  Vt. 
Adhesive. 

709,651  September  23,  1902.  John  T.  Slough,  Woodstock,  Canada. 
Adhesive  cement. 

717,085  December  30,  1902.  Henry  V.  Dunham,  New  York,  N.  Y. 
Assignor  to  Casein  Company  of  America,  N.  Y.  Casein 
compound. 

725,094  April  14,  1903.  Winfield  M.  Kimberlin,  Akron,  O.  Assignor 
to  Goodyear  Tire  and  Rubber  Co.,  of  the  same  place.  Process 
of  cementing  substances. 

729,220  May  26,  1903.  Frederick  Renken,  New  York,  N.  Y.  Method 
of  gluing  articles  together. 

739,657  September  22,  1903.  Andrew  A.  Dunham,  New  York,  N.  Y. 
Assignor  to  Casein  Company  of  America,  N.  Y.  Sizing 
and  process  of  producing  same. 

750,048  January  19,  1904.  Hezekiah  K.  Brooks,  Bellows  Falls,  Vt. 
Assignor  to  Casein  Company  of  America,  N.  Y.  Casein 
compound  and  process  of  producing  same. 

768,274  August  23,  1904.  Pedro  Fargas-Oliva,  Barcelona,  Spain. 
Process  of  manufacturing  glue  or  size. 

788,857  May  2,  1905.  Gaston  A.  Thube  and  Louis  Preaubert,  Nantes, 
France.  Adhesive  and  method  of  making  same. 

790,821  May  23,  1905.  Henry  V.  Dunham,  Bellows  Falls,  Vt.  As- 
signor to  Casein  Company  of  America,  N.  Y.  Paint  and 
process  of  preparing  same. 

799,599  September  12,  1905.  Frances  X.  Covers,  Owego,  N.  Y.  As- 
signor to  Americus  Manufacturing  Co.,  N.  Y.  Casein  glue. 

809,731  January  9,  1906.  Ottokar  Henry  Nowak,  Chicago,  111.  Water- 
proofing compound. 

814,594  March  6,  1906.  Henry  V.  Dunham,  Bellows  Falls,  Vt.  As- 
signor to  the  Casein  Company  of  America,  N.  Y.  Process 
of  precipitating  and  preserving  casein. 

838,785  December  18,  1906.  Mone  R.  Isaacs,  Philadelphia,  Pa.  Glue 
or  sizing. 

845,681  February  26,  1907.  Alexander  Bernstein,  Berlin,  Germany. 
Process  of  making  glue  substitutes. 

845.790  March  5,  1907.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Method 

of  treating  proteids. 

845.791  March  5,  1907.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

compound  glues,  etc. 
848,746     April  2,  1907.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

compound. 
852,915     May  7,  1907.     Julius  Talman,  Philadelphia,  Pa.     Assignor  of 

one-half  to  Charlton  H.  Royal,  Philadelphia,  Pa.     Process  of 

treating  casein  and  compound  obtained  therefrom. 
868,445     October  15,  1907.     John  A.  Just,  Syracuse,  N.  Y.     Process  of 

preparing  casein  soluble  to  a  neutral  solution. 
879,967     February    25,     1908.     Mone     R.     Isaacs,     Philadelphia,     Pa. 

Method    of    treating  albumoid  and  the  compository  matter 

obtained  therefrom. 


WATER-RESISTANT  GLUES 


343 


897,885 

920,959 
959,348 
974,448 

1,015,365 
1,018,559 

1,063,974 
1,141,951 

1,167,434 
1,192,783 
1,209,221 

1,209,222 

1,235,784 
1,244,463 

1,291,396 

1,310,706 
1,347,845 

607,281 

838,785 


September  8,  1908.     Henry  V.  Dunham,  Bainbridge,  New  York. 

Assignor  to  Casein  Company  of  America,  N.  Y.     Manufacture 

of  casein. 
May  11,  1909.     Charles  Jovignot,  Paris,  France.     Compository 

for  hermetically  closing  vessels. 
May  24,  1910.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

compound  or  sizing. 
November  1,  1910.     Frederick  Supp,  N.  Y.     Assignor  to  the 

Arabol   Manufacturing  Co.,  of  New  York,   N.   Y.     Process 

of  liquefying  organic  colloids. 
January  23,  1912.     William  A.  Weelands  and  Eugene  Williams, 

Chicago,  111.     Assignors  to  Adanio  &  Elting  Company  of  the 

same  place.     Size. 
February  27,  1912.     Richard  Heim,  Canastota,  N.  Y.     Assignor 

one-half  to  Frederick  Behrend,  New  York,  N.  Y.     Process 

for  producing  adhesives. 
June  10,   1913.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Coating 

composition. 

June  8,   1915.     Andrew  A.  Dunham,  Bainbridge,  N.  Y.     As- 
signor to  Casein  Company  of  America,  N.  Y.     Paper  coating 

composition  and  method  of  making  same. 
January    11,     1916.     Ludwig    H.    Reuter,    Torreon,    Mexico. 

Process  of  producing  casein  and  product  therefrom. 
July  25,  1916.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

composition. 
December  19,  1916.     Noel  Statham,  Boonton,  N.  J.     Assignor 

to  the  Industrial  Chemical  Co.,  New  York,  N.  Y.     Paper  and 

process  of  making  same. 
December  19,  1916.     Noel  Statham,  Boonton,  N.  J.     Assignor 

to  Industrial  Chemical  Company,  N.  Y.     Glazed  paper  and 

coating  composition  therefor. 
August    7,    1917.     Daniel    F.    Ferney,    Grand    Rapids,    Mich. 

Glue  or  adhesive. 

October  30,  1917.     George  H.  Brabrook,  Boston,  Mass.     As- 
signor of  one-half  to  Albert  T.  Fletcher,  of  the  same  place. 

Adhesive. 

January    14,    1919.     Samuel   Butterman,    Madison,   Wis.     As- 
signor   to    United    States    of    America.     Process    of    manu- 
facturing waterproof  adhesives. 
July  22,  1919.     Alfred  C.  Lindauer,  Madison,  Wis.     Assignor 

to  the  United  States  of  America.     Waterproof  glue. 
July  27,  1920.     Henry  V.  Dunham,  Mt.  Vernon,  N.  Y.     Stable 

casein  and  process  for  making  same. 

FLUORIDES  IN  ADHESIVES 

July  12,  1898.     Percy  Gerald  Sanford,  London,  England.     Mode 

of  preserving  albuminous  matter. 
December  18,  1906.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Glue 

or  sizing. 


344  GELATIN  AND  GLUE 

845.790  March  5,  1907.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Method 

of  treating  proteids. 

845.791  March  5,  1907.     Mone  R.  Issacs,  Philadelphia,  Pa.     Adhesive 

compound,  glue,  etc. 
848,746     April  2,  1907.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

compound. 
879,967     February     25,     1908.     Mone     R.     Isaacs,     Philadelphia,     Pa. 

Method  of  treating  albuminoids  and  composition  of  matter 

produced  therefrom. 
897,885     September  8,   1908.     Henry  V.   Dunham,   Bainbridge,   N.   Y. 

Assignor  to  Casein  Company  of  America,  N.  Y.  City.     Manu- 
facture of  casein. 
920,959     May  11,  1909.     Charles  Jovignot,  Paris,  France.     Composition 

for  use  in  hermetically  closing  vessels. 
959,348     May  24,  1910.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

compound  or  sizing. 
1,192,783     July  25,  1916.     Mone  R.  Isaacs,  Philadelphia,  Pa.     Adhesive 

composition. 
1,244,463     December    30,    1917.     George    H.    Brabrook,    Boston,    Mass. 

Assignor    one-half    to    Albert   T.    Fletcher,    Boston,    Mass. 

Adhesive. 
1,347,845     July  27,  1920.     Henry  V.  Dunham,  Mt.  Vernon,  N.  Y.     Stable 

casein  solution  and  process  for  making  same. 

2.  Blood  Albumin  Glue. — Blood  albumin  glue,  like  casein 
glue,  was  not  generally  used  prior  to  1918,  but  the  sudden  and 
imperative  demand  for  water-resistant  plywood  for  military 
purposes  resulted  in  an  intensive  study  of  all  possible  sources  of 
waterproof  glues,  and  casein  and  blood  albumin  were  found  to  be 
most  satisfactory  for  this  purpose. 

Blood  albumin  glue  is  not  placed  upon  the  market  in  a  prepared 
form  chiefly  on  account  of  a  decrease  in  the  solubility  of  the 
albumin  with  age.  But  it  may  be  prepared  without  any  diffi- 
culty by  the  consumer  from  either  fresh  blood  or  the  dried  albu- 
min. If  fresh  blood  is  to  be  used  the  supply  must  be  readily 
accessible  to  the  consuming  plant,  for  decomposition  takes  place 
very  rapidly  which  unfits  the  material  for  glue  purposes.  It  is 
the  more  commonly  prepared  from  the  dried  soluble  albumin 
which  is  made  by  coagulating  red  blood  corpuscles  and  the 
fibrin  and  subjecting  the  clear  serum  to  evaporation  under 
reduced  pressure.  The  temperature  must  not  be  allowed  to  rise 
above  160°F.,  as  at  that  temperature  the  albumin  coagulates. 
The  resulting  solid  residue  is  ground  and  sold  as  dried  soluble 
blood  albumin.  As  the  solubility  diminishes  with  age  the  albu- 
min should  be  used  as  a  glue  only  when  reasonably  fresh. 


WATER-RESISTANT  GLUES  345 

Preparation  of  the  Glue. — A  glue  may  be  made  that  is  satis-  ^ 
factory  for  some  purposes  by  simply  dissolving  the  albumin  in 
water,  but  the  desirable  qualities,  are  improved  by  the  addition 
of  ammonium  hydroxide  and  lime,  and  still  other  ingredients 
may  be  added.  If  too  much  lime  is  used,  however,  the  glue  will 
set  very  rapidly  to  a  stiff  jelly  that  is  not  workable. 

In  putting  the  material  into  solution  it  is  advisable  to  allow  it  to 
soak  for  about  two  hours  before  stirring.  Water  at  about  70  to 
80°F.  should  be  used.  After  it  is  thoroughly  soaked  it  should  be 
agitated  until  it  is  of  a  uniform  consistency.  There  will  usu- 
ally be  some  insoluble  material  that  will  not  go  into  solution,  but 
unless  it  is  excessive  in  amount  or  is  particularly  coarse  it  may  be 
neglected.  It  is  sometimes  advisable  however,  to  strain  the 
mixture  through  a  screen  of  about  30  mesh  wire  before  using. 
Blood  albumin  glues  must  not  be  heated  in  their  preparation. 

A  formula  developed  by  the  Forest  Products  Laboratory1  for  the  prepara- 
tion of  blood  albumin  glue  has  been  found  very  satisfactory.  This  is 
specified  as  follows:2 

6  parts  of  black  soluble  blood  albumin  (90  per  cent  solubility). 
11  parts  of  water  at  about  80  degrees  F. 
Y±  parts  of  ammonium  hydroxide  (sp.  gr.  0.90). 

%  part  of  hydrated  lime  (from  2  to  3  per  cent  of  the  weight  of  albumin). 
After  the  blood  has  been  put  into  solution,  the  ammonia  is  added  while 
stirring  the  mixture  slowly.  The  lime  is  then  added  in  the  form  of  a  thick 
cream,  and  agitation  should  be  continued  slowly  for  a  few  minutes.  Care 
should  be  exercised  in  the  use  of  the  lime,  inasmuch  as  a  small  excess  will 
cause  the  mixture  to  thicken  and  become  a  jelly-like  mass.  The  glue 
should  be  of  moderate  consistency  when  mixed  and  should  be  suitable  for 
use  for  several  hours.  The -exact  proportions  of  albumin  and  water  may 
be  varied  to  produce  a  glue  of  greater  or  less  consistency  or  to  suit  an  albumin 
of  different  solubility  than  that  specified. 

The  Application  of  Blood  Albumin  Glue. — Glues  made  from 
blood  albumin  tend  to  foam  in  the  spreading  machines,  but  if  the 
spreader  is  allowed  to  remain  stationary  except  while  panels  are 
actually  being  coated  with  glue,  they  may  be  satisfactorily 
operated.  Since  these  glues  are  used  almost  entirely  upon  ply- 
wood it  is  important  that  the  glue  should  be  spread  by  machinery. 

The  finishing  of  a  joint  glued  with  blood  albumin  differs  from 
that  of  all  other  types  in  that  a  high  temperature  in  the  press  is 
imperative  for  the  setting  of  the  glue.  The  high  temperature 

1  Patent  applied  for  in  the  name  of  S.  B.  HENNING,  Forest  Products 
Laboratory. 

2  Cf.  National  Advisory  Committee  for  Aeronautics,  Report  66  (1920),  16. 


346  GELATIN  AND  GLUE 

brings  about  the  coagulation  of  the  albumin,  and  it  is  this 
insoluble  coagulum  which  renders  the  product  resistant  to  the 
action  of  either  hot  or  cold  water.  The  heat  is  most  conveniently 
applied  to  the  wood  by  pressing  the  glued  plywood  in  an  hydrau- 
lic press  supplied  with  hollow  platens  heated  by  steam.1 
The  coagulation  may  also  be  accomplished  by  placing  the 
clamped  joints  in  a  dry  kiln  or  hot  chamber  at  the  proper  tempera- 


FIG.  59. — Hot  press  for  making  experimental  panels  with  blood  glue.      (Kindness 
of  F.  L.  Browne,  Madison,  Wisconsin.) 

ture.  The  temperature  of  the  press  or  the  drying  chamber 
must  not  be  less  than  160°F.,  but  in  order  to  hasten  the  process 
it  is  customary  to  use  temperatures  of  from  200  to  220°F.  If 
temperatures  much  higher  than  these  are  used,  however,  the 
moisture  in  the  wood  is  converted  to  steam  and  in  forcing  its  way 
out  of  the  wood  produces  blisters  or  steam  pockets  between  the 
plies. 

1  F.  L.  BROWNE,  Chem.  Met.  Eng.,  21  (1919),  136. 


WATER-RESISTANT  GLUES  347 

The  pressure  employed  is  commonly  from  50  to  100  pounds  per 
square  inch,  and  for  a  single  three-ply  panel  with  ^{Q  inch  face 
plies  three  minutes  under  the  press,  at  a  temperature  of  212°F.,  is 
sufficient.  The  time  required  will  be  greater  with  lower  tem- 
peratures, and  with  increasing  thickness  of  the  material  joined. 
Where  thick  blocks  are  joined  the  steam-heated  press  cannot  be 
used,  but  the  clamped  blocks  are  placed  in  the  heated  chamber. 
The  temperature  is  usually  lower  than  that  used  in  the  press  so  as 
to  prevent  an  excessive  loss  of  moisture  from  the  wood.  On 
account  of  this  difficulty  in  heating  the  joint  under  pressure,  the 
service  of  blood  albumin  glues  is  mostly  confined  to  plywood 
manufacture.  A  press  used  for  experimental  work  at  the  Forest 
Products  Laboratory  is  shown  in  Fig.  59. 

The  water-resistant  qualities  of  blood  albumin  glue  surpass 
those  even  of  casein.  After  soaking  in  cold  water  for  several 
days,  or  in  boiling  water  for  several  hours,  the  shearing  strengths 
of  plywood  joints  show  that  from  50  to  75  per  cent  of  the  dry 
strength  has  been  retained. 

A  number  of  special  precautions  upon  the  use  of  blood  albumin 
glue  have  been  urged  by  the  Forest  Products  Laboratory:1 

1.  Weigh  out  all  constituents;  do  not  measure  them. 

2.  Add  cold  water  to  blood  albumin  and  do  not  heat  mixture. 

3.  Do  not  stir  blood  until  it  has  soaked  for  from  one  to  two  hours. 

4.  Avoid  excessive  stirring  of  the  glue  or  agitation  on  the  spreader,  since 
this  produces  foamy  glue. 

5.  Load  press  and  apply  pressure  quickly  to  prevent  coagulation  of  the 
blood  before  pressure  is  secured. 

6.  Pressures  ranging  from  50  to  100  pounds  per  square  inch  are  advisable, 
depending  upon  the  glue  consistency,  nature  of  wood,  etc. 

7.  Excessively  high  temperatures  of  the  platens  of  the  press  produce 
steam,  causing  blisters.     A  range  of  200  to  212°F.  is  advisable. 

8.  Panels  should  be  left  in  the  press  until  the  heat  has  penetrated  so  as 
to  raise  all  parts  to  at  least  160°F. 

9.  Be  careful  not  to  use  an  excessive  amount  of  lime  or  a  strongly  alkaline 
water. 

Dry  Glue  Process  for  Thin  Veneer. — A  very  interesting  and 
important  application  that  has  been  made  of  blood  albumin  glues 
is  the  practicability  of  gluing  together  very  thin  veneer  ranging 
from  ^30  to  >f25  of  an  inch  in  thickness.  When  such  thin  plies 
are  glued  in  the  ordinary  way  the  glue  usually  penetrates  through 
the  face  plies,  and  a  curling,  wrinkling,  and  overlapping  occurs 

1  National  Advisory  Committee  for  Aeronautics,  Report  66  (1920),  17. 


348  GELATIN  AND  GLUE 

due  to  the  excessive  and  uneven  swelling  of  the  thin  veneer  from 
rapid  absorption  of  water. 

The  blood  albumin  glue  has  been  adapted  to  this  service  by  the 
Forest  Products  Laboratory.  The  details  of  the  formulas  have 
not  been  made  public,  but  the  glue  mixture  is  reported  to  vary 
from  the  standard  formula  previously  given  "  principally  in  the 
addition  of  a  substance  which  makes  the  glue  hygroscopic,  or 
capable  of  attracting  and  retaining  moisture,  sufficiently  to  give 
a  contact  with  wood." 

The  procedure  differs  from  the  ordinary  operation  chiefly  in 
that  the  blood  albumin  glue  is  coated  very  thinly  upon  tissue 
paper  or  cloth,  and  in  the  application  this  layer  is  merely  spread 
between  the  face  plies  and  pressure  and  heat  applied. 

To  obtain  good  results,  the  glue  must  be  mixed  thin  and  be  free  from 
lumps  or  undissolved  particles.  Straining  through  a  sieve  is  absolutely 
necessary  in  this  case.  A  thin,  porous  tissue  paper  is  used  for  coating  and  is 
placed  in  a  machine  geared  to  run  it  through  the  glue  bath  at  a  rate  of 
approximately  1  foot  per  minute.  The  tissue  paper  passes  over  a  roller  in 
the  glue  bath,  and  upward  into  a  drying  chamber  to  a  worm  roller  upon 
which  there  are  strips  of  felt  to  prevent  it  from  wrinkling.  It  then  passes 
over  a  third  roller  and  through  pinch  rolls  to  a  final  dry  roll.  Cloth  may 
be  used  as  the  medium  upon  whih  the  glue  is  dried  and  then  be  made  to 
serve  as  one  ply  in  panel  construction. 

In  manufacturing  plywood  with  the  glue,  sheets  of  it  are  placed  between 
the  plies  of  the  wood  and  pressed  in  a  hot  press.  A  pressure  of  from  150 
to  200  pounds  per  square  inch  is  necessary  in  order  to  bring  about  good  con- 
tact between  the  glue  layer  and  the  wood.  If  the  moisture  of  the  veneer 
is  low,  the  water  resistant  properties  of  the  plywood  may  be  increased  by  a 
slight  sprinkling  or  sponging  of  the  veneer  immediately  before  placing 
in  the  press. 

The  many  advantages  of  this  form  of  glue  over  the  ordinary 
wet  process  are  summarized  by  the  Forest  Products  Laboratory: 

1.  Veneer  as  thin  as  K  25  inch  may  be  glued  up  successfully. 

2.  Overlaps,  wrinkling,  open  joints,  etc.,  are  overcome. 

3.  Gluing  with  the  addition  of  little  or  no  moisture  overcomes  cupping 
and  twisting  of  panels. 

4.  Drying  of  plywood  is  largely  eliminated. 

5.  Subsequent  trouble  in  checking  of  veneer  in  drying  is  eliminated. 

6.  Glue  is  always  ready  for  use  and  keeps  for  a  long  time. 

7.  It  can  probably  be  used  more  rapidly  and  with  less  labor  than  the  wet 
glue  process. 

8.  No  spreader  is  required. 


WATER-RESISTANT  GLUES  349 

II.  GLUES  OF  MARINE  ORIGIN 

Two  very  different  types  of  glue  are  made  from  parts  of  the  fish. 
The  very  highest  grades  of  gelatin  may  be  prepared  from  the 
swimming  bladders.  This  product  is  more  nearly  water  white 
than  any  animal  gelatin,  but  it  is  difficult  to  entirely  remove  from 
it  a  fishy  odor  that  makes  it  less  desirable  for  culinary  purposes 
than  the  animal  product.  The  uncooked  sounds  are  sold  as 
isinglass,  but  after  these  have  been  dissolved  in  water  and  con- 
verted into  gelatin  it  is  better  known  as  fish  gelatin,  isinglass 
gelatin  or  refined  isinglass.  On  account  of  its  greater  cost  it  has 
not  been  able  to  compete  as  an  ordinary  adhesive  with  animal 
glue, 

A  less  desirable  product  is  produced  from  the  skin,  scales, 
heads,  and  other  refuse  of  fish.  These  parts  may  contain  much 
material  of  a  non-protein  nature,  and  are  likewise  often  con- 
taminated with  salts,  blood,  flesh,  etc.  It  is  very  difficult  if  not 
impossible  with  the  use  of  skin  stock,  even  though  selected  and 
treated  with  great  care,  to  obtain  a  product  of  sufficiently  high 
gelatin  content  to  even  set  to  a  firm  jelly  at  ordinary  tempera- 
tures. For  this  reason  the  material  is  commonly  sold  in  liquid 
form  as  rather  thick  viscous  fluids.  A  firmer  jelly  may  be  pro- 
duced from  the  fish  head  stock.  The  natural  odor  is  always 
strong  and  offensive,  and  to  mask  it  and  make  it  less  objectionable 
some  aromatic  substance  of  a  strong  odor  as  creosote,  oil  of 
sassafras,  or  wintergreen  is  added. 

It  should  be  pointed  out  that  all  liquid  glues  are  not  fish  prod- 
ucts, but  that  animal  glues  are  sometimes  treated  with  a  chem- 
ical which  destroys  the  power  of  the  glue  to  form  a  jelly.  Acetic 
acid  and  nitric  acid  are  used  for  this  purpose.  It  is  doubtful  if 
this  practice  is  profitable,  however,  for  although  a  glue  is  ob- 
tained that  may  be  used  without  warming  or  other  preparation, 
yet  the  strength  per  unit  concentration  of  dry  glue  is  diminished, 
and  the  joint  is  apt  to  decrease  in  strength  with  age.  The  dimin- 
ished strength  is  usually  compensated  by  increasing  the  con- 
centration at  which  it  is  applied,  but  this  can  hardly  be  regarded 
as  an  economic  procedure.  For  small  repair  work  and  household 
use  it  is,  however,  convenient.  The  use  of  calcium  or  magne- 
sium salicylate  and  of  thiourea  have  recently  been  proposed  for 
the  preparation  of  a  liquid  animal  glue.1 

1  D.  K.  TRESSLER,  U.  S.  Pats.  1,394,653  and  1,394,654,  Oct.  25,  1921. 


350  GELATIN  AND  GLUE 

1.  Isinglass. — Isinglass  (from  the  Dutch  huisenblas,  German 
hausenblase,  meaning  sturgeons  bladder)  is  as  its  name  signifies 
a  product  composed  of  the  air-  or  swimming-bladders  of  certain  of 
the  fishes.  Most  important  of  these  is  the  sturgeon,  and  for 
centuries  the  material  has  been  obtained  and  exported  from 
Russia.  In  many  of  the  fishes  this  bladder  is  too  small  or 
too  securely  fastened  to  the  backbone  or  abdominal  wall  to  make 
its  removal  a  practical  proposition,  but  the  sturgeon,  the  catfish, 
and  the  carp  have  long  been  utilized  for  this  purpose. 

More  recently  many  other  of  the  fishes  have  been  made  use  of 
for  the  manufacture  of  isinglass.  The  siluridse  are  used  in 
Brazil  and  Venezuela.  Iceland  exports  a  good  quality  made 
from  cod  sounds,  and  the  cod  is  also  used  to  some  extent  in 
Norway  and  in  Canada  for  this  purpose.  The  North  American 
product  is  obtained  chiefly,  however,  from  the  large  hake  that  are 
found  in  deep  water.  Over  100  tons  of  hake  sounds  were 
obtained  annually  on  the  New  England  coast  alone  a  few  years 
ago,  but  the  production  is  much  less  at  present.  The  squeteague 
has  also  been  much  used,  and  the  tilefish  has  been  demonstrated 
to  be  well  adapted  to  the  production  of  isinglass.1  The  yield 
and  quality  of  the  product  varies  greatly,  however,  with  the 
specie  of  fish.  The  large  deep-water  hake  yield  from  40  to  50 
pounds  of  dry  isinglass  per  ton  of  fish,  and  the  product  contains 
about  85  per  cent  of  gelatin.  The  smaller  shallow-water  hake 
yield  only  about  30  pounds  of  isinglass  per  ton  of  fish.  The  cod 
yields  but  15  to  20  pounds,  and  the  squeteague  20  pounds.  The 
gelatin  content  of  the  product  from  the  latter  two  species  is  also 
low,  being  only  about  50  per  cent. 

The  Manufacture  of  Isinglass. — Isinglass  appears  on  the  market 
in  several  different  forms.  The  best  Russian  product  is  known  as 
staple  isinglass  and  may  be  obtained  as  long  or  as  short  staple. 
It  is  made  by  rolling  each  bladder  and  folding  around  a  few  pegs 
set  in  the  form  of  a  horseshoe.  The  leaves  are  sometimes 
twisted  like  ropes.  When  the  bladders  are  merely  placed  one 
upon  the  other  in  sheets  it  is  known  as  leaf  isinglass.  If  these 
leaves  are  folded  before  they  are  completely  dry,  and  covered 
with  a  damp  cloth,  they  constitute  book  isinglass. 

The  preparation  of  the  material  for  the  market  is  very  simple. 
The  Russian  method  is  as  follows:  The  sounds  are  allowed  to 
remain  in  water  for  several  days  with  frequent  changes  of  water  to 

1  G.  F.  WHITE,  U.  S.  Bureau  of  Fisheries  Doc.  852  (1917),  7. 


WATER-RESISTANT  GLUES  351 

remove  the  blood  and  fatty  matter  present.  After  the  washing 
is  complete  the  sounds  are  cut  longitudinally  into  sheets.  These 
are  laid  out  to  dry  by  exposure  to  the  sun  and  air  upon  boards  of 
linden  or  bass-wood.  During  this  process  the  inner  layer,  which 
is  the  layer  consisting  of  pure  isinglass,  is  uppermost,  and  after  a 
partial  drying  it  may  be  removed  from  the  coarser  external 
lamellae.  The  finer  sheets  are  placed  between  cloths  to  protect 
them  from  the  flies,  and  are  subjected  to  heavy  pressure  to  flatten 
them.  After  thoroughly  drying  they  are  assorted  and  tied  up 
into  packages  for  export  containing  10  to  15  sheets  and  weighing 
about  \Y±  pounds.  These  packages  are  commonly  shipped 
in  lots  of  eight  sewed  up  in  a  cloth  bag  or  inclosed  in  sheet 
lead. 

The  external  layers  are  softened  with  water  and  a  considerable 
amount  of  gluey  material  scraped  off  which  is  moulded  and  dried. 
This  is  used  as  an  inferior  isinglass.  The  residue,  together  with 
trimmings  from  the  sounds  and  other  parts  of  the  fish,  is  boiled 
with  water  to  produce  a  fish  glue. 

The  preparation  of  isinglass  in  this  country  differs  from  that 
outlined  above  chiefly  in  the  introduction  of  machinery  for  the 
hand  labor.  The  sounds,  especially  from  the  hake,  are  often 
detached  on  the  fishing  vessels  when  dressing  the  fish,  and  are 
then  salted  in  barrels  so  that  they  will  not  decompose.  They 
are  sometimes  air-dried  in  that  condition.  Upon  delivery  to  the 
isinglass  manufacturer  they  are  first  soaked  for  several  hours  in 
water  to  soften  them,  washed,  slit  open  and  the  black  outer 
membrane  scraped  off,  and  again  thoroughly  washed.  The 
pure  sounds  are  then  usually  run  into  a  cutting  machine  provided 
with  a  roller  and  a  set  of  knives  in  which  they  are  chopped  into 
small  pieces.  This  material  is  mixed  and  macerated  between 
rollers,  and  then  passed  to  the  sheeting  rollers.  These  are 
hollow  iron  rollers  through  which  cold  water  is  allowed  to  circu- 
late to  prevent  a  softening  and  sticking  of  the  sounds.  A  thick 
sheet  is  formed  which  passes  in  turn  to  the  ribbon  rollers  where  it  is 
drawn  out  into  a  thin  uniform  ribbon  ^4  inch  in  thickness  and 
6  to  8  inches  in  width.  The  ribbons  are  suspended  in  warm 
rooms  where  they  dry  out  in  a  few  hours,  and  are  then  rolled 
on  wooden  spools  into  coils  weighing  less  than  a  pound  each. 
The  recovery  of  isinglass  by  this  method  is  about  80  per  cent  of 
the  weight  of  the  original  sounds. 

The  drying  of  the  sounds;  the  rolling  of  the  sounds;  the  drying 


352 


GELATIN  AND  GLUE 


FIG.  60. — Drying  Hake  sounds  for  isinglass  manufacture.     (By  permission  of 
Bureau  of  Fisheries,  Washington,  D.  C.) 


FIG.  61. — Rolling  Hake  sounds  for  isinglass.     (By  permission  of  the  Bureau  of 
Fisheries,  Washington,  D.  C.) 


room;  and  final  rolling  of  the  isinglass  into  coils  are  illustrated  in 
the  accompanying  photographs,  Figs.  60,  61,  62,  63. 


WATER-RESISTANT  GLUES 


353 


A  product  obtained  by  dissolving  New  England  isinglass  in 
water,  straining  off  the  insoluble  material,  and  spreading  in  very 


FIG.  62. — Drying  room  of  isinglass  factory.     (By  permission  "of  the  Bureau  of 
Fisheries,  Washington,  D.  C.) 


FIG.  63. — Wooden  spool  for  rolling  into  coils.     (By  permission  of  the  Bureau 
of  Fisheries,  Washington,  D.  C.) 


thin  sheets  on  oil  cloth  or  glass,  is  known  in  the  trade  as  trans- 
parent or  refined  isinglass. 

23 


354 


GELATIN  AND  GLUE 


The  Composition)  Properties,  and  Uses  of  Isinglass. ] — The  corn- 
composition  of  isinglass  from  various  sources  has  been  reported 
by  Prollius,2  and  is  given  in  the  following  table. 


TABLE  46. — COMPOSITION  OF  ISINGLASS 


Source  of  isinglass 

Ash,  per  cent 

Water, 
per   cent 

Residue  insol- 
uble   in   hot 
water,  per  cent 

Astrakhan  (Russian)  

0.20 

16.0 

2.8 

Astrakhan  (Russian)     

0.37 

18.0 

0  7 

Astrakhan  (Russian)  
Astrakhan  (Russian)  

0.20 
0.80 

17.0 
19.0 

1.0 
3.0 

Astrakhan  (Russian)   

0.50 

19.0 

0  4 

Astrakhan  (Russian)  
Hamburg  

0.40 
1.30 

17.0 
19.0 

1.3 
2.3 

Hamburg  

0.13 

19.0 

5  2 

Iceland 

0  60 

17  0 

21  6 

East  India  
Yellow,  unknown  source 

0.78 
2.30 

18.0 
17.0 

8.6 
15.6 

The  high  reputation  which  the  Russian  product  enjoys  is 
shown  by  the  above  table  to  be  quite  justified.  The  insoluble 
matter  and  ash  are  low,  and  the  analyses  run  reasonably  uniform. 
A  certain  amount  of  matter  insoluble  in  hot  water  will  always  be 
found,  but  in  the  best  grades  it  is  low. 

On  soaking  in  cold  water,  isinglass  swells  uniformly  but  slowly, 
and  does  not  take  up  nearly  the  amount  of  water  that  will  be 
absorbed  by  a  similar  weight  of  gelatin.  The  isinglass,  it  must 
be  remembered,  is  not  gelatin  but  rather  collagen,  and  in  order  to 
convert  it  into  gelatin  a  brief  heating  in  water  is  necessary. 
If  a  gelatin  is  made  and  allowed  to  dry  out,  the  product  will  then 
possess  all  of  the  characteristics  commonly  associated  with 
gelatin.  Its  jelly  strength  will  be  fairly  high,  it  will  be  clear  and 
almost  colorless,  and  produce  a  glue  of  high  adhesive  value.  A 
distinctive  odor  will  always  be  present  unless  some  material  has 
been  added  which  serves  to  mask  that  odor  by  imparting  to  it  a 
stronger,  although  less  disagreeable  one  of  another  character. 

1  For  chemical  composition,  see  pages  28,  43,  46,  and  435.     For  the  strictly 
medicinal  uses  of  isinglass  the  reader  is  referred  to  Potter's  "  Materia  Medica. " 

2  F.  PROLLIUS,  Dingier' s  polytech.  J.,  249  (1884),  425. 


WATER-RESISTANT  GLUES  355 

In  its  several  tests  it  responds  in  a  way  entirely  similar  to  animal 
gelatin. 

Since  isinglass  is  more  expensive  than  gelatin,  attempts  have 
frequently  been  made  to  adulterate  it.  One  of  these  methods  is 
by  rolling  a  layer  of  gelatin  between  layers  of  isinglass.  White1 
reports  that  such  adulteration  may  easily  be  detected  by  treating 
with  water  and  observing  the  nature  of  the  colloidal  solution 
under  the  microscope.  The  isinglass  will  retain  its  characteristic 
fibrous  structure  which  is  not  present  in  a  gelatin  solution,  and 
the  gelatin  becomes  more  transparent  than  before,  the  shreds 
becoming  disintegrated. 

In  addition  to  adulteration,  imitation  is  sometimes  attempted 
by  putting  out  material  that  is  intended  to  simulate  the  true 
isinglass,  but  which  is  made  from  altogether  different  material. 
Thus  blood  fibrin,  calves'  foot  gelatin,  agar  agar,  etc.,  have  been 
manufactured  to  resemble  natural  isinglass. 

One  of  the  oldest  and  best  established  uses  of  isinglass  is  as  a 
clarifying  agent  for  various  beverages  as  wine,  cider  and  malt 
liquors.  The  efficacy  of  the  isinglass  for  this  service  lies  in  the 
purely  mechanical  property  it  possesses  of  maintaining  a  fibrous 
structure  in  the  solution,  and  as  this  settles  slowly  to  the  bottom 
it  entangles  in  its  netlike  meshes  the  colloidal  bodies  that  produce 
the  undesirable  turbidity.  For  clarifying  wine  the  isinglass  is 
first  swollen  in  water  and  then  in  the  wine  until  it  is  completely 
swollen  and  transparent.  It  is  then  thoroughly  beaten  into  a 
small  amount  of  the  wine,  strained  through  a  linen  cloth,  and 
stirred  into  the  rest  of  the  wine.  The  temperature  is  kept  low 
and  the  isinglass  does  not  go  into  solution,  but  only  into  a 
very  finely  divided  suspension.  Thus  the  original  fibrous 
structure  of  the  sounds  has  at  no  time  since  it  came  from  the  fish 
been  lost.  In  this  lies  the  difference  in  the  action  of  isinglass 
and  gelatin  for  fining.  If  isinglass  were  heated  and  made  into  a 
true  gelatin  it  would  then  have  lost  the  properties  which  make  it 
so  valuable  for  this  service.  A  single  ounce  of  isinglass  will 
clarify,  under  the  optimum  conditions,  500  gallons  of  wine  in 
10  days. 

In  the  manufacture  of  beer  and  ale  the  starch  granules, 
bacteria,  and  protein  matter,  which  do  not  settle  in  the  tanks 
after  the  primary  fermentation,  are  gotten  out  by  either  filtra- 
tion, adsorption  upon  wood  chips,  or  fining.  In  the  latter 

1  G.  F.  WHITE,  loc.  cit. 


356  GELATIN  AND  GLUE 

procedure  the  isinglass  is  treated  with  sour  beer  and,  after  swelling, 
macerated  as  in  the  fining  of  wine.  After  straining  it  is  mixed 
thoroughly  with  the  rest  of  the  beer.  One  pound  of  isinglass 
will  fine  from  100  to  500  barrels  of  beer. 

Other  uses  of  isinglass  are  somewhat  scattered.  Court 
plaster  frequently  employs  isinglass  instead  of  gelatin,  the 
proportions  used  being  10  grams  of  isinglass,  40  grams  of  alcohol, 
1  gram  of  glycerine,  and  water  to  make  a  total  weight  of  120 
grams.  Taffeta  is  first  treated  with  successive  layers  of  isinglass 
in  water,  then  with  the  isinglass,  water,  alcohol  and  glycerin 
solution,  and  the  reverse  side  covered  with  tincture  of  benzoin. 
This  is  sufficient  for  a  piece  38  cm.  square. 

Various  cements  for  repairing  glass,  pottery,  etc.,  are  made  of 
isinglass  with  alcohol  and  gums.  A  formula  for  a  cement  of  the 
following  composition  has  been  published : 

10  grains  isinglass 

5  grams  gum  ammoniac 

5  grams  mastic 
80  grams  alcohol 

The  isinglass  and  gums  are  said  to  be  dissolved  separately  in  the 
alcohol  and  then  heated  together  over  boiling  water.1 

Where  a  high  luster  is  desired  on  textile  goods  isinglass  is 
sometimes  used,  mixed  with  gum,  as  a  size.  Some  silks  are 
sized  in  this  way.  Mixed  with  pyroxylin  and  dissolved  in  acetic 
acid,  with  the  addition  of  a  little  formaldehyde  or  potassium 
bichromate,  it  has  been  employed  as  a  waterproofing  composition 
for  textiles.  Isinglass  dissolved  in  water  is  used  to  repair  leather 
belts  that  have  torn  apart. 

2.  Liquid  Fish  Glue.2 — Fish  glue  is  marketed  usually  in  the 
form  of  liquid  glue  and  it  is  the  most  important  liquid  glue. 
Dry  fish  glues  are  soluble  in  water  at  ordinary  room  temperatures, 

1  A  vast  number  of  formulas  for  the  preparation  of  special  cements,  insol- 
uble or  water-resistant  glues,  and  glues  for  special  purposes  have  been 
published.     Some  of  these  are  undoubtedly  workable  and  satisfactory,  but 
a  considerable  number  of  them  are  either  unworkable  or  unsatisfactory. 
For  example,  the  above  formula  calls  for  a  solution  of  isinglass  in  alcohol. 
Since  alcohol  is  a  precipitant  for  gelatin,  it  is  obvious  that  such  a  solution 
cannot  be  obtained.     It  is  probable  that  an  emulsion  of  an  aqueous  solution 
of  the  isinglass  in  the  alcohol  is  indicated. 

2  By  DONALD  K.  TRESSLER,  Ph.    D.,    Industrial  Fellow  of   the    Mellon 
Institute  of  Industrial  Research,  University  of  Pittsburgh,  Pittsburgh,  Pa. 


WATER-RESISTANT  GLUES  357 

whereas  hide  and  bone  glues  merely  swell  but  do  not  dissolve 
under  these  conditions.  Hide  and  bone  glues  are  occasionally 
made  into  liquid  glue;  but,  in  order  to  do  this,  the  hard  glues 
must  be  either  dissolved  in  a  solution  of  a  gel-inhibiting  substance 
or  so  treated  that  their  chemical  composition  and  properties  are 
changed. 

Source  of  Raw  Materials. — The  bulk  of  the  fish  glue  manu- 
factured today  is  made  from  the  waste  products  of  the  cod, 
haddock,  cusk,  hake  and  pollock  industries.  These  fish  are  the 
so-called  "ground"  fish  which  are  caught  on  the  banks,  usually 
together  in  the  same  nets,  and  cleaned  on  the  same  wharves. 
Consequently,  most  of  the  fish  glue  stock  comes  to  the  glue 
factory  already  mixed;  that  is  to  say,  the  waste  from  the  various 
species  of  fish  have  been  dumped  into  the  same  containers. 

Some  other  species  of  fish  than  those  mentioned  above  are 
used  in  the  manufacture  of  glue — indeed,  any  fish  might  be  used  for 
the  making  of  glue — but  for  certain  practical  and  economical 
reasons  only  small  quantities  of  glue  are  manufactured  from  other 
fish.  The  quality  of  the  glue  prepared  from  these  ground  fish  is 
higher  and  the  yield  is  greater  than  in  the  case  of  glue  made 
from  most  other  fish.  Many  species  of  fish — e.g.,  menhaden,— 
yield  such  small  quantities  of  glue  that  it  is  not  economically 
practicable  to  use  them  for  the  manufacture  of  glue.  Other 
fish  such  as  the  herring  and  mackerel  contain  such  large  quantities 
of  fat  that  special  procedures  must  be  followed  to  remove  the  fat 
from  the  fish  in  the  glue  making  process.  Many  fish  which 
would  otherwise  be  used  are  not  caught  largely  in  any  one  locality 
and  consequently  the  supply  of  fish  waste  at  any  particular 
point  is  not  large  enough  to  justify  the  establishment  of  a  glue 
factory.  Other  fish  are  caught  only  for  short  seasons,  which 
would  cause  the  glue  factories  to  be  idle  most  of  the  year. 

The  ground  fish  waste  ordinarily  is  divided  into  three  classes: 
viz.,  (1)  fish  heads,  (2)  waste,  i.e.,  salt  fish  trimmings  and  bones, 
and  (3)  skin  from  the  dried  salted  fish.  The  fish  heads  are  fresh 
and  are  hauled  from  the  wharves  where  the  ground  fish  are 
cleaned.  With  the  exception  of  the  exported  salt  fish,  most  of 
the  dried  salt  fish  is  skinned  before  it  is  packed  for  shipping.  The 
cod  and  cusk  skins  are  not  mixed  with  the  skins  of  the  haddock, 
hake  and  pollock.  The  cod  and  cusk  skins  which  have  a  small 
amount  of  salt  fish  adhering  to  them  constitute  the  skin-glue 
stock.  Most  of  the  salt  fish  sold  in  this  country  is  cut  into  strips, 


358  GELATIN  AND  GLUE 

trimmed  of  the  outer  3^ellow  portion  and  freed  from  bones.  The 
trimmings,  the  bones,  and  the  haddock,  hake  and  pollock  skins 
constitute  the  salt-fish  waste  glue-stock  and  is  termed  "  waste." 

Methods  of  Manufacture. — The  glue  stock,  regardless  of  its 
source,  must  be  freed  from  salt  or  freshened  before  being  made 
into  glue.  The  fish  skin  and  waste  stock  being  a  waste  product 
of  the  salt  fish  industry,  contains  a  much  greater  percentage  of 
salt  and  consequently  more  care  must  be  used  in  freshening  it 
than  in  freshening  the  fish  head  stock.  The  fish  skin  and  waste 
stock  ordinarily  are  agitated  in  running  water  in  large  tanks  for 
a  period  of  twelve  hours  or  more,  or  until  a  sample  of  the  wash- 
water  on  analysis  shows  a  low  percentage  of  chlorides.  The 
stock  is  then  thrown  into  false-bottomed  tanks,  called  "  cookers," 
which  usually  have  a  layer  of  excelsior  on  their  false  bottoms. 
The  stock  is  covered  with  water  and  a  slow  stream  of 
steam  is  passed  into  the  tanks.  The  length  of  the  cooking 
period  varies  with  the  nature  of  the  glue  stock,  fish  waste  requir- 
ing longer  cooking  than  fish-skin  stock.  Usually  two  runs  are 
made;  that  is,  the  liquor  formed  by  the  cooking  of  the  stock  is 
drawn  off  when  it  becomes  sufficiently  concentrated,  more  water 
is  added  and  the  cooking  is  continued.  The  average  concentra- 
tion of  the  glue  liquors  is  about  five  per  cent.  The  first  run  of 
glue  liquor  is  the  better. 

After  6  to  10  hours  cooking,  when  nearly  all  the  glue  has  been 
removed  from  the  stock,  the  cooking  is  stopped  and  the  second 
run  of  glue  liquor  is  withdrawn.  The  residue  in  the  cookers 
usually  is  put  in  large  hydraulic  presses,  where  most  of  the  remain- 
ing glue  liquor  is  pressed  out.  This  press-glue  liquor  is  added  to 
the  second  run  liquor. 

Preservatives  are  added  to  the  glue  liquor  to  prevent  any 
bacterial  action.  Fish  glue  and  glue  liquors  decompose  very 
rapidhr  if  any  considerable  amount  of  bacterial  growth  in  them 
is  permitted.  The  preservatives  added  by  various  glue  makers 
include  phenol,  cresol  and  boric  acid.  The  finished  product 
contains  from  0.5  to  3  per  cent  of  preservatives,  the  amount 
depending  upon  the  nature  of  the  preservative  added. 

The  glue  liquor,  drained  from  the  cookers,  is  next  pumped  to 
the  evaporators.  The  types  of  evaporators  used  vary  in  different 
factories.  Some  plants  use  open  pans  heated  with  steam  coils, 
others  use  open  pans  containing  revolving  copper  coils,  and  still 
others  use  vacuum  pan  evaporators.  The  glue  liquors  usually 


WATER-RESISTANT  GLUES  359 

are  strained  through  a  coarse  wire  screen.  The  liquors  are 
evaporated  to  a  uniform  viscosity  and,  just  before  the  glue  is 
run  into  the  storage  tanks,  a  sufficient  amount  of  some  essential 
oil  dissolved  in  ethyl  alcohol  is  added,  to  prevent  the  growth  of 
moulds.  The  essential  oils  used  depend  upon  the  custom  of  the 
glue  maker  and  on  the  use  to  which  the  glue  is  to  be  put;  oils  of 
cassia,  camphor,  clove,  wintergreen  and  sassafras  are  among 
those  employed.  These  essential  oils  have  a  dual  purpose,  for, 
in  addition  to  preventing  the  growth  of  moulds,  they  mask  the 
fishy  odor  of  the  glue.  Some  fish  glues  also  are  made  opaque  by 
the  addition  of  zinc-white  or  some  other  white  pigment. 

The  processes  by  which  the  fish  heads  are  converted  into  glue 
usually  are  kept  more  or  less  secret.  The  processes  are  for  the 
most  part  similar  to  that  outlined  above,  except  that  the  glue 
stock  is  digested  with  dilute  acids,  usually  hydrochloric  or  acetic 
acid,  instead  of  cooking  with  steam  alone.  Moreover,  the  stock 
and  glue  liquors  usually  are  bleached  well.  Sulphur  dioxide  and 
sodium  bisulphite  are  the  common  bleaching  agents.  Fish-head 
glues  generally  are  made  opaque  with  a  white  pigment. 

The  residue,  "chum,"  from  the  hydraulic  presses  is  dried  and 
marketed  either  as  chicken  feed  or  as  a  fertilizer.  This  material 
makes  a  very  satisfactory  chicken  feed  as  it  contains  approxi- 
mately fifty  per  cent  of  protein.  Then,  too,  the  fish  head  and 
waste  chum  contain  a  high  percentage  of  calcium  phosphate 
which  supplies  lime  for  the  egg  shell  and  phosphorus  for  the 
egg  yolk. 

Various  fish  glue  makers  market  their  glue  in  different  ways. 
Some  cater  to  the  trade  buying  liquid  glue  in  bulk,  others  market 
it  chiefly  in  small  bottles  and  cans;  but  the  following  three  grades 
can  be  purchased  on  the  market:  (1)  photo-engraving  glue,  which 
is  made  from  the  first-run  glue  liquors  from  fish  skin.  (2) 
fish  skin  and  fish  waste  glue  which  is  usually  sold  in  small  bottles 
and  small  cans;  and  (3)  fish  head  glue,  which  is  prepared  from 
fish  heads  and  ordinarily  is  marketed  in  large  cans  and  barrels. 

Practical  Tests  to  Determine  the  Quality  of  Fish  Glue. — Fish 
glue  of  the  ordinary  viscosity  contains  from  50  to  55  per  cent  of 
glue  and  weighs  from  9J£  to  10  pounds  to  the  gallon. 

There  is  a  considerable  quantity  of  fish  glue  on  the  market 
which  is  of  rather  doubtful  quality.  Consequently,  if  the  glue 
user  does  not  test  his  glue,  it  is  wise  to  buy  only  from  manufac- 
turers with  well-established  reputations. 


360  GELATIN  AND  GLUE 

The  best  fish  glues  have  a  gel  point  of  about  7.5°C.  A 
higher  gel  point  is  satisfactory  in  warm  weather,  but  is  unsuited 
for  outdoor  use  in  cool  weather.  Glues  with  lower  gel  points 
are  usually  weak.  Fish  glues  should  not  contain  more  than  0.2 
per  cent  of  sodium  chloride,  as  a  higher  salt  content  indicates  a 
poor  drying,  hygroscopic  glue  which,  while  affording  satisfactory 
joints  in  cool  dry  weather,  probably  will  weaken  in  humid 
weather:  it  is  for  this  reason  that  the  purchaser  should  be  very 
careful  of  the  quality  of  fish  glue  which  he  buys.  The  titration 
of  an  ashed  sample  of  dried  fish  glue  with  a  standard  silver  nitrate 
solution  using  potassium  chromate  as  an  indicator  will  determine 
the  chloride  content.  All  fish  glues  should  be  slightly  acid  to 
phenolphthalein. 

One  of  the  most  instructive  tests  which  may  be  conducted  in 
the  examination  of  a  fish  glue  is  the  drying  test.  This  is  carried 
out  preferably  by  spreading  a  uniform  layer  of  glue,  about  J£ 
inch  in  depth,  on  a  glass  plate  and  placing  the  plate  in  a  constant- 
humidity  and  temperature  room,  together  with  a  similar  layer 
of  a  standard  glue  of  known  hygroscopic  properties.  A  room 
having  a  constant  temperature  of  20°C.,  and  a  constant  humidity 
of  20  per  cent  will  be  satisfactory.  The  time  of  drying  and  the 
hardness  of  the  dried  film  are  noted  and  compared  with  the 
standard.  The  dried  films  should  then  be  placed  in  a  room 
having  a  higher  humidity  and  temperature.  A  very  exacting 
test  may  be  conducted  by  choosing  a  room  having  a  temperature 
of  25°C.,  and  80  per  cent  humidity.  Under  such  conditions 
most  fish  and  bone  glues  will  soften  slightly.  If  the  dried  glue 
film  becomes  liquid  or  sticky  under  these  conditions,  a  poor  glue 
is  indicated.  If  constant  temperature  and  humidity  rooms  are 
not  available,  large  humidors  containing  sulphuric  acid  of  the 
proper  dilution  may  be  used. 

The  joint  strength  tests,  as  ordinarily  applied,  are  not  of 
much  value  in  determining  the  quality  of  a  given  sample  of  fish 
glue,  inasmuch  as  the  temperature  and  humidity  at  which  the 
tests  are  conducted  are  the  controlling  factors  in  the  strength 
of  the  joints.  The  personal  equation  is  also  an  important  factor 
which  should  be  considered  in  comparing  results  of  joint  tests. 
However,  if  the  laboratory  worker  conducts  all  the  joint  strength 
tests  under  the  same  conditions  of  temperature  and  humidity, 
the  results  of  these  tests  become  valuable.  The  results  are 
particularly  useful  if  these  tests  are  made  under  humid  condi- 


WATER-RESISTANT  GLUES  361 

tions.  A  constant  temperature  and  humidity  room  should  be 
so  regulated  that  the  temperature  is  in  the  neighborhood  of 
25°C.,  and  the  humidity  about  80  per  cent.  The  wood  blocks 
and  the  joints  should  be  kept  in  this  room  or  in  a  humidor  having 
similar  conditions  of  temperature  and  humidity.  Under  the 
conditions  mentioned  above,  good  fish-skin  and  fish-waste  glues 
possess  about  the  same  tensile  strength  as  high  grade  bone  and 
low  grade  hide  glues,  whereas  fish-head  glues  are  about  as  strong 
as  medium  grade  bone  glues. 

Composition. — Until  more  work  has  been  done  on  the 
composition  of  fish  glue,  a  complete  analysis  of  the  50  per  cent 
of  dry  matter  contained  in  liquid  fish  glue  will  be  of  little  value 
in  indicating  the  quality  of  the  glue.  Fish  glues  differ  in  compo- 
sition from  hide  and  bone  glues,  in  that  fish  glues  are  composed 
chiefly  of  proteoses  and  peptones  with  a  smaller  proportion  of 
proteins,  whereas  the  higher  grade  of  hide  glues  are  nearly  pure 
gelatin,  and  bone  glues  consist  mainly  of  gelatin  and  proteoses.1 
The  proteins  of  fish  glues  are  higher  in  ammoniacal  nitrogen, 
melanin  and  non-amino  nitrogen  than  the  proteins  of  either 
hide  or  bone  glues.  The  composition  of  the  proteins  of  fish 
glue  resembles  more  closely  that  of  the  proteins  of  bone  glues 
than  that  of  the  proteins  of  hide  glues.2 

Dry  fish-skin  and  fish-waste  glues  contain  about  one  per  cent 
of  ash.  The  amount  of  ash  contained  in  fish-head  glues  varies 
widely,  depending  on  the  method  of  manufacture  used  and  the 
amount  of  pigment  or  other  inorganic  material  added  during  the 
manufacture  of  the  glue.  Samples  which  have  been  analyzed  by 
the  writer  contained  from  1  to  5  per  cent  of  ash  in  the  dry  glue. 
A  representative  analysis  of  a  sample  of  ash  from  a  fish  skin  glue 
is  given  below: 

PER 

CENT 

Ash  in  dry  matter 0. 96 

Analysis  of  ash 

Silica  (SiO2) 12.7 

Calcium  oxide  (CaO) 10. 5 

Magnesia   (MgO) trace 

Potash  and  soda  (K2O  and  Na2O) 13. 9 

Sulphur  trioxide  (SO3).- 34. 0 

Phosphorus  pentoxide  (P2O6) 24. 9 

Chlorine  (Cl) 3.2 

Ferric  oxide  (Fe2O3) trace 

99.2 
1  See  table  10  on  page  28.      2  See  table  14  on  page  48. 


362 


GELATIN  AND  GLUE 


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364  GELATIN  AND  GLUE 

As  has  been  mentioned  previously,  the  chlorine  content  of  the 
ash  is  often  an  index  of  the  hygroscopic  properties  of  the  fish 
glue.  Ash  anatyses  of  fish  glues  may  be  of  value  in  detecting 
adulteration;  moreover,  it  is  often  possible  to  distinguish  be- 
tween liquid  glues  prepared  from  hard  glues  and  liquid  fish  glues 
by  the  differences  in  the  composition  of  the  ash. 

Properties. — The  color  of  liquid  fish  glue  depends  upon  the 
nature  of  the  raw  material,  the  method  of  manufacture  and  the 
clarity  of  the  product.  Fish-skin  glues,  as  they  are  ordinarily 
produced,  possess  the  greatest  degree  of  clarity.  Fish-waste  and 
fish-head  glues  are  more  or  less  opaque.  Most  clear  fish  glues 
make  a  dark  joint  when  used  with  light  colored  woods,  and  con- 
sequently much  of  the  liquid  glue  on  the  market  contains  some 
white  pigment.  This  gives  the  glue  a  lighter  color  and  also 
makes  the  joint  less  conspicuous. 

The  odor  and  taste  of  fish  glues  depend  largely  upon  the  nature 
and  amount  of  preservatives  and  essential  oils  added.  Upon 
heating  for  some  time,  the  essential  oil  is  driven  off  and  the  true 
odor  of  the  fish  glue  becomes  more  apparent. ' 

The  " speed  of  set,"  or  the  time  elapsing  after  the  application 
of  a  coat  of  glue  until  the  glue  becomes  a  gel,  depends  upon  the  gel 
point  and  viscosity  of  the  liquid  glue,  the  amount  of  glue  applied, 
the  nature  of  the  wood,  and  also,  to  some  extent,  the  humidity 
of  the  atmosphere.  " Setting"  is  caused  by  a  partial  withdrawal 
of  the  moisture  of  the  glue,  thus  causing  the  gelling  of  the  liquid 
glue.  The  higher  the  viscosity  and  gel  point  of  the  liquid  glue, 
the  less  the  amount  of  glue  applied,  the  more  absorbent  the 
surface  to  which  the  glue  is  applied,  and  the  lower  the  tempera- 
ture and  humidity  of  the  atmosphere,  the  more  rapidly  does  the 
glue  "set." 

At  any  given  temperature  and  humidity  the  rate  at  which  a 
fish  glue  dries  depends  upon  the  source  of  the  glue,  the  method  of 
manufacture,  and  the  salt  content.  As  a  rule,  fish-skin  and  waste 
glues  dry  more  rapidly  than  fish-head  glues,  although  if  the  fish- 
skin  and  waste  glues  contain  an  abnormally  high  salt  content  this 
may  be  reversed. 

The  viscosity  of  liquid  fish  glue  depends  upon  the  source  of  the 
glue,  the  method  of  manufacture,  the  percentage  of  dry  glue  in 
the  liquid  glue,  the  temperature,  and  the  addition  of  substances 
other  than  fish  glue,  e.g.,  boric  acid,  hard  glue,  phenol  and  cresol. 
The  addition  of  boric  acid  increases  the  viscosity  of  liquid  fish 


WATER-RESISTANT  GLUES  365 

glue  to  some  extent;  whereas  the  addition  of  phenol  and  cresol 
decrease  the  viscosity.  Small  amounts  of  hard  glues,  i.e., 
animal  glues,  sometimes  are  added  to  increase  the  viscosity. 

Fish-head  glues  are  usually  more  flexible  than  skin  and  waste 
glues.  Glycerine  and  glucose  often  are  added  to  increase  the 
flexibility  of  glues. 

Properly  preserved  liquid  fish  glues  will  keep  indefinitely  in  an 
air-tight  can  or  well-stoppered  bottle.  If  the  glue  is  stored  in  a 
cold  room  it  will  gel.  This  gel  melts  quickly  as  soon  as  the  glue 
has  been  warmed  above  its  gel  point.  When  liquid  glue  con- 
taining phenol  or  cresol  is  put  up  in  tin  cans,  after  a  time  a  black 
ring  is  formed  around  the  top  of  the  can  where  the  phenol  or 
cresol  has  attacked  the  iron.  However,  this  does  not  injure  the 
quality  of  the  glue.  Precipitates  sometimes  settle  out  from 
poorly  prepared  liquid  glue,  but  this  settling  does  not  injure  the 
strength  of  the  glue. 

Uses. — The  best  grade  of  fish -skin  glue  is  very  satisfactory  for - 
the  production  of  half-tone  plates  for  photo-engraving  work.  It 
is  also  used  to  some  extent  in  the  production  of  zinc  line  plates 
for  photo-engraving  work.  Fish  glues  are  used  largely  where 
flexible  glues  are  required,  e.g.,  in  the  manufacture  of  court- 
plaster,  labels  and  stamps,  and  in  the  binding  of  books.  Where 
small  amounts  of  a  strong,  ready-to-use,  adhesive  is  needed, 
fish  glues  are  universally  used;  e.g.,  for  small  repair  jobs  about  the 
house,  for  shoe  repairing  and  general  repair  work.  Some  fish 
glue  is  blended  with  hide  glue  and  used  as  belt  cement  for  leather 
belts.  Large  quantities  of  fish-head  glue  are  used  in  various 
sizing  operations,  for  this  glue  stiffens  material  yet  is  somewhat 
flexible.  Some  fish  glue  is  used  in  the  chipping  of  glass  in  the 
production  of  translucent  glass.  Large  quantities  are  used  in 
box  making,  furniture  making,  and  for  general  joining  work. 


A  SELECT  BIBLIOGRAPHY  ON  FISH  GLUES 

ANON,  Isinglass,  ichthycolla,  fish  glue,  Farben-Ztg.,  26  (1921),  1913; 
Chem.  Abstracts,  16  (1921),  2562. 

CLARK,  E.  D.  and  ALMY,  L.  H.,  A  study  of  food  fishes,  the  complete 
analysis  of  twenty  common  food  fishes  with  special  reference  to  a  seasonable 
variation  in  composition,  J.  Biol.  Chem.,  29  (1917),  xxii;  Chem.  Abstracts, 
11  (1917),  3065. 

DIETERICH,  K.,  The  pharmacodiacosmy  and  chemical  analysis  of  sturgeon 
and  fish  bladders,  Chem.  Ztg.,  33  (1909),  357;  368. 


366  GELATIN  AND  GLUE 

EKMAN,  C.  D.,  Extraction  of  glue  from  fish,  hides,  or  bones,  Brit.  Pat. 
2,680  (1883). 

GAMBLE,  C.  W.,  On  the  properties  of  certain  collagenous  bodies,  and  their 
behavior  towards  light  when  associated  with  alkali  bichromates,  J.  Soc. 
Chem.  Ind.,  29  (1910),  65-9. 

GREEN,  E.  H.  and  TOWER,  R.  W.,  The  organic  constituents  of  the  scales 
of  fish,  U.  S.  Fish  Commission  Bull.  21  (1901),  97. 

GROTH,  L.  A.,  Preparation  of  liquid  glue  from  fish,  Brit.  Pat.  5,786  (1882). 

KNUDSEN,  Manufacture  of  fish  glue,  Brit.  Pat.  153,526  (1920). 

LINDEMUTH,  J.  and  PARKER,  E.,  Analysis  of  fish  scrap,  /.  Ind.  Eng. 
Chem.,  6  (1913),  388. 

MORNER,  A  study  of  fish  scales,  Z.  physiol.  Chem.,  24  (1897),  125;  37 
(1902),  88. 

Naamlooze  Vennootschap  Allgemeene  Uitrinding  Exploitatie,  Preparation 
of  protein  from  fish,  Brit.  Pat.  7,700  (April  1,  1913). 

Naamlooze  Vennootschap  Allgemeene  Uitrinding  Exploitatie,  Preparation 
of  proteoses  from  fish,  Brit.  Pat.  22,462  (Oct.  6,  1913). 

OKUDA,  Y.,  Hydrolysis  of  fish  gelatin,  J.  Coll.  Agr.  Imp.  Univ.  Tokyo,  5 
(1916),  355-63;  J.  Soc.  Chem.  Ind.,  36  (1917),  513. 

OKUDA,  Y.,  Quantitative  determination  of  creatine,  creatinine,  and  mono- 
amino-acids  in  some  fishes,  mollusca,  and  Crustacea,  Orig.  Com.  8th  Intern. 
Congr.  Appl.  Chem.,  18,  275-81;  discussion  of  same,  ibid.,  27,  161-2. 

OKUDA,  Y.  and  OYAMO,  K.,  Hydrolysis  of  fish  muscle,  J.  Coll.  Agr.  Imp. 
Univ.  Tokyo,  5  (1916),  365-72;  Expt.  Sta.  Rec.,  40  (1919),  171;  Chem. 
Abstracts,  14  (1920),  1373. 

PROLLIUS,  F.,  The  composition  of  isinglass,  Dingier' s.  polytech.  J.,  249 
(1884),  425. 

SCHWICKERATH,  K.,  A  proteose  preparation  from  fish,  Brit.  Pat.  1,343, 
(Jan.  20,  1908). 

VIKTORIN,  H.,  Die  Meeresproducte,  Hartlebens  Verlag,  Wein  &  Leipzig 
(1906). 

WHITE,  G.  F.,  Fish  isinglass  and  glue,  U.  S.  Bureau  of  Fisheries,  Doc. 
852  (1917). 

WILLIAMS,  K.,  Cooking  and  chemical  composition  of  some  English  fish, 
Chem.  News,  104  (1911),  271;  Chem.  Abstracts,  6  (1912),  516. 


CHAPTER  VIII 
THE  TESTING  OF  GLUE  AND  GELATIN 

All  things  recently  glued  together 

are  weak  and  easily  pulled  asunder. 

Cicero  (about  50  B.C.) 

PAGE 

1.  The  Jelly  Strength  or  Consistency 369 

The  Early  Methods  of  Making  the  Jelly  Strength  Test 369 

The  Later  Methods  for  Measuring  Jelly  Consistency 373 

2.  The  Viscosity .  380 

The  Theory  of  Viscous  Flow 381 

The  Capillary  Tube  Type  of  Viscosimeter 384 

The  Rising  Bubble  and  Falling  Sphere  Types  of  Viscosimeter 392 

The  Torsional  Type  of  Viscosimeter :  394 

3.  The  Melting  Point 399 

The  Scientific  Basis  of  the  Melting  Point  Test 400 

The  Methods  for  Measuring  Melting  Point 404 

4.  The  Adhesive  Strength 41 1 

5.  The  Tensile  Strength  and  Elasticity 411 

6.  The  Optical  Rotation x 413 

7.  The  Swelling  Capacity 415 

8.  The  Rate  of  Setting 416 

9.  The  Foam  Test 416 

10.  The  Grease  Test '.. 418 

11.  The  Reaction 419 

12.  The  Appearance,  Odor,  Color,  Keeping  Qualities,  etc 420 

The  Inspection  Test 421 

Crazed  Glue 422 

As  gelatin  and  glue  are  used  for  a  large  variety  of  purposes, 
and  as  their  application  to  the  arts  dates  back  to  the  time  of  the 
ancients,  there  have  been  proposed  from  time  to  time  a  great 
many  tests  intending  to  determine  the  relative  value  of  the 
material  for  its  several  uses.  Glue  is  not  a  simple  substance, 
fixed  and  invariable  in  its  properties.  One  may  not  order 
"pure  glue,"  as  he  would,  for  example,  pure  linseed  oil,  or  pure 
slaked  lime.  It  is  perfectly  well  known  to  everyone  who  has 
handled  the  material  that  there  is  good  glue  and  bad  glue,  strong 
glue  and  weak  glue,  sweet  glue  and  sour  glue,  hide  glue  and  bone 
glue,  brown  glue,  yellow  glue,  and  white  glue — in  fact  if  one 
orders  just  plain  glue  he  may  get  anything  from  size  to  isinglass, 

367 


368  GELATIN  AND  GLUE 

and  pay  anywhere  from  five  cents  to  a  dollar  per  pound.  And  if 
we  consider  for  a  moment  the  divers  ingredients  that  go  to 
make  up  the  various  types  of  glue,  it  will  not  seem  at  all  surpris- 
ing that  such  is  the  case. 

In  spite  of  this,  however,  the  manufacturing  process  has  vastly 
improved  in  the  last  30  years.  There  was  a  time  when  every 
conceivable  part  of  the  animal  that  could  not  be  utilized  for 
more  valuable  products  was  " dumped"  into  the  glue  kettle. 
Very  little  precaution  was  exercised  to  prevent  decomposition, 
and  the  whole  heterogeneous  bacteria-laden  mess  was  boiled 
down  for  " glue."  But,  as  an  inspection  of  the  preceding  chapters 
will  reveal,  that  method  is  a  thing  of  the  past,  and  the  industry 
is  beginning  to  operate  on  a  scientific  basis. 

Notwithstanding  this  improvement,  however,  or  perhaps  in 
part  because  of  it,  many  different  types  of  product  are  now 
produced.  The  heavy  bones,  the  light  bones,  the  "green" 
bones,  the  " steamed"  bones,  the  country  bones,  the  heads,  the 
feet,  and  the  bones  from  different  animals,  are  all,  for  the  most 
part  treated  separately,  and  result  in  different  types  of  glue,  as 
are  also  hide  pieces,  fleshings,  leather  scrap,  tendons,  and  the 
like.  And,  as  has  also  been  shown,  each  of  these  lots  is  digested 
a  number  of  times  with  fresh  portions  of  water,  which  still 
further  increases  the  number  of  different  quality  glues. 

The  tests  that  have  been  applied  are  each  based  upon  some 
property  that  has  been  believed  to  be  of  fundamental  importance 
in  estimating  the  suitability  of  the  particular  glue  to  a  particular 
usage.  These  tests  may  be  divided  into  the  two  general  classes: 
physical  tests  upon  some  property,  and  chemical  analysis  for 
some  constituent.  The  former  will  be  considered  in  this  chapter. 
Little  attempt  will  be  made  in  this  place  to  discuss  the  relative 
merits  of  the  tests  to  be  described,  nor  to  present  means  by 
which  improvement  could  be  introduced.  For  a  consideration 
of  these  points  the  reader  is  referred  to  Chap.  X. 

The  several  physical  tests  which  are  in  common,  use,  both  in 
this  country  and  abroad  or  which  have  been  proposed,  are  as 
follows : 

1.  The  Jelly  Strength  or  Consistency. 

2.  The  Viscosity. 

3.  The  Melting  Point. 

4.  The  Adhesive  Strength. 

5.  The  Tensile  Strength  and  Elasticity. 


TESTING  OF  GLUE  369 

6.  The  Optical  Rotation. 

7.  The  Swelling  Capacity. 

8.  The  Rate  of  Setting. 

9.  The  Foam  Test. 

10.  The  Grease  Test. 

11.  The  Reaction. 

12.  The  Appearance,  Odor,  Color,  Keeping  Qualities,  etc. 

These  will  be  considered  in  the  order  given  above. 


1.  THE  JELLY  STRENGTH  OR  CONSISTENCY 

In  this  country  the  jelly  strength  test  is  probably  the  most 
generally  used  test  for  the  determination  of  what  is  called  the 
grade  of  the  glue.  It  is  based  upon  the  belief  that  if  a  number  of 
glues  are  put  into  solution  at  a  given  concentration,  and  allowed 
to  chill,  or  set,  the  value  of  the  glues  will,  in  general,  be  propor- 
tional to  the  relative  consistency  of  the  jellies  so  formed.  u~~~ 

The  Early  Methods  of  Making  the  Jelly  Strength  Test.— A 
large  number  of  modi  operandae  have  been  proposed  for  carrying 
out  the  technique  of  this  test.  In  nearly  all  of  them  it  is  neces- 
sary to  have  some  kind  of  a  basis  for  comparison.  In  other 
words,  the  measurements  obtained  are  not  absolute,  but  are  of 
value  only  in  so  far  as  they  stipulate  the  relative  consistency  of 
one  jelly  as  compared  with  another  which  is  taken  arbitrarily 
as  a  standard.  By  far  the  best  known  of  these  standards  are 
those  which  were  established  by  Peter  Cooper  in  1844.  They 
consist  of  a  series  of  eleven  types  of  glue  of  a  regularly  varying 
jelly  consistency,  from  the  highest,  which  he  calls  A  Extra  to 
the  lowest,  which  he  calls  No.  2.  These  are  as  follows: 

A  Extra  1)3 

1  Extra  \%. 

No.  1  1% 

IX  1% 

1M  No.  2 
1% 

Other  American  manufacturers  have  followed  a  somewhat  similar 
system  of  grading,  but,  as  they  have  developed  under  a  regime  of 
secrecy  and  intense  competition,  they  have,  for  the  most  part, 
felt  it  incumbent  upon  themselves  to  mystify,  rather  than  to  make 
clear  to  the  purchaser  of  their  product  the  nature  of  the  substance 
he  is  buying.  Accordingly  the  present  glue  market  is  utterly 
submerged  beneath  a  chaos  of  glue  " grades,"  such  as  ex's,  Bx's, 

24 


370  GELATIN  AND  GLUE 

Ix's,  x3's,  x4's  and  the  like,  which  cannot  possibly  convey  any 
intelligent  meaning  to  the  lay  buyer.  The  action  of  Peter 
Cooper  was  a  long  step  in  the  right  direction,  however,  for  his 
were  the  first  lots  of  definitely  graded  glues,  listed  at  definite 
market  prices. 

Inasmuch  as  the  standards  which  are  used  for  making  all  com- 
parisons in  jelly  consistency  are  themselves  glues  which  have  been 
especially  selected  for  that  purpose,  and  as  such  standards  must 
occasionally  be  renewed,  it  follows  that  these  must  vary  from  time 
to  time,  and  as  the  order  of  jelly  strength  may  be  reversed  at 
slightly  different  temperatures,  and  the  same  temperature  is  not 
always  used  for  comparisons,  it  becomes  evident  that  a  system 
less  arbitrary  would  be  advantageous.  This  will  be  considered 
in  Chap.  X. 

The  Finger  Test. — Of  all  the  methods  that  have  been  suggested 
for  making  the  test  of  jelly  consistency,  the  oldest,  and  likewise 
the  most  persistent,  is  what  is  known  as  the  finger  test.  Just 
when  or  how  this  test  originated  can  not  be  ascertained.  And, 
in  spite  of  a  score  and  more  of  newer  and  more  scientific  pro- 
cedures, this  ancient  comparison  has  remained,  even  to  the 
present,  one  of  the  most  popular  of  the  jelly  strength  tests.  Fern- 
bach1  as  late  as  1907  wrote,  "For  practical  commercial  purposes, 
there  is  no  better  method  of  measuring  the  resistance  of  the  glue 
jelly  than  by  means  of  the  finger.  The  fourth  finger  of  the  left 
hand  is  used,  as  it  is  the  most  sensitive  of  all."  As  the  technique 
of  most  of  the  methods  for  measuring  jelly  consistency  is  the 
same  except  for  the  final  operation,  it  will  be  given  in  detail  at 
this  place. 

The  glues  are  made  up  of  such  strength  that,  when  chilled,  the 
consistency  will  not  be  too  great.  Otherwise  there  will  be  diffi- 
culty in  observing  slight  differences.  For  this  reason  it  is  not 
desirable  to  test  all  glues  at  the  same  concentration.  If  the 
inspection  of  the  sample  shows  it  to  be  a  very  weak  glue,  it  may 
be  made  to  about  15  per  cent  concentration.  If  it  appears  to  be 
an  unusually  strong  specimen  6  or  7  per  cent  will  be  satisfactory. 
For  other  intermediate  glues  8,  10,  and  12  per  cent  concentrations 
may  be  used. 

The  glues  are  carefully  weighed  out,  such  an  amount  being 
taken  as  will  produce  200  c.c.  of  solution  of  the  desired  concen- 
tration.    Standard  glues  which  are  expected  to  closely  correspond 
1  R.  L.  FERNBACH,  "Glues  and  Gelatine,"  New  York  (1907),  47. 


TESTING  OF  GLUE  371 

in  jelly  consistency  to  the  samples  being  tested  are  weighed  out 
and  treated  exactly  similarly  to  the  latter  throughout  the  pro- 
cedure. The  glues  are  placed  in  weighed  glasses  or  tumblers, 
covered  with  cold  water  (any  amount  not  exceeding  the  final 
necessary  quantity  may  be  added  without  measuring) ,  and  placed 
in  a  cool  place,  preferably  in  an  ice  box,  for  several  hours,  or  over 
night.  (If  the  glues  are  ground,  an  hour  or  two  will  suffice  to 
soak  them,  but  if  in  thick  pieces  they  must  stand  for  about  eight 
hours.)  After  soaking,  they  are  placed  in  a  water  bath  in  which 
the  temperature  of  the  water  does  not  exceed  60°C.  The  glues 
are  stirred  frequently,  allowed  to  reach  a  temperature  of  at 
least  50°  and,  when  thoroughly  dissolved,  placed  upon  a  balance 
and  water  added  to  make  the  necessary  concentration.  (The 
concentration  is  determined  by  the  weight  of  glue  taken,  and  the 
volume  of  solution  always  made  the  same,  e.g.,  about  200  c.c.) 
The  glues  are  then  again  put  in  a  cool  place,  which  should  be  an 
ice  box  of  a  constant  temperature  of  about  10°C.,  and  allowed 
to  remain  for  several  hours,  or  over  night.  In  making  the  test, 
the  fingers  (or  one  finger)  are  pressed  lightly  upon  the  surface  of  the 
glues  being  tested,  and  of  the  standards.  If  a  given  glue  appears 
to  offer  the  same  resistance  to  the  finger  pressure  that,  for 
example,  a  standard  1J^  glue  (using  the  Cooper  standards) 
offers,  then  the  glue  is  given  the  grade  1J^.  If  it  offers  a  slightly 
greater  resistance  than  the  standard  1J£,  but  less  than  the  1%, 
it  is  graded  as  1J£  +  or  \%—  depending  upon  which  of  the  two 
it  appears  to  be  the  nearest. 

The  finger  test  appears  to  have  held  its  position  of  favor  both  on 
account  of  its  simplicity  of  operation,  requiring  a  minimum  of 
apparatus,  and  also  on  account  of  the  psychic  " personal  factor." 
The  average  man  employed  as  a  glue  tester  has  an  aversion  to  any 
appliance  which  seems  to  lessen  his  own  responsibility  and  place 
the  same  upon  a  merely  mechanical  instrument.  But  it  must  be 
admitted  that  with  practice  a  considerable  amount  of  skill  in 
testing  by  the  fingers  may  be  attained. 

Early  Jelly-breaking  Appliances. — One  of  the  earliest  substi- 
tutes for  the  finger  test  was  the  device  of  Lipowitz.1  He  sug- 
gested a  scheme  which  has  been  the  basis  of  many  modifications, 
and  the  principle  of  which  is  used  quite  extensively  at  the  present 
time.  He  placed  upon  the  surface  of  the  jelly  a  flat  disk,  one  or 
two  inches  in  diameter,  on  the  upper  side  of  which  was  soldered 

1  LIPOWITZ,  "Neue  Chem.  tech.  Unters.,"  Berlin  (1861),  37. 


372 


GELATIN  AND  GLUE 


an  iron  rod  supporting  a -funnel.  The  rod  was  held  vertically 
over  the  jelly  by  being  passed  through  a  hole  in  a  board  which 
rested  upon  the  glass  tumbler.  Lead  shot  was  then  slowly 
poured  into  the  funnel  until  the  weight  was  just  sufficient  to  cause 
the  thin  disk  to  penetrate  the  surface  of  the  jelly,  or  until  the  disk 


FIG.  64. — The  Lipowitz  jelly 
strength  test. 


FIG.  65. — Valenta's  apparatus  for 
testing  glue. 


was  caused  to  sink  completely  through  the  jelly  and  rest  upon  the 
bottom  of  the  glass.  The  lead  shot  was  then  weighed,  and  this 
weight  plus  that  of  the  funnel  and  tube  taken  as  a  measure  of  the 
jelly  consistency.  A  sketch  of  the  apparatus  is  shown  in  Fig.  64. 
Another  substitute  for  the  finger  test  was  suggested  by  Kiss- 
ling.1  He  employed  rods  of  glass,  zinc,  and  brass  of  specified 
dimensions  and  weight,  and  measured  the  firmness  of  a  jelly  by 
noting  the  length  of  time  taken  by  certain  of  these  rods  to  pene- 
trate and  sink  through  the  jelly.  He  compared  the  results 
obtained  by  his  method  with  those  of  Stelling,2  who  rated  glues 
according  to  their  content  of  material  insoluble  in  72  per  cent 
alcohol,  and  with  those  of  Fels,3  who  graded  glues  according  to 

1  R.  KISSLING,  Chem.  Ztg.,  17  (1893),  726;  22  (1898),  171. 

2  C.  STELLING,  ibid.,  20  (1896),  461.     See  page  463. 

3  J.  FELS,  ibid.,  21  (1897),  56;  25  (1901),  23.     See  page  387-8. 


TESTING  OF  GLUE 


373 


their  viscosity.  He  reported  that  the  result  of  these  compari- 
sons was  not  satisfactory,  although  a  certain  parallelism  was 
noticeable  between  the  jelly-firmness  and  viscosity,  and  that 
Stellung's  process  showed  that  glues  yielding  the  firmest  jellies 
contained  the  smallest  proportion  of  " non-glue." 

Valenta1  improved  somewhat  upon  these  earlier  devices  of 
Lipo witz  and  Kissling.  Upon  the  j  elly 
in  a  glass  tumbler  Valenta  placed  a 
convex  piston,  into  the  upper  end  of 
which  was  soldered  a  rod,  held  in  a 
vertical  position  by  means  of  a  suitable 
frame.  Upon  the  top  of  the  rod  was 
secured  a  beaker.  Mercury  was 
allowed  to  run  slowly  into  the  beaker 
until  the  combined  weight  of  the 
piston,  rod,  beaker,  and  mercury  was 
just  sufficient  to  break  through  the 
surface  of  the  jelly.  This  weight  was 
taken  as  the  measure  of  the  jelly 
consistency,  see  Fig.  65. 

An  important  modification  of  the 
Lipowitz  instrument  was  invented  by 
Scott2  in  1907.  Scott  placed  his 
beaker  of  jelly  upon  the  pan  of  a 
spring  balance,  see  Fig.  66,  set  the 
pointer  at  the  zero  mark,  and  slowly 
forced  a  rod,  terminating  in  a  conical 
metallic  head,  down  until  the  surface 
of  the  jelly  was  broken.  The  pressure  at  this  point  was 
measured  by  the  degree  of  deflection  which  was  observed  by  the 
pointer  of  the  scale. 

The  Later  Methods  for  Measuring  Jelly  Consistency.— 
In  most  of  the  more  recent  methods  for  measuring  jelly  consis- 
tency some  attempt  has  been  made  to  overcome  the  error  which 
always  attends  the  breaking  or  compression  of  a  jelly,  due  to  the 
formation  of  a  "skin,"  an  especially  tough  layer,  at  the  surface. 
This  was  early  recognized  by  Alexander3  who  in  1906  suggested 

1  VALENTA,  Chem.  Ztg.,  33  (1909),  94. 

2  SCOTT,  Chem.  Eng.,  5  (1907),  441. 

3  J.  ALEXANDER,  J.  Soc.  Chem.  Ind.,  25  (1906),  158;  U.  S.  Patent  No. 

882,731  (1908). 


FIG.  66.— The  Scott  glue 
tester.  (Kindness  of  The 
Arthur  H.  Thomas  Company 
of  Philadelphia.) 


374 


GELATIN  AND  GLUE 


an  ingenious  instrument  which  permitted  the  free  expansion  of  a 
block  of  jelly  in  one  direction  while  being  compressed  in  another 
direction.  A  sketch  of  the  instrument  is  shown  in  Fig.  67. 
It  consists  essentially  of  a  brass  cylindrical  vessel  supported  by 
four  vertical  rods  against  which  it  slides  with  roller  bearings. 
This  cup  is  allowed  to  rest  upon  a  block  of  jelly,  in  the  form  of  a 


SectionA-A 
FIG.    67. — Alexander's     jelly      strength 
apparatus. 


FIG.  68. — Jelly  strength 
tester  used  by  the  Forest 
Products  Laboratory. 


truncated  cone,  of  specified  dimensions,  concentration,  and 
temperature.  Lead  shot  is  added  slowly  to  the  cup  which  causes 
the  jelly  to  be  compressed  downward,  while  being  free  to  expand 
outward.  Electrical  connections  are  installed  in  such  a  manner 
that  when  the  cup  has  sunk  downward  a  definite  distance  the 
circuit  becomes  automatically  closed,  and  a  bell  rings.  The 
combined  weight  of  the  cup  and  the  shot  gives  a  figure  repre- 
sentative of  the  jelly  consistency. 


TESTING  OF  GLUE  375 

Sindall  and  Bacon1  used  a  device  not  materially  different  in 
principle  from  the  older  methods  except  that  an  attempt  was 
made  to  avoid  the  error  due  to  the  "skin"  formation.  They 
blew  a  bulb  a  half  inch  in  diameter  at  one  end  of  a  short  glass 
tube,  placed  this  bulb  upon  the  abr aided  surface  of  the  jelly, 
and  very  slowly  ran  mercury  into  the  bulb  from  a  burette  until 
the  bulb  was  forced  to  a  half  inch  from  the  bottom  of  the  beaker. 

An  instrument  which  has  been  in  use  in  the  laboratories  of  the 
Armour  Glue  Works  for  a  number  of  years,  has  been  adopted, 
with  a  few  slight  modifications,  by  the  United  States  Forest 
Service  in  their  laboratory  at  Madison,  Wisconsin,  and  has  been 
described  by  them  in  their  "  Technical  Notes."2  It  consists 
essentially  of  a  light  cylindrical  frame  of  brass  which  is  permitted 
to  rest  upon  the  surface  of  the  jelly  contained  in  a  glass  tumbler  of 
specified  dimensions.  Moving  vertically  in  the  frame  is  a 
plunger,  likewise  of  brass,  and  graduated  upon  the  stem  for 
a  length  of  about  four  centimeters.  The  lower  end  consists  of  a 
blunt  conical  shaped  head,  which  may  be  either  solid,  weighing 
about  325  grams,  or  hollow,  in  which  case  a  definite  mass  of 
lead  shot  may  be  added  so  as  to  bring  the  weight  to  the  most 
sensitive  point  for  the  particular  jellies  being  tested.  A  set- 
screw  is  placed  at  the  top  so  that  the  zero  mark  on  the  scale  may 
be  correctly  adjusted  after  placing  the  whole  apparatus  upon  a 
flat  surface.  A  sketch  of  this  instrument  is  shown  in  Fig.  68. 

Perhaps  the  most  conspicuous  advantages  of  this  instrument 
are  (1)  the  great  rapidity  of  manipulation,  (2)  the  expression  of 
the  jelly  consistency  in  terms  of  a  numerical  value,  and  (3)  the 
reliability  and  duplicability  of  the  readings  obtained.  The  error 
due  to  skin  formation  is  however  not  overcome. 

In  1909  a  patent  was  granted  to  E.  S.  Smith3  for  a  glue-tester 
which  differs  materially  from  any  hitherto  proposed.  A  sketch 
is  shown  in  Fig.  69.  In  principle,  it  measures  the  hydrostatic 
pressure  necessary  to  force  a  rubber  diaphragm  downward  into  a 
jelly  a  stipulated  amount.  It  consists  of  a  thistle-tube  containing 
water,  the  mouth  of  which  is  covered  with  a  thin  rubber  dia- 
phragm, and  placed  downward  upon  the  surface  of  a  jelly  con- 
tained in  a  tumbler.  It  is  connected  by  a  T  tube,  on  the  one 
side,  to  a  manometer  gage  containing  mercury,  and  on  the  other 

1  SINDALL  and  BACON,  Analist,  39  (1914),  20. 

2  Forest  Products,  Lab.  Technical  Notes,  No.  F  32  (1919). 
3E.  S.  SMITH,  U.  S.  Patent  No.  911,277  (1909). 


376 


GELATIN  AND  GLUE 


to  a  bulb  by  which  pressure  may  be  applied.  On  the  stem  of  the 
thistle-tube  is  placed  a  graduated  scale.  In  making  the  measure- 
ment the  jelly  is  brought  up  against  the  rubber  diaphragm  until 
the  water  stands  at  the  upper  graduation  mark,  and  both  tubes 
opened  to  the  air.  The  system  is  then  closed,  and  pressure 


FIG.  69. — The  apparatus  of  E.  S. 
Smith  for  measuring  jelly  strength. 


FIG.  70. — Hulbert's  jelly  strength 
apparatus. 


applied  by  the  bulb  until  the  water  in  the  thistle-tube  has  been 
forced  down,  by  the  displacement  of  the  jelly,  to  the  lower 
graduation  mark,  and  the  pressure  simultaneously  read  on  the 
manometer  scale. 

A  modification  of  this  instrument  was  suggested  by  Hulbert1 
in  1913. 

His  apparatus  is  shown  in  Fig.  70.  It  consists  of  a  thistle- 
tube,  the  stem  of  which  is  twice  bent,  and  contains  three  bulbs. 
The  two  larger  serve  as  safety  traps,  and  are  of  about  2  cm.  in 


1  E.  HULBERT,  /.  Ind.  Eng.  Chem.,  5  (1913),  235. 


TESTING  OF  GLUE 


377 


diameter.  The  smaller  middle  one  is  graduated  to  contain  1  c.c. 
A  diaphragm  of  thin  rubber  is  stretched  over  the  mouth  of  the 
thistle-tube.  A  few  c.c.  of  water  are  placed  in  the  bulbs  and  the 
thistle-tube  brought  down  upon  a  jelly  until  the  water  just 
reaches  the  upper  mark  in  the  graduated  bulb.  The  further  end 
of  the  tube  is  connected  by  a  4  way  stop-cock  to  (1)  the  air,  (2)  a 
gage  consisting  of  a  U  tube  containing  mercury  and  a  long  open 


Suction 


Manometer 


FIG.  71.— C.  R.  Smith's  jelly  strength  test. 

fine-bore  tube  containing  water,  and  (3)  a  bulb  by  which  air 
may  be  forced  into  the  system.  After  opening  both  parts  of  the 
tube  to  the  air,  the  stop-cock  is  turned  to  connect  the  mano- 
meter, the  tube,  and  the  bulb,  and  air  is  forced  in  until  the  water 
in  the  graduated  bulb  has  dropped  to  the  lower  mark.  The 
reading  of  the  column  of  water  in  the  manometer  is  now  noted 
as  the  measure  of  the  jelly  consistency. 

W.  H.  Low1  has  reported  that,  in  the  hands  of  a  careful  worker, 
the  original  apparatus  of  Smith,  with  a  few  minor  modifications, 
is  capable  of  giving  concordent  results,  and  is  more  suitable  for 
general  work  than  the  modification  of  Hulbert. 

An  altogether  different  principle  for  measuring  jelly  consistency 
has  been  suggested  by  C.  R.  Smith,2  and  is  shown  in  Fig.  71. 
A  glass  funnel  80  mm.  across  the  top,  with  a  short  stem  accurately 
formed  at  a  60°  angle  is  used.  One  hundred  and  twenty  grams  of 

1  W.  H.  Low,  /.  Ind.  Eng.  Chem.,  12  (1920),  355. 

2  C.  R.  SMITH,  ibid.,  12  (1920),  878. 


378 


GELATIN  AND  GLUE 


mercury  are  poured  into  the  funnel,  which  is  closed  at  the  end, 
giving  a  surface  diameter  of  3  cm.  to  the  mercury.  Fifty  c.c. 
of  the  gelatin  or  glue  solution  are  then  poured  over  the  mercury, 
and  allowed  to  solidify  at  10°C.,  care  being  taken  to  maintain 
the  funnel  in  a  perfectly  vertical  position.  The  mercury  is  then 
drawn  off,  and  suction  applied  to  the  tube  of  the  funnel  to  the 
extent  of  6  dm.  of  water,  measured  by  a  manometer.  A  depres- 
sion in  the  jelly  is  produced  which  is  measured  by  a  micrometer 
depth  gage  to  a  thousandth  of  an  inch.  Smith  has  succeeded 
in  obtaining  some  remarkably  suggestive  results  by  the  use  of 
this  simple  appliance. 


A  Arm   _ 
B  Scale0 
C  Pulleu 
D  Molds 
E  Gelatin 
F  Pendulum 

0  Watertight  Case 
H  Dental  Floss 

1  Scale- Grams 
K  Scale  ° 

L  Worm  Gear 
M  Standard  _ 
N  Counterweights 
0  Celluloid 
P  Overflow 
Q  Outlet 
R  Revolving  Cone 
S  Grooved  Rod 

FIG.  72. — Sheppard's  jelly  strength  testing  machine. 

The  only  instrument  that  has  been. devised  for  measuring  jelly 
consistency  that  may  be  regarded  as  truly  scientific,  and  capable 
of  producing  results  that  are  not  only  relative  but  are  absolute, 
is  the  ingenious  apparatus  developed  by  S.  E.  Sheppard  and  his 
colaborators1  at  the  laboratories  of  the  Eastman  Kodak  Company. 
They  report  that  "in  seeking  for  correlation  between  viscosity 
coefficients  and  elasticity  coefficients  or  moduli,  it  is  desirable 
that  the  elastic  values,  e.g.,  limit  of  elasticity  and  tensile  strength, 
should  be  obtained  for  pure  shear.  This  condition  is  secured  by 
submitting  cylinders  of  the  material  to  be  tested  (the  jelly)  to 


1  SHEPPARD,  SWEET  and  SCOTT,  J.  Ind.  Eng.  Chem.,  12  (1920),  1007; 
Am.  Chem.  Soc.,  43  (1921),  539. 


,7. 


TESTING  OF  GLUE  379 

torsional  stress."     A  sketch  of  the  instrument  is  shown  in  Fig. 
72,  and  is  self  explanatory. 

The  gelatin  or  glue  is  cast  in  a  cylindrical  mold  having  a  split 
jacket,  which  is  removed  after  the  cylinder  has  been  fixed  in 
position  in  the  instrument.  The  grips  are  inserted  as  a  part  of 
the  mold.  The  gelatin  solution  is  poured  in  at  a  temperature  of 
40°C.,  and  allowed  to  chill  for  3  hours  at  zero  degrees.  The 
entire  mold  is  then  placed  in  the  instrument,  the  grips  clamped  in, 
the  sides  of  the  mold  removed,  the  scales  all  set  at  zero,  and  the 
base  of  the  cylinder  rotated  at  a  constant  speed.  The  upper 
part  of  the  cylinder  is  also  free  to  rotate,  but  this  upper  rotation 
is  opposed  by  a  weight,  in  the  form  of  a  weighted  arm  moving 
in  an  arc,  causing  a  constantly  increasing  weight  for  each  incre- 
ment of  rotation  of  the  upper  end  of  the  cylinder.  At  the  point 
where  the  cylinder  of  jelly  breaks,  the  reading  on  this  arc  is  taken 
as  the  breaking  load,  and  the  difference  between  the  rotation  of 
the  lower  and  the  upper  ends  of  the  cylinder,  measured  by  two 
scales,  is  the  torsional  "twist"  which  the  jelly  has  sustained. 
The  break  should  be  in  the  form  of  a  hectical  cleavage,  at  a  45° 
angle,  extending  from  the  base  to  the  upper  end  of  the  column, 
with  no  sign  of  imperfect  adhesion.  If  the  break  is  imperfect 
the  test  should  be  rejected.  For  investigational  purposes  it 
seems  desirable  to  use  the  expression 

Breaking  load  X  per  cent  twist 
Cross-section  area 

as  the  measure  of  the  jelly  strength,  rather  than  the  breaking 
load  alone. 

Oakes1  has  recently  described  the  Schweizer  jelly  testing 
apparatus  of  the  United  Chemical  and  Organic  Products  Co., 
of  Chicago.  This  is  somewhat  similar  to  the  Scott  apparatus 
previously  described.  It  consists,  as  shown  in  the  accompanying 
photograph,  of  a  balance,  in  one  pan  of  which  is  placed  an  empty, 
beaker,  and  to  the  bottom  of  which  is  soldered  a  blunt  plunger. 
These  are  counterpoised  so  that  the  pointer  rests  at  zero.  The 
jelly  in  a  tumbler  is  brought  into  contact  with  the  plunger,  and 
water  then  allowed  to  run  at  a  constant  slow  rate  into  the  beaker 
until  the  pointer  has  reached  an  arbitrarily  fixed  deflection, 
when,  by  an  electrical  contact,  a  light  or  bell  announces  the 
selected  deflection. 

1  E.  T.  OAKES,  62nd  Meeting,  Am.  Chem.  Soc.,  N.  Y.,  Sept.  6-10,  1921. 


380 


GELATIN  AND  GLUE 
2.   THE  VISCOSITY 


A  very  large  number  of  instruments  have  been  described  for 
the  measurement  of  viscosity,  but  for  use  with  solutions  of 
gelatin  and  glue  only  a  few  of  these  are  applicable.  They  may  be 
divided  into  two  groups:  (1)  those  that  measure  the  viscosity 


FIG.  73. — The  Schweizer  jelly  testing  apparatus. 

by  permitting  a  given  amount  of  the  liquid  to  flow  through  a 
capillary  tube  of  stated  dimensions,  and  (2)  those  in  which  the 
viscosity  is  measured  by  the  resistance  offered  by  the  gelatin  or 
glue  to  the  movement  within  it  of  a  disk,  a  cylinder,  or  a  sphere. 
The  type  of  instrument  which  is  used  in  any  given  case  will 
depend  upon  the  fluidity  of  the  solutions  which  are  to  be  meas- 
ured, and  upon  the  degree  of  accuracy  which  it  is  desired  to 
attain.  For  highly  accurate  work  upon  very  dilute  solutions, 
the  Ostwald  or  Bingham  tubes  are  most  satisfactory;  for  more 


TESTING  OF  GLUE  381 

concentrated  solutions  the  Couette  instrument  and  the  rolling 
sphere  device  of  Flowers,  have  given  good  results.  Where 
rapidity  of  operation  is  necessary,  and  only  approximate  results 
are  desired,  the  various  modifications  of  the  pipette  have  been 
most  widely  used,  but  the  MacMichael  viscosimeter  has  been 
found  to  be  very  satisfactory.  A  few  of  the  more  important 
types  of  viscosimeter  are  described  below,  following  the  develop- 
ment of  the  laws  pertaining  to  the  resistance  of  fluid  motion. 

The  Theory  of  Viscous  Flow.  —  The  first  laws  defining  the 
resistance  of  liquid  flow,  which  were  based  upon  careful  experi- 
mentation, were  developed  by  Coulomb1  in  1784.  He  employed 
a  cylinder,  and  later  a  disk,  oscillating  in  the  liquid,  and  sus- 
pended by  a  fine  wire,  and  showed  that  the  resistance  of  a  liquid 
to  a  body  moving  in  it  was  proportional  to  the  surface  area  of  the 
body  and  the  velocity  of  the  latter.  He  further  showed  that 
fluid  resistance  consisted  of  two  components:  internal  friction 
and  inertia.  The  resistance  due  to  inertia  was  pointed  out  to  be 
proportional  to  the  density.  This  resulted  in  the  mathematical 
expression  :2 


where  R%  is  the  total  resistance  offered,  K2  is  a  constant,  7  the 
density  of  the  liquid,  and  V  the  velocity. 

Poiseuille,3  in  a  series  of  classic  experiments  upon  the  flow  of 
liquids  through  capillary  tubes,  demonstrated  that  the  flow  of 
liquid  varied  directly  as  the  pressure,  directly  as  the  time,  directly 
as  the  fourth  power  of  the  diameter  of  the  capillary,  and  inversely 
as  the  length  of  the  capillary,  or 


where  Q  is  the  outflow  in  cubic  centimeters;  K,  a  constant;  p,  the 
pressure  in  dynes  per  sq.  cm.;  t,  the  time  of  outflow  in  seconds;  d, 
the  diameter  of  the  capillary  in  centimeters;  and  L,  the  length  of 
the  capillary  in  centimeters. 

The  value  of  K  may  be  calculated  by  applying  the  equations  of 
Coulomb,  which  give,  according  to  Brillouin:4 

1  COULOMB,   Historic  de  1'Academie,  Soc.  Francaise  Phys.  (1784). 

2  Cited  from  Flowers,  Proc.  Am.  Soc.  for  Test.  Mat.,  14  (1914),  565. 

3  POISEUILLE,  Acad.  Sci.  Rec.  Sav.  Strangers  (1842-1846). 

4  BRILLOUIN,  "  Viscosite  des  Liquides  et  des  Gaz.,"  Paris  (1970),  vol.  1,  p. 
56-135. 


IW,(L  +  *\Q 


382  GELATIN  AND  GLUE 

0-      ^^> 
"       1287?    L 

where  TT  =  3.14159,  and  r;  is  the  viscosity  of  the  liquid. 

By  further  applying  the  corrections  of  Couette1  for  the  end 
effects  due  to  acceleration,  and  of  Brillouin  for  the  resistance  due 
to  the  converging  stream  lines  at  the  entrance  to  the  capillary, 
the  following  equation  for  viscosity  is  obtained: 


rj   = 


In  1851  Stokes2  developed  the  law  which  bears  his  name,  which 
defines  the  relations  between  the  velocity  and  resistance  to  the 
movement  of  a  sphere  falling  through  a  viscous  fluid.  His 
equation  is  written: 


- 

~  18    f,      ° 

where  V  is  the  limiting  velocity;  d  the  diameter  of  the  sphere  in 
centimeters;  g  the  acceleration  of  gravity  in  dynes;  ?)  the  absolute 
viscosity  in  dynes;  7S  the  density  of  the  sphere  in  grams  per 
c.c.;  and  ym  the  density  of  the  medium  in  grams  per  c.c. 

Reynolds3  has  shown  that  Poiseuille's  law  which  states  that  the 
outflow  of  liquid  through  a  capillary  tube  varies  directly  as  the 
pressure,  was  applicable  only  for  a  certain  range  of  pressures. 
If  the  pressure  applied  is  gradually  increased,  a  point  will  be 
reached  where  the  flow  quite  suddenly  becomes  turbulent  and 
filled  with  eddies.  As  the  pressure  is  increased  above  this  point, 
still  another  critical  point  is  eventually  reached  above  which  the 
flow  is  again  regular,  but  the  increase  in  flow  with  increase  in 
pressure  is  less  than  it  is  below  the  first  appearance  of  turbulent 
flow.  At  pressures  between  the  two  critical  points  the  variation 
in  outflow  with  pressure  is  irregular:  The  critical  velocity  at 
which  turbulence  begins  is  given  by  the  equation: 

1  COUETTE,  Ann.  Chem.  et  Phys.,  6  (1890),  21;  502. 

2  STOKES,  Mathematical  and  Physical  papers,  Cambridge,  Univ.,  3  (1880), 
55. 

3  REYNOLDS,  Trans.  Royal  Soc.,  London,  174  (1883),  935;  186   A    (1895), 
123. 


TESTING  OF  GLUE  383 


Vs  =  2,000  -       or          =  2,000, 
yd         yd 

where  Fs  is  the  velocity  in  meters  per  second;  77,  the  absolute 
viscosity  in  dynes;  7,  the  density  in  grams  per  c.c.;  and  d,  the 

diameter  of  the  tube  in  centimeters.     The  value  —  V  is  known  as 

Reynolds'  criterion.  It  has  been  shown  however  by  Flowers1 
and  by  Hayes  and  Lewis2  that  the  value  2,000  is  too  high  for 
short  tube  viscosimeters,  and  that  the  flow  of  water  at  20°C. 
through  the  tubes  of  technical  viscosimeters,  as  the  Engler, 
Saybolt,  and  Redwood  type,  is  such  that  the  critical  velocity  is 
exceeded,  and  turbulent  flow  results.  It  becomes  necessary, 
therefore,  in  using  these  instruments,  or  others  of  the  short 
capillary  type,  that  some  material  more  viscous  than  water 
must  be  used  for  standardization,  and  that  only  relatively 
viscous  liquids  may  be  accurately  determined  by  them. 

Bingham3  states  the  most  generally  accepted  formula  for  the 
viscous  flow  of  a  liquid  through  a  capillary  of  uniform  circular 
cross-section  to  be: 

irgr*pt  mnpv 


X) 
in  which  17  =  the  viscosity  in  absolute  c.g.s.  units; 

TT  =  3.1416; 

g  =  the  gravitation  constant  of  the  locality; 

r  =  the  radius  of  the  capillary  in  centimeters; 

p  =  the  pressure  in  grams  per  sq.  cm.; 

t  =  the  time  in  seconds; 

v  =  the  volume  in  cubic  centimeters; 

I  =  the  length  of  the  capillary  in  centimeters; 

X  =  the  correction  to  capillary  length  for  "end  effect;" 

m  =  a  coefficient,  probably  1.12; 

n  =  the  number  of  capillaries  in  use  ;  and 

p  =  the  density  of  the  liquid. 

Deeley  and  Parr4   and   Herschel5  have   suggested   that  the 

1  FLOWERS,  loc.  cit. 

2  HAYES  and  LEWIS,  J.  Am.  Soc.  Mech.  Eng.,  38  (1916),  629. 

3  E.  C.  BINGHAM,  Proc  Am.  Soc.  for  Test.  Mat.,  18  (1918),  373. 

4  DEELEY  and  PARR,  Phil.  Mag.,  26  (1913),  87. 

5  W.  H.  HERSCHEL,  U.  S.  Bureau  of  Standards,  Tech.  Paper,  No.  112 
(1918). 


384  GELATIN  AND  GLUE 

absolute  viscosity  in  c.g.s.  units  be  designated  by  the  term  poise, 
and  that  this  viscosity  divided  by  the  density  be  known  as  the 
kinematic  viscosity.  Herschel  has  further  shown  that  the  equa- 
tion of  Bingham  given  above  may  be  reduced,  for  all  viscosi- 
meters  of  the  capillary  tube  type,  to  the  general  form  : 


in  which  /*  is  the  viscosity  in  poises;  7  the  density  in  grams  per 
c.c.;  t,  the  time  of  discharge  in  seconds;  and  A  and  B  are  instru- 
mental constants  which  may  be  found  either  by  theory  or  experi- 
ment. The  values  of  these  constants  which  have  been  reported 
by  Herschel  for  different  instruments  are  as  follows: 


A 

B 

Saybolt  '.  
Engler 

0.00220 
0  00147 

1.80 
3  74 

Redwood  

0.00260 

1.561 

The  Capillary  Tube  Type  of  Viscosimeter.  The  Saybolt, 
Engler  and  Redwood  Instruments. — The  saybolt,  Engler,  and 
Redwood  viscosimeters  are  three  technical  instruments  of  very 
much  the  same  type  which  were  developed  in  the  United  States, 
Germany,  and  England,  respectively,  at  about  the  same  time, 
and  before  the  relations  of  viscous  flow  had  been  thoroughly 
developed  mathematically  and  understood.  In  consequence 
they  are  not  altogether  scientific  in  their  design.  They  consist, 
as  shown  in  the  accompanying  illustrations,  of  a  reservoir  in 
which  the  liquid  to  be  tested  is  placed,  enclosed  in  a  water  or  oil 
bath  for  maintaining  a  definite  temperature.  From  the  center 
of  the  reservoir  a  short  capillary  tube  about  2  cm.  in  length 
permits  the  outflow  of  the  liquid.  The  water  bath  of  the  Engler 
and  Redwood  instruments  is  inadequate  and  requires  frequent 
attention  in  maintaining  a  constant  temperature.  When 
duplicate  determinations  are  desired,  the  process  becomes  very 
time-consuming,  for  the  liquid  must  again  be  heated  to  the 
necessary  temperature  before  runrfing  through.  It  is  not  easy 
to  clean  thoroughly  the  capillary  tubes  in  the  Engler  and  Red- 
wood instruments,  on  account  of  their  shape  and  position. 


TESTING  OF  GLUE 


385 


There  has  appeared  a  considerable  amount  of  opposition  to 
the  several  types  of  commercial  viscosimeter,  which  has  been- 
based  for  the  most  part  upon  theoretical  considerations  that  have 
shown  the  unreliability  of  those  instruments  as  the  means  of 


FIG.   74. — The   Saybolt  universal   viscosimeter.     (Kindness  of  The  Arthur  H. 
Thomas  Company  of  Philadelphia.) 

measuring  actual  viscosity.     Among  the  objections  raised  by 
Bingham1  are  the  following: 

1.  The  formula  is  valid  only  when  the  velocity  is  below  the 
Reynolds'  criterion. 

2.  Since  the  capillary  is  very  short — 2.0  cm. — the  end  correc- 
tion is  appreciable  and  variable. 

3.  The   time   of   outflow   is   not   strictly   proportional  to  the 
viscosity  alone,  but  varies  inversely  as  the  density,  so  that  the 
common  practice  of  expressing  viscosity  as  seconds  of  outflow  is 
incorrect   and   misleading.     This   applies   to   glue   and    gelatin 

1  E.  C.  BINGHAM,  loc.  tit. 

25 


386  GELATIN  AND  GLUE 

measurements  as  well  as  to  any  other  commercial  substance. 
Not  only  to  avoid  confusion,  but  for  the  sake  of  accuracy,  the 
viscosity  should  be  expressed  in  absolute  units,  or  poises. 


FIG.  75. — The  Engler  viscosimeter.  FIG.  76. — The  Redwood  viscosimeter. 
(Kindness  of  The  Arthur  H.  Thomas  (Kindness  of  The  Arthur  H.  Thomas 
Company  of  Philadelphia.)  Company  of  Philadelphia. )- 

4.  In  very  viscous  liquids  the  outflow,  especially  near  the  end 
of  the  operation,  may  be  in  the  form  of  drops  instead  of  a  steady 
stream.     This  drop  formation  results  in  a  surface  tension  effect 
which  tends  strongly  to  repress  the  flow. 

5.  The  instruments  lack  a  proper  temperature  control.     The 
bath  is  small  and  inadequate,  and  the  volume  of  liquid  large  so 
that  it  takes  much  time  for  it  to  reach  the  proper  temperature, 
and  it  is  difficult  to  maintain  the  correct  temperature  throughout. 
Duplicate    determinations    are    very    time-consuming,    as    the 


TESTING  OF  GLUE 


387 


chilled  liquid  must  be  again  heated  before  another  run  can  be 
made. 

6.  The  variation  in  pressure  obtainable  is  small  while  the  varia- 
tion in  viscosity  may  be  very  large. 


FIG.  77. — Kahrs'  viscosimeter  glue  pot. 


The  Pipette  Viscosimeter s. — Most  of  the  viscosimeters  that  have 
been  proposed  for  use  by  the  glue  trade  are  of  the  short  capillary 
tube  type.  Fels1  in  1897  proposed  that  the  Engler  instrument 

1  J.  FELS,  Chem.  Ztg.,  21  (1897),  56;  25  (1901),  23. 


388 


GELATIN  AND  GLUE 


be  adopted  as  a  standard  for  glue  evaluation  in  Germany.  He 
first  employed  a  temperature  of  30°  but  in  1901  raised  it  to  35°C. 
In  1905  Kahrs1  suggested  a  crude  combination  instrument  by 
which  could  be  measured  the  time  of  outflow,  through  a  small 
stop  cock,  of  about  9  oz.  of  glue  solution.  From  30  to  60  seconds 


V 


FIG.    78. — Fernbach's   viscosimeter.      (From   Fernbach's    "Glues   and  Gelatine," 
D.  Van  Nostrand  Co.,  New  York,  1907.) 


sufficed  for  the  measurement.  In  1906  Fernbach2  proposed  the 
use  of  an  ordinary  50  c.c.  pipette  which  could  be  enclosed  in  a 
water  bath  if  desired.  Alexander,3  in  the  same  year,  suggested  a 
very  similar  instrument,  but  urged  that  the  dimensions  should  be 
carefully  specified  in  order  that  " standard"  readings  might  be 

1  F.  KAHRS,  "Glue  Handling,"  East  Haddan,  Conn.  (1905-6),  58. 

2  FERNBACH,  " Glues  and  Gelatin,"  New  York  (1906),  38. 

3  J.  ALEXANDER,  /.  Soc.  Chem.  Ind.,  25  (1906),  158. 


TESTING  OF  GLUE 


389 


obtained.     The  United  States  Forest  Service1  has  recommended 
the  Engler  instrument  as  recently  as  1920. 

It  will  be  immediately  obvious  that  the  objections  which  have 
been  enumerated  for  the  Saybolt,  Engler,  and  Redwood  instru- 
ments upon  theoretical  grounds  apply  also,  and  to  a  multiplied 


FIG.  79. — The  Ostwald  viscosimeter. 


F  (O       r 
FIG.  80. — The  Bingham  viscosimeter. 


degree,  to  the  pipette  types  mentioned  above.  But  of  even 
greater  moment,  as  it  applies  to  the  glue  trade — the  manu- 
facturers, jobbers,  and  users  of  glue  and  gelatin — must  be  men- 
tioned the  impossibility  of  a  uniform  expression  for  viscosity, 
intelligible  to  everyone,  so  long  as  the  present  system  prevails, 
of  each  tester  making  his  viscosity  measurements  with  tubes  of 
his  own  specifications.  Most  of  the  glue  trade  employ  a  pipette 
type  of  instrument;  but  the  dimensions,  the  volume  of  outflow, 
and  the  method  of  expressing  the  results,  are  almost  as  diverse  as 
the  number  of  companies  or  individuals  who  make  the  test. 
Some  one  standard  instrument  should  by  all  means  be  employed, 
and  the  viscosity  expressed  always  in  absolute  units. 

The  Ostwald  and  Bingham  Viscosimeter s. — Most  of  the  objec- 
tions that  have  been  raised  against  the  use  of  the  short  capillary 

1  National  Advisory  Committee  for  Aeronautics,  Report  66  (1920),  26. 


390 


GELATIN  AND  GLUE 


tube  type  of  viscosimeter  are  eliminated,  in  the  case  of  not  too 
viscous  liquids,  in  the  simple  classic  instrument  of  Win.  Ostwald,1 
shown  in  Fig.  79.  This  consists  of  a  tube,  one  side  of  which 
is  about  10  mm.  in  diameter.  The  other  side  consists  of  a 


FIG.  81. — The  Rideal-Slotte 
viscosimeter. 


FIG.  82. — The  viscosimeter 
of  Baume  and  Vigneron. 


capillary  tube  about  8  cm.  in  length  at  either  end  of  which  is 
blown  a  bulb.  The  upper  bulb  holds  about  2  c.c.,  and  a  gradua- 
tion mark  is  placed  on  a  capillary  tube  just  above  and  below  this 
bulb.  In  making  the  measurement,  the  tube  is  immersed  in  a 
constant  temperature  bath,  about  3^  c.c.  of  liquid  which  has 
reached  the  desired  temperature  is.  pipetted  into  the  wide  tube 
and,  by  applying  suction,  this  is  drawn  up  through  the  capillary 
tube  until  the  upper  bulb  is  filled,  and  the  liquid  has  passed 
the  upper  graduation  mark.  The  suction  is  then  released  and,  by 
the  use  of  a  stop-watch,  the  time  noted  between  the  passing 

1  WM.  OSTWALD,  " Physico-Chemical  Measurements,"  translated  by 
Walker,  1894.  See  also  FINDLAY  "Practical  Physical  Chemistry"  (1911), 
72. 


TESTING  OF  GLUE  391 

of  the  liquid  from  the  upper  to  the  lower  graduation  mark.  For 
more  accurately  recording  the  time  of  outflow  of  this  instrument, 
Rankin  and  Taylor1  have  modified  the  instrument  by  the 
attachment  of  connections  to  an  electromagnetic  clock  which 
automatically  records  the  time. 

Bingham2  has  further  improved  upon  the  Ostwald  model  by 
devising  an  instrument  such  that,  by  applying  pressure  to  either 
side,  the  measurements  may  be  repeated,  under  varying  pressure 
control,  at  will,  see  Fig.  80. 


Opening  \          tClamp 
\ 


Solenoid  !        '''Spring 

<5 oft  Iron  Core 

FIG.  83. — Cope's  centrifugal  viscosimeter. 

Rideal3  employed  a  capillary  tube  above  which  was  sealed  a 
larger  tube  having  three  bulbs  blown  in  it  as  shown  in  the  dia- 
gram. These  were  surrounded  with  a  water  jacket.  The  glue 
solution  which  was  of  1  per  cent  concentration  at  20°C.  was 
drawn  up  into  the  upper  bulb  and  allowed  to  run  out  slowly,  by 
placing  the  finger  lightly  at  the  upper  opening  of  the  tube,  until 
the  level  had  reached  the  constriction  between  the  upper  and 
middle  bulbs.  The  finger  was  then  removed  and  the  time  taken 
for  the  solution  to  run  out  to  the  lower  constriction  of  the  middle 
bulb  measured  by  a  stop  watch.  In  case  the  liquid  was  very 
viscous  the  time  could  be  shortened  by  applying  a  definite  reduc- 
tion of  pressure  to  the  lower  end  of  the  tube  as  shown,  and  sub- 
sequently correcting  for  this  reduced  pressure. 

Baume  and  Vigneron4  have  suggested  an  instrument  (see  Fig.  82) 
by  which  any  form  of  viscosimeter  tube  of  the  capillary  type  is 
immersed  at  one  end  of  the  liquid  under  examination.  This  is 
surrounded  by  a  thermostatic  jacket  in  which  a  liquid  of  a  con- 
stant boiling  point  is  kept  boiling.  After  allowing  15  minutes  for 
the  contents  to  reach  a  constant  temperature,  the  solution  is 

1  RANKIN  and  TAYLOR,  Trans.  Roy.  Soc.  Edinburg,  45  (1905-7),  397. 

2  E,   C.   BlNGHAM,  IOC.   Clt. 

3  S.  RIDEAL  and  W.  YOULE,  J.  Soc.  Chem.  Ind.,  10  (1891),  615;  S.  RIDEAL, 
"Glues  and  Glue  Testing"  (1901),  130. 

4  BAUME  and  VIGNERON,  Ann.  Chim.  anal,  appl.,  1  (1919),  379. 


392  GELATIN  AND  GLUE 

forced  up  into  the  viscosimeter  tube  by  compressing  the  bulb  con- 
nected at  T.  The  time  in  seconds  taken  by  the  solution  in  falling 
between  two  points,  a  and  6,  marked  off  on  the  tube  is  taken  as 
the  viscosity. 

Cope1  has  suggested  a  device  by  which  a  centrifugal  force  may 
be  applied  to  the  liquid,  and  varied  at  will.  This  gives  a  con- 
stant pressure  upon  the  liquid,  and  by  an  electromagnetic 
arrangement  the  outlet  to  the  capillary  may  be  opened  or 
closed  while  the  centrifuge  is  in  operation,  thus  permitting  of  an 
accurate  control  of  the  time  allowed  for  a  given  test.  The 
weight  of  liquid  transpiring  in  the  given  time  and  at  the  given 
pressure,  is  used  as  the  measure  of  viscosity,  see  Fig.  83. 

The  Rising  Bubble  and  Falling  Sphere  Types  of  Viscosi- 
meter. The  "Bubble"  Method. — A  method  known  as  the  bubble 
method  for  the  determination  of  relative  viscosities  has  occasion- 
ally been  used.  C.  R.  Smith2  used  a  bubble  of  air  about  4.5  mm. 
in  diameter  admitted  at  the  bottom  of  a  polariscope  tube,  and 
noted  the  rate  of  ascent.  Faust,3  in  a  study  of  this  method, 
reported  that  the  time  of  ascent  of  a  bubble  of  air  was  independ- 
ent of  its  size,  but  that  the  tube  must  be  not  less  than  18  to  24 
mm.  in  diameter  in  order  that  the  calculated  and  experimental 
results  should  be  in  harmony. 

The  Falling  Sphere  Method. — The  basis  of  the  measurement  of 
viscosity,  by  noting  the  velocity  of  fall  of  a  sphere  through  a 
liquid,  was  established  by  Stokes4  more  than  seventy  years  ago, 
but  the  application  of  this  principle  to  commercial  practice  has 
developed  only  recently.  In  1907  Ladenburg5  suggested  the 
first  practicable  application,  and  carefully  defined  the  conditions 
and  the  constants  involved.  Ladenburg's  work  was  followed 
shortly  by  a  treatise  by  Arnold6  upon  the  same  subject.  In  1917 
Sheppard7  made  use  of  this  method  for  the  determination  of  the 
viscosity  of  very  viscous  solutions  of  nitrocellulose,  and  more 
recently  has  used  it  with  gelatin  sols.  He  employed  for  the 
purpose  steel  ball  bearings  of  J£,  %  and  J^  inch  diameter  and 
permitted  them  to  drop  axially  into  the  center  of  a  tube  contain- 

1  W.  C.  COPE,  J.  Ind.  Eng.  Chem.,  9  (1917),  1046. 

2  C.  R.  SMITH,  J.  Am.  Chem.  Soc.,  41  (1919),  135. 

3  O.  FAUST,  Z.  physik.  Chem.,  93  (1919),  758. 

4  STOKES,  op.  cit. 

5  R.  LADENBURG,  Ann.  physik.,  22  (1907),  287;  23  (1907),  447. 

6  H.  D.  ARNOLD,  Phil  Mag.,  22  (1911),  755. 

7S.  E.  SHEPPARD,  J.  Ind.  Eng.  Chem.,  9  (1917),  523. 


TESTING  OF  GLUE 


393 


ing  the  liquid,  40  cm.  in  height,  standing  in  a  water  bath  main- 
tained at  20°C.  The  time  occupied  by  the  sphere  in  passing 
from  one  graduation  mark  on  the  cylinder  to  another  lower  down 
was  noted  by  a  stop-watch,  and  the  absolute  viscosity  calculated 
by  Stocks'  law.  This  held  sufficiently  closely  for  industrial 
purposes  provided  the  ratio  of  the  diameters  of  the  tube  to  the 
ball  was  at  least  10. 


FIG.  84.— The  Gib- 
son -  Jacobs  falling 
sphere  viscosimeter. 


FIG.  85. — Flowers'  rolling  sphere  viscosimeter. 


Gibson  and  Jacobs1  have  used  the  same  method  also  upon 
solutions  of  nitrocellulose,  but  the  equations  which  they  have 
developed  differ  from  those  of  Sheppard.  They  contend  that 
the  diameter  of  the  sphere  should  be  much  smaller  than  those 
used  by  Sheppard,  and  recommend  a  size  of  0.15  centimeter  in 
diameter.  They  used  a  tube  (Fig.  84)  29  qm.  in  height  and  2  cm. 
in  diameter.  The  measurements  were  made  in  the  same  manner 
as  those  of  Sheppard,  but  a  different  form  of  release  for  the  sphere 
is  suggested. 

Fischer2  has  applied  this  method  to  the  determination  of  the 
viscosity  of  glues. 


1  GIBSON  and  JACOBS,  /.  Chem.  Soc.,  117  (1920),  473. 

2  R.  FISCHER,  Chem.  Ztg.,  44  (1920),  622. 


394  GELATIN  AND  GLUE 

The  Rolling  Sphere  Method. — Flowers1  has  conceived  of  a  new 
method  for  measuring  viscosity  by  rolling  a  sphere  down  an 
inclined  tube  filled  with  the  liquid  under  examination.  Spheres  of 
different  density,  as  normal  Jena  glass,  hard  steel,  and  platinum, 
were  used  depending  upon  the  relative  density  and  viscosity 
of  the  liquid.  He  finds  that  "the  resistance  may  be  very  great 
indeed,  if  the  sphere  nearly  fills  the  tube  bore;  that  it  decreases 
at  first  very  rapidly  as  the  bore  of  the  tube  is  increased,  and  then 
more  slowly.  It  is  shown  that  for  a  given  ratio  of  diameters  of 
sphere  and  bore  of  tube,  the  velocity  attained  increases  more 
rapidly  than  the  sine  of  the  slope  angle  of  the  slanted  tube,  but 
the  rate  of  change  of  velocity  is  the  same  for  all  viscosities  above 
a  certain  minimum  value,  and  that  at  any  given  slope  the  time 
to  roll  a  given  distance*  increases  directly  with  the  viscosity. 
He  finds  the  rolling  sphere  type  of  viscosimeter  entirely  practi- 
cable for  commercial  purposes,  and  that  it  may  be  used  for  the 
comparison  of  absolute  viscosity  over  a  wide  range  with  a 
moderate  error. 

Flowers  found  that  the  absolute  viscosity  could  very  easily 
be  calculated  by  the  simple  equation: 

rj  =  KKJ, 

where  ij  is  the  absolute  viscosity  in  dynes;  K  a  constant  depend- 
ent upon  the  diameters  of  the  sphere  and  the  tube,  the  slope, 
and  the  roll  distance ;  K2,  the  correcting  factor  due  to  the  density 
of  the  liquid  tested;  and  t,  the  time  of  roll  in  seconds. 

The  Torsional  Type  of  Viscosimeter.  Oscillating  Cylinder 
or  Disk  Viscosimeter. — In  1893  Doolittle2  revived  a  modification 
of  the  instrument  employed  by  Coulomb  in  1784.  A  hollow 
cylinder  was  suspended  by  a  wire,  about  8  inches  in  length,  into 
the  liquid  to  be  tested,  which  was  contained  in  a  brass  cup. 
By  means  of  a  suitable  device  this  cylinder  was  rotated  one 
complete  revolution  of  360°,  and  released.  Due  to  the  torque  on 
the  wire,  the  cylinder  would  revolve  back  to  the  zero  point  and 
continue  in  the  opposite  direction  a  certain  distance  which  was 
dependent  upon  the  viscosity  of  the  liquid.  Several  oscillations 
were  permitted,  and  the  average  difference  between  succeeding 
turns  taken  as  a  measure  of  the  viscosity.  Garrett,3  in  1903, 

1  A.  E.  FLOWERS,  Proc.  Am.  Soc.  for  Test.  Mat.,  14  (1914),  565. 

2  DOOLITTLE,  J.  Am.  Chem.  Soc.,  16  (1893),  173. 

3  GARRETT,  Phil  Mag.  (6),  6  (1903),  374. 


TESTING  OF  GLUE 


395 


used  a  similar  device  for  determining  the  viscosity  of  gelatin 
solutions,  but  employed  a  disk  instead  of  a  hollow  cylinder, 
submerged  in  the  liquid. 

The  Couette  Viscosimeter.— The  Couette,  Stormer,  and  the 
MacMichael  viscosimeters  may  be  regarded  as  special  modifica- 
tions of  these  older  oscillating  types,  but  they  differ  from  the 
latter  in  that  the  shear  is  a  constant  one,  dependent  upon  a 
rotation  of  the  liquid,  rather  than  one  of  oscillation. 

n 


c 

4 

B 

6 

f 

1 

I 

—  * 

A 

^  . 

a 

mppip 

•—  •    ?i 

FIG.  86.  —  The 
Doolittle  torsion 
viscosimeter. 


FIG.  87. — Hatschek's  modification  of 
the  Couette  viscosimeter. 


The  Couette  instrument1  consists  of  a  hollow  cylinder  sus- 
pended by  a  fine  wire  into  a  slightly  larger  one,  coaxial  to  it,  in 
which  is  placed  the  liquid  to  be  tested.  The  outer  cylinder  is 
provided  with  a  water  jacket.  This  outer  cylinder  with  its 
water  jacket  is  caused  to  revolve  at  any  desired  speed,  and  the 
deflection  of  the  suspended  cylinder,  which  soon  becomes  con- 
stant, is  noted.  Hatscheck2  has  improved  the  original  design 

1  COUETTE,  J.  phys.,  9  (1890),  414;  Ann.  Chem.  Phys.,  6  (1890),  21;  502. 

2  E.  HATSCHECK,  Trans.  Faraday  Soc.,  9  (1913),  80. 


396 


GELATIN  AND  GLUE 


of  the  instrument,  see  Fig.  87,  and  made  it  adaptable  for  highly 
accurate  measurements.  He  uses  the  product  of  the  deflection 
times  seconds  per  revolution,  as  the  viscosity,  and  reports 
highly  satisfactory  results.  It  should  be  pointed  out  that  in 
this  apparatus  the  clearance  between  the  suspended  and  the 
rotating  cylinder  is  very  slight,  and  that  the  velocity  of  rotation 
is  very  low,  e.g.,  from  3  to  96  seconds  being  taken  for  one  revolu- 
tion. In  these  respects  it  differs  greatly  from  the  MacMichael 
instrument. 

The  Stormer  Viscosimeter.—The  Stormer  viscosimeter,  Fig.  88, 

is  an  instrument  in  which  a 
cylinder  is  caused  to  rotate  in 
the  liquid  under  examination 
through  the  influence  of  a  weight. 
As  the  rotation  of  the  cylinder 
under  the  influence  of  any  given 
weight  is  assumed  to  be  pro- 
portional to  the  viscosity  of  the 
liquid,  the  time  in  seconds 
taken  for  100  revolutions  of  the 
cylinder  is  taken  as  the  measure 
of  viscosity.  If  water  is  taken 
as  unity  then  the  quotient 
obtained  by  dividing  the  time 
taken  in  any  given  liquid  by  the 
time  taken  in  water  gives  the 
relative  viscosity  in  terms  of 
water.  For  very  viscous  liquids 
a  heavier  weight  is  used,  and 
the  viscosity  of  some  "intermediate"  liquid  used  for  calculating 
back  to  water. 

Rogers  and  Sabin1  have  recommended  the  Stormer  instrument 
as  being  especially  well  adapted  for  the  comparison  of  the  vis- 
cosities of  paints,  but  Rigg  and  Carpenter2  have  pointed  out- 
several  sources  of  probable  error  in  its  use.  First,  the  friction  of 
the  instrument  must  be  measured  and  a  correction  applied. 
This  may  vary  from  day  to  day.  Second,  in  order  to  secure 
comparable  figures  for  different  liquids,  a  constant  weight  must 


FIG.  88. — The  Stormer  viscosi- 
meter. (Kindness  of  The  Arthur  H. 
Thomas  Company  of  Philadelphia.} 


1  A.  ROGERS  and  A.  SABIN,  J.  Ind.  Eng.  Chem.,  3  (1911),  737. 

2  G.  RIGG  and  J.  CARPENTER,  ibid.,  4  (1912),  901. 


TESTING  OF  GLUE  397 

be  used.  Third,  the  use  of  the  "  intermediate  "  liquid  may  result 
in  an  error  of  over  20  per  cent.  They  deprecate  the  use  of  the 
instrument  except  within  the  narrow  limits  where  only  one  weight 
shall  be  used  throughout. 

Higgins  and  Pitman1  have  recently  applied  the  Stormer 
instrument  to  the  measurement  of  the  viscosity  of  pyroxylin 
solutions  with  apparant  success.  They  compared  several  types 
and  concluded  that  the  Stormer  was  the  most  suitable.  Vis- 
cosities ranging  from  10  times  that  of  water  to  200  times  that  of 
castor  oil  at  20°C.  were  measured  with  no  difficulty.  They  find 
that  the  general  equation  expressing  absolute  viscosity  is  of  the 
type: 

V  =  At  +  B, 

where  V  is  the  viscosity  in  centipoises;  A  and  B  are  instrumental 
constants,  which  in  this  case  vary  as  the  weight  used;  and  t,  the 
corrected  time  for  100  revolutions.  B  is  the  value  of  V  when 

V  —  B 
t  =  0,    and   A  =  — The   following   equations   with   the 

values  for  A  and  B  inserted  were  found  for  the  different  weights 
used: 

150  gram  counterweight,  V  =  4.6£  —  25, 
300  gram  counterweight,  V  =  9.3*  -  30. 

At  values  of  V  less  than  15  the  formula  is  not  linear  and  does  not 
hold.  Above  15  it  is  linear. 

The  MacMichael  Viscosimeter. — The  MacMichael  torsional 
viscosimeter,2  Fig.  89,  is  the  most  recent  of  the  series,  having 
been  first  described  in  1915.  It  consists  of  a  brass  disk,  sus- 
pended by  a  fine  gold-plated  steel  wire,  into  a  brass  cup  which 
contains  the  liquid  under  examination.  The  cup  is  caused  to 
revolve  at  a  constant  speed  by  a  motor,  and  the  friction  of  the 
liquid  causes  the  suspended  disk  also  to  rotate  until  the  tor- 
sional force  in  the  suspending  wire  just  balances  the  viscous  re- 
sistance, and  then  it  remains  in  a  fixed  position.  The  degree  of 
deflection  may  be  read  by  means  of  a  dial  which  is  divided  into 
300°  to  the  circle.  These  are  known  as  MacMichael  degrees,  or 
degrees  M.  The  wires  for  use  with  the  instrument  are  about  10 

1  E.  HIGGINS  and  E.  PITMAN,  ibid.,  12  (1920),  587. 

2R.  F.  MACMICHAEL,  J.  Ind.  Eng.  Chem.,  7  (1915),  961;  U.  S.  Patent 
1,281,024  (1918). 


398  GELATIN  AND  GLUE 

inches  long  and  of  several  sizes.  It  is  inadvisable  to  permit  more 
than  two  complete  revolutions  of  the  disk,  as  the  wire  may  be 
strained  beyond  its  limit  of  elasticity  by  so  doing. 

A  critical  study  of  this  instrument  has  been  made  by  Herschel.1 
He  reports  a  number  of  possible  and  inevitable  sources  of  error. 
The  lines  of  flow  above  and  below  the  disk  are  spiral  instead  of 


FIG.  89. — The  MacMichael  viscosimeter.     (Kindness  of  The  Arthur  H.  Thomas 
Company  of  Philadelphia.) 

circular,  due  to  the  centrifugal  action  of  the  rotating  liquid,  and 
an  error  is  introduced  on  account  of  an  acceleration  of  the  liquid.2 
A  high  speed  results  in  a  marked  turbulence  of  the  liquid,  espe- 
cially if  of  low  viscosity.  The  deflection  of  the  disk  will  be 
influenced,  not  only  by  the  speed  of  the  cup  and  the  viscosity  of 
the  liquid,  but  also  by  the  density  of  the  solution.  For  these 
reasons  it  becomes  impossible  to  employ  a  mathematical  equation, 
which  will  define  the  absolute  viscosity,  calculated  from  the 
dimensions  of  the  instrument. 

1  W.  H.  HERSCHEL,  J.  Ind.  Eng.  Chem.,  12  (1920),  282. 

2  HAYES  and  LEWIS,  J.  Am.  Soc.  Mech.  Eng.,  38  (1916),  626;  1002. 


TESTING  OF  GLUE  399 

For  a  given  approximately  uniform  density  of  liquid,  a  given 
speed  of  rotation,  and  a  given  size  of  wire,  it  is  however  both 
possible  and  practicable  to  obtain  readings  that  are  very  nearly 
straight  line  functions  of  the  absolute  viscosity.  Since  the 
absolute  viscosity  may  not  be  accurately  calculated  from  the 
dimensions  of  the  instrument,  it  is  necessary  to  obtain  some  kind 
of  a  standard  of  known  viscosity  for  calibration.  This  should 
have  very  nearly  the  same  density  and  the  same  order  of  viscosity 
as  the  liquids  that  are  to  be  examined.  Such  standard  liquids 
may  be  obtained  from  the  Bureau  of  Standards  in  Washington. 
In  calibrating  the  instrument,  a  wire  of  such  size  should  be 
selected  that,  at  the  required  temperature,  a  speed  of  from  20  to 
60  revolutions  per  minute  is  required  to  obtain  the  same  reading 
on  the  dial  as  corresponds  to  the  certified  viscosity  of  the  liquid 
in  poises  or  centipoises.  Several  different  temperatures  should 
be  used  with  the  standard  in  order  that  the  scale  interval  corre- 
sponding to  centipoises  may  be  definitely  established.  Then  by 
substituting  any  other  liquid  of  approximately  the  same  density 
the  absolute  viscosity  in  centipoises  may  either  be  read  directly 
from  the  dial,  or  obtained  by  a  very  simple  calculation.1 

Too  much  stress  cannot  be  laid  upon  the  desirability  of  express- 
ing all  viscosity  values  in  terms  of  the  absolute  unit — the 
centipoise.2  When  this  is  not  done  the  values  reported  have  only 
a  very  limited  significance,  and  it  becomes  impossible  to  compare 
figures  even  between  two  instruments  of  the  same  make — much 
less  between  types  as  far  apart  as  the  pipette  and  the  torsional 
apparatus.  By  taking  the  time  to  make  this  simple  calculation, 
however,  all  values  are  in  the  same  " language,"  and  the  reports  of 
any  one  investigator  or  tester  become  intelligible  to  every  other 
investigator  and  tester. 

3.   THE   MELTING   POINT 

The  melting  point  of  the  jelly  of  a  given  gelatin  or  glue  has 
often  been  regarded  of  importance  in  the  estimation  of  the  value 
of  the  material.  Some  investigators  have  considered  this  factor 
to  be  of  fundamental  importance,  while  others,  perhaps  most  of 
them,  have  regarded  a  melting  point  determination  only  as  a  con- 

1  Cf.  Appendix,  page  608,  for  the  calibration  of  the  Mac  Michael  Viscosi- 
meter. 

2  Vide  page  384. 


400  GELATIN  AND  GLUE 

venient  means  of  measuring  the  more  generally  conceded  funda- 
mental property  of  jelly  consistency.  The  assumption  is  made, 
in  these  cases,  that  the  melting  point  and  the  jelly  consistency 
are  always  parallel  functions. 

The  Scientific  Basis  of  the  Melting  Point  Test.— The 
methods  that  have  been  employed  for  measuring  this  property 
are  for  the  most  part  unscientific,  and  give  only  rough  approxima- 
tions of  the  true  value.  It  is,  of  course,  by  no  means  a  simple 
matter  to  obtain  accurate  measurements  of  the  melting  point  of 
such  an  amorphous  substance  as  a  gelatin  jelly.  For  upon 
applying  heat  to  such  a  body  it  gradually  softens,  gradually 
loses  its  shape,  and  after  passing  through  all  stages  of  a  semi- 
solid  and  a  semi-liquid,  it  finally  melts  to  the  consistency  of  a 
real  liquid.  The  exact  temperature  at  which  the  solid  becomes 
liquid,  or  the  liquid  becomes  solid  is  not  easy  to  determine.  In 
lieu,  therefore,  of  this  exact  figure,  certain  arbitrary  degrees 
of  softening  or  congealing  are  usually  taken  as  expressions  of  the 
melting  point. 

The  reasons  for  the  difficulty  experienced  in  locating  an  exact 
melting  point  are  probably  to  be  found  in  the  relations  which 
Smith1  has  shown  to  exist  between  the  sol  and  gel  forms  of  the 
gelatin.  Smith  has  found  that  from  0.6  to  1.0  per  cent  of  the 
gel  form  must  be  present  in  any  gelatin  mixture  at  any  tempera- 
ture before  gelation  will  take  place,  but  that  the  presence  of  that 
amount  will  produce  gelation.  He  accordingly  gives  a  new  defini- 
tion to  melting  point  as  applied  to  gelatin,  namely,  that  tempera- 
ture at  which,  for  any  given  concentration  of  gelatin,  there  will 
be  0.6  to  1 .0  per  cent  only  of  the  gel  form  present.  At  tempera- 
tures of  15°C.  and  below  the  gelatin  will  be  entirely  in  the  form  of 
the  gel.  But  as  the  temperature  rises  from  15  to  35°C.  the  ratio 
of  percentage  of  gelatin  in  the  sol  form  increases  until  at  the 
latter  temperature  it  is  entirety  in  the  sol  condition.  Conse- 
quently a  pure  gelatin  of  a  concentration  of  0.6  to  1.0  per  cent 
will  have  a  melting  point  at  about  15°.  Lower  concentrations 
will  not  gel  at  any  temperature.  Every  other  concentration 
will  have  its  melting  point  between  15  and  38°C.,  i.e.,  that  tempera- 
ture at  which  0.6  to  1.0  per  cent  only  of  the  gel  form  is  present. 
Obviously,  the  greater  the  concentration,  the  higher  will  be  the 


1  C.  R.  SMITH,  /.  Am.  Chem.  Soc.,  41  (1919),  135;  /.  Ind.  Eng    Chem., 
12  (1920),  878. 


TESTING  OF  GLUE  401 

temperature,  not  exceeding  38°,  at  which  the  above  condition 
will  be  realized.1 

Smith  has  furthermore  shown  that  in  order  for  a  true  equili- 
brium to  be  established  between  the  two  phases,  considerable 
time  may  be  required — 8  hours  or  more.  If  this  amount  of 
time  is  allowed,  then  it  will  be  found  to  make  no  difference  from 
which  direction  this  temperature  is  approached;  the  melting 
point  and  the  setting  point  will  be  identical.  This  is  of  impor- 
tance, for  statements  have  often  appeared  that,  for  example,  the 
melting  point  was  28°  and  the  setting  point  24°  for  a  given 
concentration. 

Effect  of  Hydrolysis  on  Melting  Point. — A  further  point  must 
be  brought  to  attention  in  this  place.  Gelatin  which  has  been 
subjected  to  hydrolysis  by  the  action  of  a  high  temperature, 
acids,  alkalies  or  other  means  is  converted,  in  proportion  to  the 
extent  of  such  treatment,  into  a  nongelatinizing  substance 
consisting  no  longer  of  the  protein  gelatin,  but,  in  its  place,  the 
cleavage  products  proteose  and  peptone.  These  have  been 
called  j8  gelatin.  This  material,  being  nongelatinizing,  possesses 
of  course  no  melting  point  in  the  sense  in  which  we  have  used 
the  term.  Low  grades  of  gelatin,  and  nearly  all  glues,  contain 
more  or  less  of  this  &  gelatin,  and  the  larger  the  amount  of  this 
form  present,  the  lower  will  be  the  melting  point  of  the  whole. 
If,  for  example,  a  10  per  cent  solution  of  pure  gelatin  contained 
the  necessary  1  per  cent  only  of  gel  form  (i.e.,  had  its  melting 
point)  at  20°,  a  10  per  cent  solution  of  glue  which  consisted  of 
half  |8  gelatin  would  require  a  lower  temperature  to  give  the 
necessary  1  per  cent  of  gel  form.  That  is,  the  melting  point 
would  be  lower. 

This  explains  the  value  of  the  melting  point  test  for  glues. 
The  greater  the  percentage  of  the  pure  gelatin  present,  the  higher 
will  be  the  melting  point.  The  greater  the  amount  of  hydrolyzed 
material  present,  the  lower  will  be  that  value. 

The  findings  of  Smith  are  of  especial  significance  in  that  they 
fully  corroborate  results  obtained  in  the  author's  laboratory2 
through  an  entirely  different  procedure.  These  results  indicate 
that  the  melting  point  and  the  jelly  strength  are  parallel  func- 
tions, other  conditions  being  constant,  and  that  the  ratio  between 
the  protein  gelatin  and  its  products  of  hydrolysis  vary  propor- 

1  See  Section  on  Structure,  Chap.  III. 

2R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  64;  105. 

26 


402 


GELATIN  AND  GLUE 


tionately  with  the  jelly  strength.  That  is,  the  melting  point  is 
proportionate  to  the  ratio  of  a  to  /S  gelatin  in  the  glue,  or,  which 
is  the  same  thing,  to  the  " grade"  of  the  glue. 


'15  10  65  60  55  50  45  40  35  30  25  20  15 
TemperaturejCentkjrade 

FIG.  90. — The  relation  of  normal  viscosity  to  melting  point. 


4          8         )l         16        20         24 
Substance  Added,  per  cent 

FIG.  91. — The  effect  of  added  substances  upon  jelly  strength. 

The  relation  between  the  viscosity  and  melting  point  is  illus- 
trated in  Fig.  90,  and  the  effect  of  the  addition  of  various  sub- 
stances on  jelly  strength  is  shown  in  Fig.  91. 


TESTING  OF  GLUE 
Reasoning  from  Clerk  Maxwell's  elasticity  theory 


403 


'T' 

where  E  =  the  clastic  modulus,  17  =  the  coefficient  of  viscosity, 
and  T  =  time  of  relaxation,  i.e.,  time  for  a  deformation  to  fall 
to  1/6  of  its  initial  value,  Sheppard  and  Sweet1  point  out  that  the 
"melting  point"  is  the  temperature  at  which  the  elastic  modulus 
becomes  very  small.  Since  rj  remains  of  considerable  magni- 
tude, this  can  only  be  by  T  becoming  very  large.  Hence  they 


FIG.  92. — The  relation  between  setting  point  and  tensile  strength,  plotted  against 

concentration. 

define  both  " melting  point"  and  "solidification  point"  (setting 
point)  as  "the  convergence  temperature  at  which  the  'time  of 
relaxation'  becomes  infinite." 

Comparative  curves  obtained  by  the  use  of  a  special  apparatus 
(see  below)  indicate  that  setting  point  (or  melting  point)  con- 
centration curves  of  different  gelatins  may  cut  each  other  at 
varying  points.  This,  they  argue,  makes  it  dangerous  to  attempt 

1  S.  E.  SHEPPARD  and  S.  SWEET,  /.  Ind.  Eng.  Chem.,  13  (1921),  423. 


404  GELATIN  AND  GLUE 

to  evaluate  gelatins  by  melting  points  at  any  given  fixed  gelatin 
concentration,  but  they  suggest  that  a  comparison  of  the  curves 
over  a  range  of  concentrations,  as  from  0  to  50  per  cent,  would 
adjust  the  difficulty. 

They  also  show  that  the  above  curves  are  not  parallel  to  the 
jelly  strength-concentration  curves,  and  that  the  same  order  of 
grading  would  not  result  by  the  two  methods.  Figure  92  shows 
a  comparison  of  the  two  sets  of  curves  presented  by  Sheppard 
and  Sweet. 

The  use  of  the  melting  point  as  a  criterion  for  the  evaluation 
of  a  glue  is  based,  of  course,  upon  the  hypothesis  that  the  melting 
point  is  a  measure  of  the  adhesive  strength  of  the  material. 
This  assuredly  does  not  apply  to  all  kinds  of  glues,  since  liquid 
marine  glues,  and  animal  glues  which  have  been  rendered  non- 
gelatinizing  by  the  addition  of  some  foreign  substance,  still 
may  possess  a  high  degree  of  adhesive  strength.  And  Her  old1 
has  even  gone  so  far  as  to  state  that,  even  in  animal  glues,  the 
presence  of  gelatinizing  gelatin  is  actually  a  drawback  to  the 
adhesive  power  of  the  glue.  He  regards  as  the  best  glue  that 
which  contains  a  minimum  of  gelatin,  and  a  maximum  of  non- 
gelatinizing  proteose.  Consequently  Herold  considers  a  low 
melting  point  a  low  viscosity,  and  a  low  jelly  strength  as  the 
desiderata  in  glue  evaluation.  We  admit,  because  of  the  high 
adhesive  strength  of  some  nongelatinizing  glues,  that  further 
investigation  is  necessary  before  this  point  can  be  definitely 
settled,  but  it  is  surely  the  common  experience  of  most  investiga- 
tors and  users  of  glue  that,  in  general,  a  high  melting  point,  a 
high  viscosity,  and  a  high  jelly  strength  are  expressive  of  a  high 
adhesive  strength,  and  that,  as  these  factors  decline,  the  adhesive 
strength  will  also  become  less.  Experiments  made  in  the  author's 
laboratory2  point  very  emphatically  to  this  conclusion.  No 
other  factor  was  found  so  correctly  to  parallel  the  actual  adhesive 
strength  of  a  glue  as  its  melting  point. 

The  Methods  for  Measuring  Melting  Point.  Chercheffski's 
Method. — A  method  suggested  by  Chercheffski3  in  1901  is  based 
upon  a  measurement  of  the  temperature  at  which  small  cubes 
of  the  jelly  become  soft  enough  to  lose  their  cohesion.  His 
apparatus,  Fig.  93,  consists  of  a  250  c.c.  beaker  containing  pale 

1  J.  HEROLD,  Chem.  Ztg.,  35  (1911),  93. 

2  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  64;  197. 

3  CHERCHEFFSKI,  Chem.  Ztg.,  25  (1901),  413. 


TESTING  OF  GLUE 


405 


petroleum  oil.  Across  the  top  of  this  is  placed  a  glass  rod.  One 
end  of  a  Z-shaped  brass  wire  is  coiled  around  the  rod  and  the 
other  horizontal  end  is  immersed  in  the  oil  near  a  thermometer. 
Small  cubes  of  gelatin  or  glue,  of  about  5  mm.  to  a  side,  and  of  a 
standard  concentration  are  skewered  onto  the  wire,  and  the 
temperature  of  the  oil  slowly  raised  until  the  cubes  liquefy  and 
drop  off.  This  temperature  is  noted  as  the  melting  point. 


L 

FIG.  93. — Chercheffsky's  method 
or    the    measurement    of    melti  ng 
point. 


8$* 


FIG.  94. — Cambon's 
fusiometer. 


Method. — The  temperature  at  which  a  glue  became 
sufficiently  mobile  to  flow  was  measured  by  Kissling.1  He 
dissolved  15  grams  of  glue  in  30  c.c.  of  water  and  poured  the 
solution  into  a  wide  test  tube.  The  contents  were  chilled  by 
immersing  in  water  at  15°C.  for  one  hour.  The  melting  point 
was  then  determined  by  placing  the  tube  horizontally  in  warm 
water,  and  noting  the  temperature  at  which  the  surface  of  the 
glue  became  inclined. 


1  KISSLING,  Z.  angew.  Chem.,  17  (1903),  398. 


406  GELATIN  AND  GLUE 

Winkelblectis  Method. — Winkelblech1  placed  400  c.c.  of  a 
standard  glue  solution  in  a  500  c.c.  flask  and  shook  in  cold  water 
until  a  consistency  of  jelly  was  reached  at  which  a  thermometer 
placed  therein  remained  stationary  for  some  time.  This  tem- 
perature he  recorded  as  the  gelation  point. 

Combon's  Fusiometer. — The  method  of  determining  the  melting 
point  by  the  use  of  Combon's  fusiometer,  as  described  by  Kiit- 
tner  and  Ulrich,2  has  been  regarded  as  more  or  less  of  a  standard 
procedure  for  many  years.  In  principle  it  is  much  the  same  as 
,the  foregoing  methods.  The  apparatus  consists  of  a  metal  bowl 
22  mm.  high,  17  mm.  in  diameter  at  the  top,  and  15  mm.  at  the 
bottom.  The  prescribed  weight  is  exactly  7  grams.  The  solu- 
tion of  glue  or  gelatin  to  be  tested  is  poured  into  the  bowl,  and  a 
rod  inserted  and  maintained  in  an  upright  position  until  the 
contents  have  congealed.  The  apparatus  is  then  suspended  in  a 
beaker  of  water  at  15°C.,  the  beaker  placed  in  a  water  bath  at  50°, 
and  the  water  in  the  latter  slowly  heated  (about  1°  per  minute) 
until  the  bowl  falls  off  from  the  rod.  The  temperature  of  the 
water  in  the  beaker  when  this  occurs  is  taken  as  the  melting 
point  of  the  substance.  A  concentration  of  20  per  cent  is  recom- 
mended. By  this  procedure  the  best  glues  show  a  melting  point 
of  30°  or  above,  and  intermediate  grades  from  24  to  29°C. 

Herald's  Method. — Herold3  employed  a  somewhat  similar,  but 
less  accurate,  method.  A  thermometer,  graduated  to  tenths  of 
a  degree,  was  allowed  to  be  congealed  into  a  test  tube  of  gelatiniz- 
ing jelly  and,  when  the  gelation  was  complete,  the  test  tube  was 
suspended  in  warm  water  by  means  of  the  thermometer.  The 
temperature  at  which  the  tube  fell  away  was  noted  as  the  melting 
point.  Herold  claims  that  by  careful  manipulation  it  is  possible 
to  obtain  results  which  may  be  duplicated  to  0.1  to  0.2°.  He 
specifies  that  the  tube  should  be  small,  the  space  between  the 
bulb  of  the  thermometer  and  the  wall  of  the  tube  being  about 
1  mm.  The  temperature  of  the  water  bath  should  not  be  more 
than  1.5  to  2.0°  higher  than  the  expected  melting  point  of  the 
jelly.  He  reported  that  the  melting  point  concentration  curve 
is  a  straight  line,  and  that  the  relative  values  of  two  samples  are 
proportional  to  the  tangents  of  the  angles  of  inclination  of  these 
lines  to  the  horizontal. 

i  WINKELBLECH,  ibid.,  19  (1906),  1260. 

2KtiTTNER  and  ULRICH,  Z.  offent.  Chem.,  13  (1C07),  121. 

3  J.  HEROLD,  Chem.  Ztg.,  34  (1910),  203. 


TESTING  OF  GLUE  407 

Sammet's  Method. — Sammet1  suggested  that,  for  comparative 
purposes,  the  relative  order  in  which  several  glue  jellies  liquefied 
when  heated  on  a  brass  strip  served  as  an  indication  of  jelly 
strength,  and  hence  of  the  value  of  the  glue.  His  method  is  to 
soak  the  granulated  glues  in  cold  water  for  one  minute,  then 
place  small  amounts  of  the  swollen  substance  near  the  end  of  a 
brass  strip,  and  place  the  end  of  the  strip,  bent  at  an  angle,  into 
water  at  40°C.  Similarly  treated  portions  of  standard  glues  are 
placed  beside  the  samples,  and  the  comparative  order  of  the 
melting  point  noted  by  observing  the  order  in  which  the  several 
portions  melt  and  run  off  the  strip. 

Clark  and  DuBois'  Method. — The  procedure  of  Clark  and 
DuBois2  has  as  its  object  the  determination  of  " jelly  value"  in 
terms  of  the  minimum  percentage  of  glue  or  gelatin  which  will 
remain  in  solid  phase  when,  after  putting  into  solution  and 
cooling  to  well  below  10°,  it  is  brought  gradually  up  to  10°C. 
This  is,  in  reality,  a  determination  of  melting  point.  The  several 
samples  are  made  up  in  a  number  of  different  concentrations, 
and  the  lowest  concentration  that  will  produce  a  jelly  at  the 
specified  temperature  (10°)  is  taken  as  the  "jelly  value."  Obvi- 
ously, the  higher  the  quality  of  the  material  examined,  the  lower 
will  be  the  "jelly  value." 

Smith's  Method. — The  procedure  adopted  by  C.  R.  Smith3  for 
determining  the  exact  melting  point  was  as  follows:  He  cooled 
the  sol  to  2  or  3°  below  the  expected  temperature,  left  it  at  this 
temperature  until  the  gel  was  produced,  and  then  transferred  to  a 
constant-temperature  bath  held  at  the  expected  temperature. 
He  selected  an  arbitrary  standard  of  rather  high  viscosity  as  the 
state  most  accurately  representative  of  the  melting  point.  At 
the  correct  temperature  the  gel  should  show  continuously  for 
several  hours  the  selected  condition  of  viscosity.  The  particular 
viscosity  employed  by  Smith  as  the  transition  point  from  sol  to 
gel  was  determined  by  permitting  a  bubble  of  air  4.5  mm.  in 
diameter  to  move  vertically  upward  through  a  polariscope  tube 
containing  the  gelatin.  At  the  specified  viscosity  the  bubble 
should  move  with  a  scarcely  perceptible  motion  of  about  1  cm. 
in  4  seconds. 

Sheppard  and  Sweet's  Method. — The  apparatus  of  Sheppard  and 

1  C.  F.  SAMMET,  /.  Ind.  Eng.  Chem.,  10  (1918),  595. 

2  A.  W.  CLARK  and  L.  DuBois,  /.  Ind.  Eng.  Chem.,  10  (1918),  707. 

3  C.  R.  SMITH,  /.  Am.  Chem.  Soc.,  41  (1919),  146. 


408 


GELATIN  AND  GLUE 


Sweet1  is  designed  to  measure  directly  the  melting  point  and  the 
setting  point  of  gelatins.  The  principle  is  as  follows:  "An 
intermittent  stream  of  air  bells,  under  constant  pressure,  is 
passed  through  the  test  solution,  the  latter  being  cooled  with  ice 
water.  A  thermometer  is  immersed  with  its  bulb  next  to  the  air 


FIG.  95. — The  melting  point  apparatus  of  Sheppard  and  Sweet. 

passage,  and  the  temperature  at  which  the  bubbles  cease  to  pass 
is  taken  as  the  'setting  point.'  Inversely,  after  sufficient  under- 
cooling, the  set  jelly  is  surrounded  with  water  at  a  definite  higher 
temperature,  and  the  'melting  point'  taken  as  the  temperature 
at  which  bubbles  again  pass  through."  The  operation  of  the 
instrument  will  be  evident  from  Fig.  95. 

"  Compressed  air  passes  manometer  A  and  the  manostat  bottle 
B  to  the  first  U-tube  E,  containing  mercury.  This  tube  is  used 
as  a  valve  to  produce  intermittence  in  the  delivery  of  air.  A 
solenoid,  D,  the  current  through  which  is  made  and  broken  by  the 
timer  C  every  15  seconds,  effects  this  interruption  by  operating 

1  S.  E.  SHEPPARD  and  S.  SWEET,  J.  Ind.  Eng.  Chem.,  13  (1921),  423. 


TESTING  OF  GLUE 


409 


an  iron  plunger.  From  this  U-tube  E  the  air  passes  the  com- 
pensating U-tube  F  to  the  setting  or  melting  tube  K.  The  outlet 
in  K  is  shown  in  detail  at  G.  To  obtain  satisfactory  and  repro- 
ducible results  with  this  apparatus  the  following  precautions  are 
necessary : 

1.  Fifteen-second  intervals  between  passage  of  air  bells. 

2.  Slow  flow  (i.e.,  slight  overpressure). 

3.  Exit  at  definite  depth  below  surface. 

4.  Water  in  compensation  tube  at  same  level  throughout  the 
test." 

An  alternative  melting  point  apparatus  is  also  described  by 


annular 
weight 

FIG.  96. — The  alternative  melting  point  apparatus  of  Sheppard  and  Sweet. 

Sheppard  and  Sweet,  designed  for  more  rapid  but  less  accurate 
measurements.     This  is  shown  in  Fig.  96. 

"The  jelly  is  set  in  a  test  tube  with  a  thermometer  centrally 
embedded,  the  bulb  being  just  below  the  surface.  Round  this 
thermometer  slips  a  small  test  piece,  resting  on  the  jelly  by  three 
equidistant  wedge-shaped  feet.  The  test  tube  is  air  jacketed  and 


410  GELATIN  AND  GLUE 

heated  at  a  constant  rate,  and  the  temperature  read.  The  point 
at  which  the  tester  just  begins  to  penetrate  the  jelly  surface  is 
taken  as  the  softening  or  yield  point,  and  the  temperature  at 
which  the  tester  has  sunk  just  above  the  feet  as  the  melting 
point.'7  There  is  a  slight  error  in  the  use  of  this  instrument  due 
to  the  skin  formation,  but  it  is  very  small. 

Bogue's  Method. — The  author1  has  pointed  out  that,  if  the 
melting  point  may  be  assumed  to  be  represented  by  the  tem- 
perature at  which  a  solution  of  glue  or  gelatin  will  no  longer  flow 
at  all  from  the  orifice  of  a  viscosimeter  tube,  then  it  becomes  a 
comparatively  easy  matter,  by  taking  a  series  of  viscosity  read- 
ings at  decreasing  temperatures,  to  plot  the  temperature  at  which 
the  viscosity  would,  with  that  instrument,  reach  infinity,  i.e., 
would  cease  altogether  to  flow.  In  principle,  this  is  similar  to  the 
method  of  Smith,  but  it  makes  use  of  a  more  easily  measurable 
property.  The  author  also  found  that,  at  temperatures  near  the 
setting  point,  the  higher  the  viscosity,  the  higher  also  would  be 
the  temperature  of  gelation.  It  therefore  becomes  permissible 
to  employ  the  actual  viscosity  measurement  at  temperatures 
near  the  setting  point,  e.g.,  30  to  35°C.,  as  a  measure  of  melting 
point.  Advantage  has  accordingly  been  taken  of  this  relation- 
ship by  making  viscosity  determinations  at  these  temperatures. 

The  advantages  gained  by  such  a  procedure  are,  first,  that  an 
easily  measurable  property  is  determined,  rather  than  the  more 
difficult  and  rather  hypothetical  melting  point,  and  second,  that 
a  much  wider  range  of  values  are  obtainable.  That  is,  the 
melting  points  are  limited  to  between  15  and  35°C.,  accurate 
to  perhaps  1  or  2°,  while  the  viscosity,  when  measured  by 
the  MacMichael  instrument,  and  expressed  in  centipoises, 
was  found  to  vary  from  about  10  in  the  lowest  to  above  150 
in  the  highest  grade  glues,  at  35°C.,  and  are  easily  duplicatable 
to  1°. 

By  the  above  method,  therefore,  the  melting  point  may  be 
either  computed  in  terms  of  actual  temperature  of  gelation,  by 
taking  several  viscosity  readings  at  different  temperatures  near 
the  gelation  point,  or  the  viscosity  in  centipoises  at  some  stated 
low  temperature  may  be  determined,  and  expressed  as  an  indirect 
measure  of  melting  point.  Such  determinations  were  found  very 
accurately  to  parallel  the  adhesive  strength  of  the  glue. 

JR.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  64;  J.  Ind.  Eng.  Chem., 
14  (1922),  435. 


TESTING  OF  GLUE  411 

4.  THE  ADHESIVE  STRENGTH 

Whenever  glue  is  to  be  used  as  a  binding  agent  in  joint  or 
veneer  work,  the  actual  strength  which  may  be  developed  in  the 
joint  or  panel  by  the  use  of  a  given  glue  is  of  course  of  the  greatest 
importance.  It  is  essential  for  good  results  that  a  certain  mini- 
mum of  strength  shall  always  be  produced. 

On  account  of  the  difficulties  encountered  in  making  the  test 
such  that  the  results  will  be  reliable  and  always  comparable, 
there  have  been  very  few  attempts  on  a  commercial  or  laboratory 
scale  to  include  the  adhesive  test  in  specifications.  The  most 
noteworthy  exception  to  this  is  the  specifications  of  the  United 
States  War  Department  for  glue  to  be  used  on  airplanes.  In 
general,  however,  recourse  has  been  had  to  other  more  easily 
performed  tests,  such  as  the  jelly  consistency,  viscosity,  and 
melting  point,  in  the  belief  that  these  properties  represented, 
to  a  certain  extent  at  least,  the  order  of  differentiation  that  would 
be  found  by  the  actual  joint  strength  test. 

A  detailed  discussion  of  the  adhesive  strength  of  glues,  and  the 
factors  affecting  the  same  is  given  in  Chap.  XI,  pages  517  to  534.. 

5.  THE  TENSILE  STRENGTH  AND  ELASTICITY 

The  tensile  strength  of  the  glue  has  frequently  been  suggested 
as  the  property  upon  which  the  adhesive  properties  ultimately 
depended,  but  very  little  advance  has  been  made  in  the  develop- 
ment of  this  test  on  account  of  the  technical  difficulties  encoun- 
tered. Most  important  of  the  difficulties  experienced  is  the 
impossibility  of  obtaining  strips  of  the  dried  glue  which  are 
quite  free  from  stresses  and  strains  set  up  during  the  drying 
process,  except  in  the  possible  instance  of  very  thin  specimens. 
And  even  in  the  latter  there  is  great  uncertainty  until  the  test 
has  been  completed  and  the  nature  of  the  break  ascertained. 

In  recognition  of  these  difficulties,  and  in  order  to  avoid  them, 
Setterberg1  used  strips  of  paper  dipped  in  the  solution  of  glue, 
dried,  and  pulled  apart.  Gill2  was  not  able  to  obtain  satisfactory 
results  by  this  method,  and  modified  the  procedure  by  deter- 
mining the  bursting  strength  of  the  glue-treated  paper  by  the  use 
of  the  Mullen  paper  tester.  He  dipped  strips  of  filter  paper 


1  SETTERBERG,  Svensk.  Kern.  Tid.,  28,  52. 

2  A.  H.  GILL,  J.  Ind.  Eng.  Chem.,  7  (1915),  102. 


412  GELATIN  AND  GLUE 

2  inches  wide  into  a  25  per  cent  solution  of  glue,  and  allowed 
them  to  dry  in  the  air  for  36  hours.  The  paper  was  then  cut 
into  2  inch  squares,  weighed,  and  broken  by  the  Mullen  paper 
tester,  which  measures  the  force  required  by  a  rubber  knob  to 
burst  through  the  paper.  The  results  were  calculated  to  a  basis 
of  the  breaking  pressure  per  100  mg.  of  glue  per  4  square  inches 
of  surface.  Gill  reports  that  the  results  were  much  more  con- 
cordant than  any  that  were  obtained  by  any  other  means,  giving 
data  that  did  not  vary  more  than  10  per  cent.  The  test,  how- 
ever, does  not  give  results  that  may  correctly  be  termed  tensile 
strength. 

A  method  by  which  this  property  may  be  more  accurately 
measured  has  been  described  by  Hopp.1  His  method  consists  in 
making  molds  of  the  glue  jelly,  allowing  to  dry,  cutting  and 
trimming  to  a  standard  size,  and  finally  pulling  apart  in  a  tensile 
machine.  He  uses  solutions  containing  60  to  80  per  cent  by 
volume  of  glue,  warms  to  160°F.,  and  pours  carefully  into  an  iron 
mold  12  by  12  by  J£  inch  deep.  The  glue  is  permitted  to  set 
for  5  hours,  and  the  jelly  then  removed  and  placed  on  a  fine  wire 
gauze.  Air  currents  are  not  permitted.  The  jelly  is  turned 
frequently,  and  strains  prevented  by  cutting  slits  Y±  inch  deep 
into  the  glue  as  it  dries  out.  When  thoroughly  dried  the  sheet 
is  cut  into  strips  by  a  hot  knife,  and  cut  or  ground  to  a  definite 
shape  and  size.  His  most  favorable  dimensions  were  0.1  inch  in 
thickness,  0.33  inch  in  width,  and  7  inches  in  length.  A  length 
of  2.5  inches  was  indented  at  the  center  of  the  strip  to  insure  its 
breaking  in  that  area.  The  tensile  strength  and  also  the  stretch 
before  breaking  were  measured  on  a  Schopper  machine. 

The  results  reported  by  Hopp  are  remarkable  in  their  uni- 
formity upon  duplicate  samples.  The  maximum  deviation  from 
the  average  of  17  determinations  upon  one  lot  was  only  about 
5  per  cent.  The  stretch  was  not  as  consistent,  but  showed 
a  distinct  tendency  to  be  greater  in  the  stronger  glues.  Much 
more  work  is  necessary  upon  the  exact  relations  which  obtain 
between  tensile  strength,  stretch,  and  adhesive  strength  before 
the  value  and  applicability  of  this  new  test  is  definitely  placed, 
but  if  corroborative  evidence  is  developed  showing  a  ready 
adaptability  to  glue  house  technique,  and  an  unquestionable 
conformity  for  all  glues,  there  is  no  reason  why  it  should  not 
constitute  a  valuable  addition  to  our  tests  for  glue  evaluation. 

1  G.  HOPP,  /.  Ind.  Eng.  Chem.,  12  (1920),  356. 


TESTING  OF  GLUE 


6.  THE  OPTICAL  ROTATION 


413 


Smith1  has  demonstrated  that  a  nicely  balanced  equilibrium 
exists  in  glues  and  gelatins2  between  the  sol  and  gel  form  of  the 
protein,  and  that  this  equilibrium  is  reflected  both  in  the  jellying 
power  of  the  material  and  in  the  degree  of  mutarotation  between 
35  and  15°C.  By  studying  a  number  of  glues  of  different  grades, 
Smith  found,  first,  that  the  minimum  concentration  of  glue 


Mutarotation  (I50-35°J 

_  —  —  ro  rO  ro 
O  rO  -F*  <S">  co  O  rJ  -F*  3 

\ 

\ 

V 

\ 

\ 

\ 

\ 

j£ 

s 

X 

X 

N 

•x. 

•^ 

< 

>. 

O     o'     O      G*     CJ     — :     — '•     — :      — "      — :     — :  — '      — :      — '     — :     C^    O* 

Minimum  Amount  to  Produce  Standard  Jelly  at  15° 
FIG.  97.- — Mutarotation  and  jellying  power.     (From  data  of  C.  R.  Smith.) 

necessary  to  produce  a  jelly  of  a  standard  consistency  decreased 
as  the  apparent  purity  and  gelatin  content  of  the  sample,  and 
second,  that  the  change  in  rotation  (mutarotation)  of  a  given 
concentration  of  the  glue  between  35  and  15°  increased  with 
increasing  purity  of  the  material.  He  suggests  therefore  that 
the  change  in  rotation  of  a  3  per  cent  solution  of  glue  between  35 
and  15°C.  be  employed  as  a  measure  of  the  jelly  strength.  For 
example,  if  a  sample  polarizes  at  —20.5°  V.  at  35°C., 
and  at  -40.0°V.  at  15°C.  in  a  concentration  of  3  grams  per  100 
c.c.,  it  is  suggested  that  the  strength  be  expressed  as!9.5  points 
at  15°C.,  the  increment  in  rotation  in  Ventzke  degrees. 

1  C.  R.  SMITH,  J.  Ind.  Enq.  Chem.,  12  (1920),  878. 

2  See  pages  117  to  119. 


414 


GELATIN  AND  GLUE 


The  significance  of  such  a  system  will  be  understood  by  a 
reference  to  the  following  table  which  is  taken  from  a  more  exten- 
sive one  by  Smith. 


TABLE  47. — MUTAROTATION  AND  JELLYING  POWER 


Minimum  amt.  to  produce 
standard  jelly  at   15°C. 

Rotation,  °V.,  3  g. 
per  100  c.c.  in  equili- 
brium 35°C. 

Rotation,  °V.,  3  g. 
per  100  c.c.  in  equili- 
brium 15°C. 

Mutarotation 

Bone  Glues 


2.10 

-19.8 

-30.2 

10.4 

1.60 

-20.2 

-32.4 

12.2 

1.45 

-20.1 

-33.4 

13.3 

1.40 

-20.6 

-34.1 

13.5 

1.40 

-21.0 

-33.8 

12.8 

0.96 

-20.3 

-36.6 

16.3 

0.88 

-20.4 

-37.6 

17.2 

0.89 

-20.4 

-36.8 

16.4 

0.77 

-20.5 

-40.0 

19.5 

0.80 

-20.7 

-39.0 

18.3 

0.78 

-21.0 

-40.2 

19.2 

0.80 

-20.7 

-40.0 

19.3 

0.72 

-20.1 

-40.0 

19.9 

0.72 

-20.5 

-41.6 

21.1 

0.70 

-21.3 

-42.0 

20.7 

0.68 

-20.7 

-43.2 

22.5 

0.67 

-21.4 

-43.9 

22.5 

0.67 

-21.0 

-42.8 

21.8 

0.58 

-20.8 

-44.4 

23.6 

0.58 

-20.3 

-44.9 

24.6 

0.55 

-21.3 

-46.2 

24.9 

Hide  and  Sinew  Glues 


0.55 

-20.9 

-45.0 

24.1 

0.69 

-20.7 

-41.2 

20.5 

0.88 

-21.1 

-38.0 

16.9 

0.64 

-21.5 

—  44.2 

22.7 

0.69 

-20.7 

-42.4 

21.7 

0.57 

-20.5 

-43.6 

23.1 

0.80 

-20.3 

-39.0 

18.7 

0.95 

-20.4 

-36.3 

15.9 

0.87 

-21.0 

-38.4 

17.4 

1.09 

-20.6 

-36.2 

15.6 

1.35 

-21.3 

-33.2 

11.9 

1.60 

-20.2 

-31.8 

11.6 

3.50 

-19.6 

-25.6 

6.0 

5.30 

-19.0 

-22.9 

3.9 

1.35 

-20.0 

-34.0 

14.0 

2.60 

-19.3 

-27.2 

7.9 

In  the  first  column  is  shown  the  minimum  percentage  of  glue 
necessary  to  produce  the  standard  jelly  at  15°C.     The  standard 


TESTING  OF  GLUE  415 

jelly  is  such  "that  a  bubble  of  air  4  to  5  mm.  in  diameter  admitted 
to  the  polariscope  tube  moves  vertically  with  a  scarcely  per- 
ceptible motion  of  4  cm.  per  second."  The  second  and  third 
columns  show  the  rotation  in  Ventzke  degrees  of  a  3  per  cent 
solution  at  35  and  at  15°  respectively.  The  last  column  shows 
the  motarotation,  i.e.,  the  change  in  rotation  suffered  by  the 
glue  on  passing  from  35  to  15°. 

It  will  be  observed  that  as  the  jellying  power  increases  the 
motarotation  also  increases.  (The  smaller  the  minimum  per- 
centage of  glue  necessary  to  produce  the  standard  jelly,  the 
greater  is,  of  course,  its  jellying  power.)  The  precise  nature  of 
the  variation  is  shown  to  better  advantage  by  the  curve  in  Fig. 
97  prepared  by  the  author  from  Smith's  figures.  It  is,  unfor- 
tunately, not  a  straight  line  curve.  It  would,  however,  be  a 
simple  matter  to  prepare  a  chart  showing  the  approximate 
relation  between  any  given  jelly  consistency,  as  ordinarily 
determined,  and  the  mutarotation. 

Smith  proposes  the  use  of  the  polariscope,  applying  the  above 
principle,  to  the  grading  of  glues  and  gelatins,  and  to  factory 
control.  A  3  per  cent  solution  is  polarized  in  a  2  dm.  tube 
at  35°C.,  then  a  portion  of  the  solution  cooled  rapidly  to  15° 
and  transferred  before  the  sample  has  jellied  to  a  cold  1  dm.  tube. 
This  is  left  in  the  ice-box  over  night,  and  the  next  day  placed 
in  a  constant  temperature  bath  at  15  ±  0.4°  for  4  to  7  hours,  and 
the  polarization  noted.  It  is  sometimes  necessary  to  clarify 
the  solution,  which  is  done  by  digesting  with  5  c.c.  of  light 
powdered  magnesium  carbonate  at  30  to  40°C.  for  one  hour  or 
longer,  and  filtered. 

Although  the  findings  of  Smith  are  of  considerable  interest 
from  a  theoretical  viewpoint,  it  seems  questionable  if  the  addi- 
tional information  gained  is  of  sufficient  importance  to  justify 
the  incorporation  of  the  polariscopic  test  into  the  glue  shop  for 
grading  or  control  purposes.  The  apparatus  is  expensive,  the 
time  for  testing  long,  and  the  skill  in  technique  much  more 
exacting  than  is  required  in  other  procedures. 


7.  THE  SWELLING  CAPACITY 

The  degree  of  swelling,  or  differently  expressed,  the  amount  of 
water  which  a  given  weight  of  a  glue  or  gelatin  will  absorb,  has 


416  GELATIN  AND  GLUE 

occasionally  been  used  as  a  test  for  evaluation.1  If  every  other 
disturbing  influence  were  eliminated,  it  would  be  found  that  the 
degree  of  swelling  would  increase  in  proportion  to  the  protein 
content  of  the  sample.  But  there  are  such  a  large  number  of 
influences  which  are  of  very  appreciable  magnitude  that  modify 
this  factor2  that  it  must  be  regarded  as  of  only  secondary  impor- 
tance in  evaluation. 

The  hydrogen-ion  concentration  of  the  glue  is  responsible  for 
wide  variations  in  swelling  capacity.  The  ions  of  many  salts 
produce  marked  changes.  The  previous  history  of  the  substance 
is  of  importance.  A  glue  dried  out  from  a  dilute  jelly  will 
absorb  more  water  than  if  the  concentration  of  the  jelly  is  high. 
The  purity  of  the  water  in  which  the  sample  is  placed,  its  tem- 
perature, and  its  relative  volume,  are  all  important  sources  of 
fluctuation  in  swelling  capacity.  For  these  reasons  the  test 
has  not  been  generally  used. 

8.  THE  RATE  OF  SETTING 

For  some  uses  to  which  a  glue  is  put  it  is  very  desirable  to 
have  some  information  upon  the  comparative  rapidity  with  which 
it  sets  to  a  jelly.  Wherever  glue  is  used  at  such  concentrations 
that  a  jelly  results  upon  cooling  to  room  temperature  it  must,  of 
course,  be  handled  with  alacrity,  for  if  the  glue  sets  before  a 
joint,  for  example,  is  completed,  the  full  value  of  the  adhesive 
cannot  be  realized.  In  general,  however,  the  rate  of  setting  is 
reasonably  well  gaged  by  the  viscosity  or  the  jelly  strength. 
Where  direct  data  are  desired,  however,  they  may  readily  be 
obtained  by  comparing  the  time  required  for  the  several  solutions 
of  equal  concentration  and  temperature  to  reach  any  given 
consistency. 

9.  THE  FOAM  TEST 

A  glue  that  foams  badly  is  regarded  with  disfavor  by  practically 
all  consumers  of  the  material.  When  the  glue  is  used  only  in 
small  amounts  and  applied  by  hand,  the  presence  of  foam  is  not 
especially  embarrassing,  but  when  used  in  large  amounts  and 
applied  by  rotary  brushes  or  rollers,  its  presence  may  be  very 

1  SHATTERMANN,  Dingler's  Polytech.  /.,  96  (1845),  115. 

2  See  page  164,  et.  seq. 


TESTING  OF  GLUE  417 

troublesome.  Manufacturers  also  find  difficulty,  in  the  evapora- 
tion in  vacuo  operation,  if  the  glue  foams  badly. 

The  most  exhaustive  study  that  has  been  made  of  this  annoying 
property  of  glues  seems  to  have  been  that  reported  by  Trotman 
and  Hackford1  in  1906.  Preliminary  to  the  investigation  proper 
they  studied  the  various  methods  that  were  in  use  for  the  meas- 
urement of  foam,  and  found  them  to  be  inadequate  for  exact 
determinations.  They  found  that  not  only  must  equivalent 
concentrations  of  glue  be  employed  but  containers  of  uniform 
dimensions  must  be  used,  as  the  foam  figure  increases  with 
decreasing  diameter  of  the  tube  or  tumbler  (from  7.5  to  17.5  on 
decreasing  the  diameter  from  2  to  1  cm.);  the  height  of  the 
liquid  in  the  tubes  must  be  the  same,  as  the  foam  increases  with 
increasing  height  of  liquid  (from  3  to  22  on  increasing  the  height 
from  5  to  25  cm.);  and  the  temperature  must  be  the  same,  as  the 
foam  figure  decreases  with  increasing  temperature  (from  30  to 
3.5  on  raising  the  temperature  from  30  to  100°C.). 

The  apparatus  and  technique  for  foam  measurements  that 
was  finally  adopted  is  as  follows:  A  graduated  tube  70  cm.  in 
length,  and  of  such  diameter  that  1  c.c.  is  measured  by  a  1  cm. 
graduation,  is  half  filled  with  the  glue  solution,  and  placed  in  a 
water  bath  maintained  at  60°C.  After  the  temperature  has 
become  constant  the  height  of  the  glue  column  is  adjusted  so 
that  it  reaches  the  zero  mark  at  which  point  there  will  be  exactly 
25  c.c.  of  solution  present.  The  tube  is  then  corked  and  shaken 
vigorously  for  a  half  minute,  and  the  upper  point  of  the  foam 
read  off.  This  represents  the  cubic  centimeters  of  foam  obtained 
under  the  conditions  employed,  and  is  called  the  foam  figure. 

The  results  of  the  investigation  of  Trotman  and  Hackford  may 
be  stated  very  briefly: 

1.  The  presence  of  peptones  in  the  glue  very  markedly 
increases  the  foam  figure,  especially  in  low  concentrations.  Ten 
parts  of  peptones  to  100  parts  of  albumose  raise  the  foam  figure 
13  c.c.,  but  100  parts  of  peptone  to  100  parts  of  albumose  raise 
the  figure  only  20.5  c.c.  This  is  taken  to  signify  that  high  grade 
glues,  which  contain  relatively  small  amounts  of  peptone,  will 
show  a  relatively  small  amount  of  foam  as  compared  with  the 
the  lower  grades  of  glues  which  are  highly  hydrolyzed.  This  was 
further  evidenced  by  an  increase  in  the  foam  figure  from  16  to 
24  resulting  from  the  boiling  of  a  glue  for  24  hours. 

1  S.  TROTMAN  and  J.  HACKFORD,  J.  Soc.  Chem.  Ind.,  25  (1906),  104. 

27 


418  GELATIN  AND  GLUE 

2.  On  boiling  with  alkalies  there  seems  to  be  little  change  in 
the  foam  figure  until  rather  large  amounts  of  the  alkali  are 
present.     Ammonium  carbonate  produces  the  greatest  increase 
in  foam  at  low  concentrations.     In  the  cold,  sodium  hydroxide 
produces  a  decided  increase.     Lime  is  practically  without  effect. 
Ammonia  produces  a  slight  decrease. 

3.  Bone  oil,  oleic  acid,  cod  oil,  and  lubricating  oil  produce  a 
decided  decrease  in  foam,  but  paraffin  oil,  olive,  castor,  neats 
foot,  rape,  and  cedar  oils  were  without  effect. 

4.  Small  amounts  (up  to  5  per  cent)  of  potash  and  soda  soaps 
effected  a  decrease  in  foam,  but  large  amounts  resulted  in  an 
increase. 

5.  Most  acids  produce  a  slight  decrease  in  foam.      On  account 
of  the  hydrolyzing  effect,  however,  they  are  not  recommended  for 
that  purpose. 

6.  Insoluble    substances    suspended    in   the   glue   invariably 
produced  an  increase  in  foam,  a  5  per  cent  admixture  of  zinc 
oxide,  for  example,  raising  the  figure  12  c.c. 

7.  The  presence  of  soluble  salts  such  as  might  be  found  in  the 
water  used  has  very  little  effect  upon  foam. 

An  excessive  amount  of  foam  appearing  during  the  evaporation 
in  vacuo  of  the  glue  liquor  may  be  prevented,  according  to  Garry,1 
by  increasing  the  temperature  at  the  foaming  point  and  decreas- 
ing it  in  the  liquor.  That  is,  full  pressure  steam  would  be  cut  off 
in  the  lower  coils. 

There  is  no  satisfactory  method  for  eliminating  foam  when  it 
appears  during  the  spreading,  the  sizing  operations,  etc.,  to 
which  it  may  ultimately  be  put.  Sometimes  a  little  grease  is 
added,  if  the  latter  will  not  produce  in  itself  serious  difficulties. 
Since  alkaline  glues  foam  more  than  acid  glues,  acids  are  some- 
times added  to  neutralize  any  alkalinity  present. 

In  some  uses  to  which  gelatin  is  put,  foam  is  desirable.  Thus 
in  the  manufacture  of  marshmallows  and  confectionery  foaming 
is  of  importance  as  it  adds  to  the  volume  and  firmness  of  the 
beaten  constituents. 

10.  THE  GREASE  TEST 

Unless  precaution  is  used  during  the  manufacture  to  remove 
grease  and  fatty  matter,  the  presence  of  this  material  will 

1  H.  GARRY,  J.  Soc.  Chem.  Ind.,  25  (1906),  108. 


TESTING  OF  GLUE  419 

always  be  manifest  in  glue.  Bones  are  sometimes  treated  with 
fat  solvents  before  boiling,  but  it  is  also  common  practice  in 
this  country  to  remove  the  fatty  substance  during  the  cooking 
of  the  stock,  or  the  subsequent  standing  of  the  same,  by  the 
simple  process  of  skimming.  As  long  as  fat  sells  for  a  higher 
price  than  glue,  it  is  certain  that  a  reasonable  effort  will  be  made 
to  remove  as  much  as  practicable  from  the  glue,  but  in  spite  of 
this  there  is  oftentimes  an  appreciable  amount  of  fat  that  has 
not  been  removed. 

The  principal  objection  to  the  presence  of  grease  in  glue  lies  in 
the  fact  that  it  is  immiscible  with  the  latter,  and  consequently  on 
spreading  leaves  a  number  of  droplets  of  oil  on  the  surface  coated. 
As  oil  is  not  an  adhesive,  the  joint  strength  of  the  glue  will  be 
diminished  in  proportion  to  the  area  occupied  by  these  droplets. 
In  the  manufacture  of  sized  or  glazed  papers,  especially  in  colored 
papers  where  the  glue  is  admixed  with  the  coloring  material,  the 
presence  of  much  grease  is  impermissible,  as  it  would  unmistakably 
manifest  its  presence  by  the  formation  of  small  round  "eyes" 
or  light  spots  in  the  product.  In  clay-treated  wall  papers  the 
use  of  a  greasy  glue  is  usually  regarded  with  disfavor,  but  Fern- 
bach1  insists  that  its  use  is,  on  the  contrary,  an  advantage,  in 
that  it  makes  for  a  more  brilliant  pigmentation,  a  greater  smooth- 
ness of  flow  from  the  roller,  and  a  minimizing  of  the  possibility  of 
foaming. 

The  presence  of  grease  is  shown  by  adding  a  water  solution  of 
a  dye,  as  turkey  red,  magenta,  or  methyl  violet,  to  the  glue  solu- 
tion, and  by  means  of  a  clean  flat  brush  making  even  streaks 
of  the  mixture  across  a  sheet  of  greaseless  paper.  The  grease 
manifests  itself  by  producing  the  typical  "eyes"  or  pale  ellip- 
tical spots  throughout  the  streak. 

11.  THE  REACTION 

A  test  that  has  worked  its  way  into  the  usual  routine  procedure 
of  glue  testing,  but  which  is  only  rarely  made  use  of  intelligently, 
is  the  simple  litmus  paper  test  for  acidity  or  alkalinity.  Strips 
of  the  red  and  blue  paper  are  dipped  into  rather  thick  solutions 
of  the  glues,  and  notation  made  as  to  whether  the  latter  appear 
to  be  acid,  alkaline,  or  neutral.  The  test  is  not  especially  sensi- 
tive in  the  presence  of  the  colloid  material  of  the  glue,  and  the 

1  R.  FEBNBACH,  "Glues  and  Gelatin,"  New  York  (1907),  31. 


420  GELATIN  AND  GLUE 

value  of  the  test,  except  as  an  indication  of  high  acidity  or 
alkalinity,  is  questionable. 

In  the  manufacture  of  glue,  the  hide  stock  is  usually  treated 
with  lime.  This  lime  may  be  washed  out  in  large  measure,  but  a 
large  enough  amount  may  be  left  in  the  stock  to  produce  an 
alkaline  reaction  in  the  glue.  If  the  lime  is  neutralized  with 
some  acid,  a  slight  excess  of  acid  will  often  remain,  and  be  the 
cause  of  an  acid  reaction  in  the  product.  Bones  that  have  been 
treated  with  acid  to  dissolve  the  calcium  phosphate  will  usually 
produce  an  acid  glue. 

For  adhesive  purposes  it  is  not  considered  to  matter  whether 
the  glue  contains  acid  or  alkali,  or  is  neutral,  unless  those  sub- 
stances are  present  in  large  amounts.  In  such  case,  an  hydrolysis 
will  take  place  upon  bringing  the  glue  into  solution,  and  an 
unusually  rapid  depreciation  in  strength  upon  maintaining  the 
solution  at  the  handling  temperature  (60°C.)  may  result.  In 
small  amounts  acid  is  considered  to  be  rather  more  desirable 
than  alkali  as  the  former  is  a  less  favorable  medium  for  the  growth 
of  bacteria  and  putrefactive  organisms. 

The  presence  of  more  than  traces  of  either  acid  or  alkali  is 
detrimental  in  glue  that  is  to  be  used  as  a  size  upon  colored 
paper  or  cloth,  on  account  of  the  possibility  of  a  reaction  taking 
place  between  the  color,  or  its  precipitating  agent  which  is 
often  incompletely  washed  out,  and  the  electrolyte.  In  papers 
where  clay  is  employed,  the  presence  of  an  excess  of  acid  or 
alkali  in  the  glue  may  also  result  in  a  flocculation  of  the  colloid 
particles  of  the  clay,  causing  the  latter  to  lose  their  smoothness 
and  become  granular  or  lumpy. 

A  much  more  satisfactory  method  for  determining  the  reaction 
of  a  glue  solution  consists  in  a  measurement  of  the  hydrogen  ion 
concentration.  (See  page  500  and  Appendix,  pages  579  to  606.) 


12.     APPEARANCE,    ODOR,    COLOR,    KEEPING    QUALITIES,    ETC. 

The  general  appearance  of  a  glue,  its  odor,  color,  brittleness, 
and  the  like,  are  the  properties  that  are  first  brought  to  the 
attention  of  any  person  who  is  examining  samples  of  glue  or 
gelatin.  Although  these  properties  are  not,  as  a  general  rule, 
made  very  much  use  of  in  actual  evaluation,  yet  to  the  man  who 
is  familiar  with  glues  they  mean  much,  and,  even  to  the  casual 


TESTING  OF  GLUE  421 

observer,  certain  extremes  cannot  pass  without  revealing  their 
significance. 

The  Inspection  Test. — A  good  glue  will  be  firm;  free  from 
the  development  of  a  large  number  of  cracks,  called  craze;  will 
preferably  be  clear  and  translucent  unless  rendered  opaque  with 
some  added  material;  it  may  be  light  amber  or  dark  brown,  but 
never  black;  it  will  not  break  easily  when  bent,  but  will  show 
resistance  to  pressure  and  will  be  elastic;  it  will  not  have  a 
disagreeable  or  decomposed  odor,  even  after  making  into  solution ; 
and  it  will  not  quickly  develop  such  an  odor  on  remaining  in 
solution. 

If  the  glue  is  badly  crazed,  it  is  a  very  weak  sample.  If  it  is 
muddy  in  appearance,  or  very  dark  brown  and  opaque,  it  signifies 
that  the  sample  was  from  the  last  runs  of  the  cooking,  and  con- 
tains the  dregs  of  the  stock  and  the  excessively  hydrolyzed 
material.  Light  amber  colored  glues,  either  clear  or  otherwise, 
are  more  likely  to  be  of  bone  origin,  while  the  light  and  dark 
brown  glues  are  more  probably  obtained  from  hide,  fleshing,  and 
sinew  stock.  By  breathing  upon  a  sample  and  noting  the  odor, 
the  bone  glues  can  usually  be  distinguished  by  a  characteristic, 
rather  pungent,  odor.  A  speckled  appearance  may  be  traced  to 
a  precipitation  of  calcium  or  barium  sulphate,  due  to  an  improper 
treatment  in  the  bleaching  process.  Flakes  of  the  highest  grade 
glues  may  be  bent  double  without  breaking,  while  the  poorer 
samples  may  be  broken,  unless  too  thick,  with  the  least  amount 
of  pressure  between  the  fingers.  Glues  that  have  undergone 
bacterial  decomposition  will  give  off  putrefactive  odors,  even 
when  dry,  but  these  will  be  highly  intensified  when  the  sample 
is  put  into  solution.  A  good  glue  should  remain  perfectly  sweet 
for  at  least  48  hours  after  being  put  into  solution. 

The  form  in  which  the  glue  is  put  on  the  market  is  not,  as 
many  have  believed,  indicative  of  some  particular  grade  or  line 
of  material,  but  is  rather  to  satisfy  the  belief  of  certain  consumers 
that  such  is  the  case.  Before  modern  methods  were  introduced 
into  glue-house  technology  glue  was  made  by  a  large  number 
of  small  plants  scattered  widely,  and  as  a  rule  all  of  the  glue  from 
any  one  plant  was  put  out  in  the  same  form.  Consumers  thus 
fell  into  the  error  of  associating  certain  desirable  properties 
of  glue  with  the  form  in  which  that  particular  glue  was  marketed, 
and  this  created  a  demand  for  certain  forms  that  has  existed  to 
the  present,  although  there  is  now  no  foundation  for  such  a 


422  GELATIN  AND  GLUE 

distinction.  A  given  glue  is  now  made,  in  a  given  plant,  into 
sheet  glue,  ribbon  glue,  thin  cut  flake  glue,  thick  cut  flake  glue, 
ground  glue  of  various  sizes,  and  powdered  glue.  The  essential 
properties  of  the  several  forms  are  identical. 

There  is,  however,  a  tendency  to  make  the  higher  grades  of 
glue  thin  cut.  The  product  has  a  clearer,  more  translucent, 
and  lighter  color,  but  the  lower  grades  cannot  be  made  very  thin, 
as  they  would  too  easily  break  into  small  fragments.  The  thick 
sheet  and  ribbon  glues  are  usually  a  rather  low  grade  bone 
product.  There  has  been  an  objection  to  the  use  of  ground  glue 
on  the  stand  that  it  was  more  easily  susceptible  of  adulteration 
with  inferior  material  when  in  that  condition,  without  the  fact 
being  made  manifest  by  the  appearance  of  the  glue.  This  is  an 
unwarranted  objection  at  present,  for  nearly  all  glue  is  now 
bought  and  sold  on  specification,  and  if  mixtures  are  made  it  is 
with  the  knowledge  and  permission  of  the  consumer.  On  the 
other  hand,  there  is  a  distinct  advantage  in  the4use  of  ground 
glue,  as  the  time  required  to  put  into  solution  is  very  materially 
shortened,  and  the  space  occupied  by  a  given  weight  is  much  less 
when  in  the  ground  condition. 

Sheppard1  has  suggested  that  such  expressions  as  "  water- 
clear"  may  verj^  advantageously  be  replaced  by  a  definite  per 
cent  of  clarity,  and  definite  colorimetric  values,  where  color  is  a 
factor.  He  proposes  to  use  for  this  purpose  the  principle  of 
crossed  gratings,  on  lines  similar  to  those  employed  by  Ives2  in 
his  test  object  for  visual  acuity.  The  apparatus  consists  of 
"two  superposed  opaque  line  gratings  arranged  to  rotate  rela- 
tively to  each  other  about  an  axis  perpendicular  to  their  plane. 
Viewed  by  transmitted  light,  at  such  a  distance  that  the  grating 
lines  are  below  the  limit  of  resolution,  parallel  dark  bands  are 
seen.  The  separation  of  these  alters  quite  continuously  as  the 
gratings  are  rotated,  so  that  we  have  a  continuous  change  from 
extreme  visibility  to  invisibility  when  the  bands  can  no  longer 
be  resolved." 

Crazed  Glue. — Some  of  the  lower  grades  of  glue  break  at 
the  slightest  pressure,  and  on  inspection  of  such  a  piece  it  is 
observed  oftentimes  to  be  traversed  by  a  large  number  of  fine 
cracks.  In  extreme  cases  the  whole  mass  will  crumble  to  small 
cubical  and  rectangular  fragments  ranging  usually  from  a  thirty- 

1  S.  E.  SHEPPARD,  J.  Ind.  Eng.  Chem.,  12  (1920),  167. 

2H.  E.  IVES,  Elec.  World  (1910),  939;  /.  Optical  Soc.  Am.  (1917),  100. 


TESTING  OF  GLUE  423 

second  to  an  eighth  of  an  inch  on  a  side.  Such  a  glue  is  spoken 
of  as  crazed,  and  since  it  is  the  farthest  removed  from  the  elastic 
and  pliable  forms  it  is  naturally  given  the  lowest  rating  by  the 
"  inspection  test." 

A  study  of  this  peculiar  property  has  been  made  by  the  author.1 
It  was  first  observed  that  the  moisture  content  of  all  crazed 
samples  had  dropped  to  an  unusually  low  value,  averaging  about 
11  per  cent.  Samples  of  the  same  lots  that  had  been  stored 
under  different  conditions,  and  had  not  been  exposed  to  the  warm 
dry  air  with  which  the  crazed  samples  had  been  brought  in 
contact,  were  still  firm,  and  on  test  were  found  to  average  about 
15  per  cent  water.  These  were  in  turn  placed  in  a  dryer  and 
warmer  atmosphere  and  the  water  content  noted  at  the  point 
when  crazing  first  became  manifest.  This  was  found  to  lie  at 
about  11.5  per  cent  water.  The  maximum  difference  in  the 
water  content  at  this,  point  was  less  than  0.4  per  cent.  The 
water  content  is  therefore  shown  to  be  an  important  factor  in 
crazing,  for  it  seems  that  just  as  soon  as  this  value  reaches  a 
certain  minimum  amount,  the  low  grade  glue  will  craze. 

But  many  glues  that  are  of  the  same  viscosity  and  jelly 
strength,  as  determined  by  the  usual  means,  will  not  become 
crazed  when  exposed  to  the  same  conditions  as  those  above 
described.  Their  moisture  content,  when  they  have  reached 
equilibrium  with  the  air,  is  higher  than  11.5  per  cent.  In  an 
attempt  to  find  out  exactly  wherein  lay  the  differences  between 
these  two  types  that  one  should  be  able  to  retain  more  water 
under  any  given  conditions  than  the  other,  a  number  of  samples 
of  both  types  were  first  examined  for  their  content  of  ash,  nitro- 
gen, and  organic  matter.  These  data  not  furnishing  any  results 
of  value,  the  glues  were  examined  for  protein,  proteose,  peptone, 
and  amino-acid.2  It  was  found  that  the  crazed  samples  con- 
tained less  protein,  and  more  proteose  and  peptone  than  the  firm 
samples.  The  averages  showed  the  following  distribution  of  the 
nitrogen: 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  154. 

2  See  pages  25  to  28. 


424 


GELATIN  AND  GLUE 


Protein  N 

Proteose  N 

Peptone  N 

Amino-acid  N 

Crazed  samples.  .  . 
Firm  samples  

38.9 
42.3 

46.2 
44.6 

12.8 
11.0 

2.1 
2.1 

The  ordinary  test,  it  was  stated,  showed  the  same  for  the  two 
sets  of  samples,  but  it  has  been  shown  that  the  jelly  consistency 
and  the  viscosity  are,  under  similar  conditions,  proportional  to 
the  protein  content.  The  above  data  would  signify,  therefore, 
that  these  tests  should  be  higher  in  the  case  of  the  firm  samples. 
The  reasons  why  they  are  not  found  to  be  higher  lie  in  the 
indelicacy  of  the  tests  by  which  they  are  determined.  The  dif- 
ferences even  between  the  tests  for  pure  water  and  these  very  low 
grade  glues  is  hardly  appreciable,  but  by  the  use  of  more  refined 
instruments  the  author  has  been  able  to  show  that  just  as  the  vis- 
cosity of  water  and  a  weak  glue  is  really  very  different,  so  also 
is  that  of  the  above  crazed  and  firm  glues  under  identical  con- 
ditions. The  data  indicate  that  the  firm  samples  should  have 
the  higher  test,  and  this  is  found  to  be  the  case  when  appropri- 
ately measured. 

Crazing  in  glues  is  therefore  found  to  be  due  to  an  exceptionally 
great  hydrolysis  of  the  protein  molecule,  and  the  consequent 
inability  of  the  resulting  mixture  to  retain  water  above  that 
minimum  content  below  which  crazing  may  occur.  The  custom- 
ary rating  of  a  crazed  glue  as  an  inferior  product,  by  the  inspec- 
tion test,  is  accordingly  shown  to  be  correct  in  principle  from 
the  standpoint  of  evaluation  by  either  the  protein  content  or  the 
usual  tests  of  jelly  consistency  and  viscosity,  provided  the  latter 
are  made  by  sufficiently  sensitive  instruments. 


CHAPTER  IX 

THE  CHEMICAL  ANALYSIS,  DETECTION,  AND  ESTIMA- 
TION OF  GELATIN  AND  GLUE 

There  are  agents  in  Nature  able  to  make  the 

particles    of   bodies   stick   together   by   very 

strong   attractions.     And  it  is  the  business 

,  of  experimental  philosophy  to  find  them  out. 

Isaac  Newton  (1700) 
%  PAGE 

I.  The  Chemical  Analysis  of  Gelatin  and  Glue 427 

1.  The  Sampling 427 

2.  Proximate  Analysis 429 

3.  Ash  Analysis 433 

4.  The  Organic  Analysis  of  Gelatin 448 

5.  Further  Analytical  Tests , 450 

6.  Traces  of  Metals  in  Gelatin 456 

II.  The  Estimation  of  Gelatin  in  Commercial  Gelatin  and  Glue 463 

1.  Alcoholic  Precipitation 463 

2.  Tannin  Precipitation 464 

3.  Picric  Acid  Precipitation 466 

4.  Indirect  Methods 467 

5.  Salt  Precipitation 468 

III.  The  Detection  and  Estimation  of  Gelatin  and  Glue  in  Foods  and 

Miscellaneous  Products 470 

1.  Gelatin  in  Meat  and  Meat  Products 470 

2.  Gelatin  in  Milk,  Cream,  and  Ice  Cream 477 

3.  Gelatin  in  Other  Food  Products 478 

4.  Glue  in  Size  and  Miscellaneous  Preparations 483 

A  chemical  examination  of  gelatin  and  glue  may  be  of  value  and 
very  necessary  for  a  number  of  purposes.  In  any  kind  of  research 
dealing  with  these  substances  it  is  very  desirable  that  as 
complete  a  knowledge  as  may  be  practicable  of  the  chemical  com- 
position should  be  obtained.  From  the  nature  of  the  material, 
commercial  gelatin  cannot  be  regarded  as  a  pure  chemical 
compound.  It  may  contain  much  ash  or  little,  and  not  only  the 
amount  but  the  composition  of  the  ash  may  be  of  vital  impor- 
tance in  determining  the  properties  of  the  product.  The  mate- 
rial may  consist  essentially  of  the  unhydrolyzed  protein,  or  it  may 
be  largely  in  the  form  of  proteoses,  peptones,  and  even  amino- 
acids.  A  determination  of  the  relative  amounts  of  each  of  these 

425 


426  GELATIN  AND  GLUE 

nitrogenous  constituents  present  should  be  of  the  greatest  value 
in  any  study  of  physico-chemical  relationships,  and  has  already 
thrown  much  light  upon  the  chemical  causes  for  variations  in 
physical  properties. 

The  determination  of  the  hydrogen-ion  concentration  is,  in  the 
opinion  of  the  author,  one  of  the  most  important  of  the  later 
contributions  to  the  physico-chemistry  of  gelatin.  By  means  of 
this  simple  test  the  position  of  the  specimen  at  hand  in  the 
curves  for  swelling,  viscosity,  jelly  consistency,  and  joint  strength 
may  be  ascertained,  and  information  obtained  as  to  whether  the 
sample  has  reached  its  maximum  for  these  properties,  and,  if  not, 
the  exact  treatment  necessary  to  bring  this  desirable  change 
about  is  indicated.1  The  applications  of  this  new  test  are  still 
in  the  period  of  infancy,  but  much  is  expected  of  them.  In  fact 
we  may  already  go  so  far  as  to  affirm  that  the  properties  of  a 
gelatin  or  glue,  including  those  mentioned  above  which  are 
ordinarily  used  as  a  basis  for  grade,  are  adequately  defined  by 
three  factors:  First,  the  concentration  of  the  unhydrolyzed 
protein  gelatin  in  the  sample;  second,  the  hydrogen-ion  concen- 
tration; and  third,  the  nature  and  amount  of  inorganic  salts 
present.  With  any  given  values  for  these  three  factors,  all  other 
variables  will  be  found  to  approach  to  definite  and  precisely 
allocated  terms.  The  action  of  salts,  the  degree  of  dispersion, 
the  solvation,  are  all  functions  of  the  ionization,  and  the  ioniza- 
tion  is  shown  to  bear  a  definite  relation  to  hydrogen-ion 
concentration. 

The  application  of  chemical  methods  of  examination  to  control 
work  in  laboratories,  or  to  the  commercial  evaluation  of  gelatin 
or  glue,  is  probably  limited.  The  hydrogen-ion  concentration 
test  will  undoubtedly  find  employment.  Occasionally  the 
chemical  test  for  fat  is  used,  and  moisture  is  often  determined. 
The  various  tests  for  acids  are  sometimes  used.  But  in  general 
it  seems  that  the  physical  tests  are  of  sufficient  accuracy,  and  are 
so  much  more  easily  made,  and  require  a  so  much  lesser 
degree  of  skill  and  chemical  intelligence,  that  they  are  not  likely 
to  be  superceded  by  chemical  tests.2 

The  requirements  of  the  Federal  and  State  Pure  Food  Laws 
make  it  necessary  that  gelatin  for  use  as  a  food  or  in  medicinal 
preparations  shall  be  practically  free  of  all  substances  of  a 

1  See  Chap.  X  for  a  further  treatment  of  this  subject. 

2  The  evaluation  of  gelatin  and  glue  is  considered  in  Chap.  X. 


CHEMICAL  ANALYSIS  OF  GELATIN  427 

poisonous,  or  in  any  sense  harmful,  nature.  Such  products 
should  accordingly  be  tested  for  sulphur  dioxide,  arsenic,  copper, 
zinc,  tin,  and  lead,  as  well  as  for  acidity  or  alkalinity  and  occa- 
sionally other  specific  substances,  in  addition  to  some  test  that 
will  indicate  the  actual  amount  of  unhydrolyzed  gelatin  present. 

The  use  of  gelatin  or  glue  in  foods  and  in  numerous  com- 
mercial preparations  makes  it  necessary  that  methods  be  devel- 
oped by  which  the  presence  of  this  animal  product  may  be 
accurately  indicated  and  estimated  when  present  only  in  small 
quantities  and  in  the  presence  of  other  proteins  or  somewhat 
similar  substances.  Gelatin  often  finds  its  way  into  meat 
extracts,  cream,  ice  cream,  and  fruit  products,  and  occasionally 
into  chocolate,  coffee,  etc.  Glue  may  be  present  in  a  hundred  or 
more  technical  preparations,  as  pastes,  paints,  size,  calsomine, 
printers'  rollers,  fillers,  etc.,  etc. 

An  attempt  is  made  in  the  present  chapter  to  present,  therefore, 
not  only  the  inorganic  and  organic  methods  of  analysis,  but  the 
tests  which  may  be  employed  in  testing  for  gelatin  and  glue  in 
various  preparations. 

I.  THE  CHEMICAL  ANALYSIS  OF  GELATIN  AND  GLUE 

The  chemical  examination  of  a  gelatin  or  glue  is  commonly 
separated  into  an  analysis  of  organic  and  of  inorganic  con- 
stituents. For  some  purposes  it  suffices  to  determine  only  the 
moisture,  ash,  organic  matter,  and  nitrogen.  Such  an  examina- 
tion is  spoken  of  as  a  proximate  analysis.  It  is  sometimes 
desirable  to  know  the  complete  mineral  content  of  the  material, 
in  which  case  the  ash  is  further  examined  by  the  methods  of 
quantitative  inorganic  analysis.  The  presence  of  even  traces  of 
arsenic,  lead,  copper,  etc.,  is  impermissible  in  edible  gelatin,  and 
special  tests  may  be  made  for  these  constituents.  It  is  often- 
times essential  that  the  nature  of  the  organic  material  of  the 
gelatin  or  glue  should  be  ascertained,  and  various  methods  may  be 
employed  for  making  appropriate  determinations. 

1.  The  Sampling. — Whenever  a  glue  is  delivered  in  carload 
lots,  or  in  barrels  by  the  carload,  it  is  inevitable  that  the  glues  will 
vary  in  different  parts  of  the  shipment.  There  are  several 
reasons  for  this.  In  the  first  place  the  glue  obtained  from  a 
single  boiling  of  a  single  kettle  would  be  insufficient  to  fill  the 
order,  and  consequently  boilings  taken  from  a  number  of  different 


428  GELATIN  AND  GLUE 

kettles  of  the  same  stock,  or  of  different  stocks  that  happen  to  give 
particular  tests,  such  as  jelly  consistency  or  viscosity,  that  are 
identical,  are  mixed  or  packed  separately,  and  constitute  the 
single  shipment.  The  similarity  of  any  given  physical  test  in  two 
or  more  lots  of  glue  cannot  be  taken  as  an  indication  that  any 
other  tests  are  likewise  similar,  and  the  chemical  analysis  might 
show  very  different  results  in  the  several  cases. 

Whenever  two  or  more  lots  of  glues  are  mixed  and  the  mixture 
ground  there  is  always  a  tendency  for  the  finer  or  heavier  flakes 
to  sift  down  toward  the  bottom  of  the  barrel,  while  the  larger 
or  lighter  flakes  remain  at  the  top.  Unless  the  flakes  of  the 
mixture  are  of  exactly  the  same  size  and  density  there  will  be  a 
separation  of  the  component  glues  in  the  barrel,  the  top  being 
richer  in  the  larger  lighter  component,  the  bottom  being  richer 
in  the  finer  heavier  type. 

A  third  cause  for  variation  in  different  parts  of  the  shipment  is 
found  in  the  readiness  with  which  glue  and  gelatin  absorb 
moisture  from  the  air  when  the  latter  is  of  high  humidity,  and 
give  off  the  moisture  when  the  humidity  falls.  The  glue  and 
gelatin  act  indeed  as  a  hygrometer,  and  there  is  a  definite  water 
content  of  any  given  specimen  at  equilibrium  at  any  given  tem- 
perature and  humidity.  The  author1  has  shown  that  a  low 
grade  glue  that  will  craze  badly  when  allowed  to  stand  for  a  few 
days  in  a  cabinet  of  samples  in  an  office  will  be,  however, 
perfectly  firm  in  the  barrels,  stored  in  a  cooler  and  moister  place. 
The  difference  in  moisture  content  was  as  much  as  6  per  cent. 
It  is  obvious  that  the  glue  at  the  center  of  a  barrel  will  have  less 
contact  with  the  external  air  and  so  less  opportunity  for  fluctua- 
tion in  moisture  content  than  the  exterior  portions.  For  this 
reason  wide  variations  have  been  observed  at  different  portions 
of  the  barrel. 

For  a  perfectly  representative  sample  it  would  be  best  to 
thoroughly  mix  the  entire  shipment  before  sampling,  but  this 
procedure  is  not  usually  possible.  Portions  of  a  pound  each 
may  be  taken  from  various  parts  of  the  barrels,  and  on  account 
of  the  possible  shipment  of  more  than  one  lot,  each  barrel  should 
receive  a  separate  inspection.  The  several  portions  removed 
from  a  barrel  are  then  thoroughly  mixed,  and  a  portion  of  about 
a  pound  taken  from  different  parts  of  this  lot  and  ground  in  a 
small  mill  (a  coffee  mill  works  well  for  the  purpose)  to  as  fine  a 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  154. 


CHEMICAL  ANALYSIS  OF  GELATIN  429 

flake  as  possible.  This  is  then  placed  in  a  tight  glass-stoppered 
bottle,  from  which  all  future  samples  for  analysis  or  test  should 
be  taken. 

2.  The  Proximate  Analysis. — The  proximate  analysis  consists 
of  a  determination  of  the  moisture,  organic  matter,  ash,  and 
nitrogen.  The  moisture,  organic  matter,  and  ash  taken  together 
give  data  that  are  often  very  useful.  Other  conditions  being 
equal,  the  moisture  content  is  found  to  vary  directly  as  the 
protein  content  of  the  substance,  and  is  thus  an  indication  of 
that  important  factor.  A  high  ash  figure  usually  signifies  the 
addition  of  some  inorganic  material,  which  may  have  been 
introduced  as  a  clarifying  agent,  or  a  filler,  or  for  a  number 
of  other  reasons.  The  nitrogen  figure  is  not  of  great  importance 
for  the  greatest  diversity  of  glues  and  gelatins  have  been  shown1 
to  contain  almost  identical  amounts  of  nitrogen.  It  is  no  indica- 
tion of  the  true  protein  present,  for  the  hydrolyzed  proteose  and 
peptone  as  well  as  amino-acids  are  all  included  in  the  one  figure. 
The  practice  therefore  of  multiplying  the  nitrogen  value  by  a 
factor  to  obtain  the  protein  content  is  unsound  and  misleading 
as  far  as  glues  are  concerned. 

Moisture. — A  5  or  10  gram  sample  of  the  finely  ground  glue  is 
placed  in  a  tared  aluminium  or  tin  dish  of  about  3  inches  in 
diameter,  and  placed  in  an  electric  oven  at  110-115°C.,  or  a 
vacuum  oven  at  80°C.,  and  allowed  to  remain  until  the  weight  is 
constant.  Not  less  than  12  hours  in  the  former  and  6  hours  in 
the  latter  case  should  be  allotted  for  the  expulsion  of  all  of  the 
moisture,  and  if  the  flake  of  the  glue  is  coarse  or  thick  a  longer 
time  may  be  necessary.  The  loss  in  weight  sustained  by  the 
sample  is  moisture.  At  a  medium  degree  of  humidity  the  mois- 
ture content  of  glues  will  vary  from  about  10  to  11  per  cent  in  the 
lowest  to  about  15  to  16  per  cent  in  the  highest  grades,  but  under 
conditions  of  high  humidity  a  low  grade  glue  may  run  as  high 
as  17  per  cent  moisture,  while  under  very  dry  conditions  a  high 
grade  of  glue  may  show  as  little  as  12  per  cent  moisture. 

Ash. — A  sample  weighing  from  3  to  5  grams,  or  the  residue 
left  from  the  moisture  determination,  provided  the  amount  taken 
for  the  latter  was  not  over  5  grams,  is  placed  in  a  quartz  or 
porcelain  crucible.  Platinum  may  be  used  with  safety  only 
when  the  absence  of  phosphorus  is  assured,  as  a  slight  reduction 
of  the  phosphates  may  result  in  a  corrosion  of  the  platinum 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  105. 


430  GELATIN  AND  GLUE 

crucible.  The  crucible  is  heated  very  slowly  over  a  bunsen 
burner  until  the  moisture  is  completely  expelled,  care  being  taken 
that  the  glue  is  not  heated  so  high  that  the  particles  explode  upon 
the  escape  of  steam.  The  temperature  is  raised  slowly,  not 
permitting  the  mass  to  catch  fire,  until  the  gases  have  escaped. 
A  slow  incineration  is  then  brought  about  by  the  application  of  a 
low  flame  for  several  hours,  or  preferably  by  placing  the  crucible 
in  a  muffle  furnace.  Too  high  a  temperature  must  be  avoided  as 
a  graphite-like  carbonaceous  residue  may  be  formed  which  is 
oxidized  exceedingly  slowly.  A  high  temperature  is  also  likely 
to  result  in  a  reduction  of  the  phosphates  and  a  volatilization  of 
alkali  chlorides.  If  the  ash  is  rich  in  silica,  as  in  glues  that  have 
been  clarified  with  sodium  silicate,  or  in  phosphates  from  the 
solution  of  too  much  of  the  bone  during  boiling,  or  the  use  of  an 
excessive  amount  of  phosphoric  acid  in  the  manufacturing 
process,  the  carbon  may  not  burn  off  at  the  low  temperature 
necessary.  In  this  case,  char  the  material,  treat  with  water  or 
dilute  acetic  acid  to  dissolve  the  soluble  salts,  filter  through  an 
ashless  filter,  dry  the  paper  and  incinerate.  When  free  of 
carbon  add  the  filtrate  to  the  incinerated  residue,  evaporate  to 
dryness,  and  ignite  at  a  low  temperature.  Place  in  a  desiccator 
until  cool,  and  weigh.  The  residue  is  recorded  as  ash.  This 
should  then  be  intimately  mixed  and  preserved  in  a  tightly 
stoppered  bottle  for  use  in  the  ash  analysis,  to  be  described  later. 

This  product  should  be  white,  or  slightly  reddish  due  to  the 
presence  of  oxides  of  iron,  but  entirely  free  from  black  particles  of 
carbon.  It  may  be  easily  fusible,  due  to  the  presence  of  phos- 
phates of  calcium  and  magnesium,  or  it  may  be  quite  infusible 
at  the  temperatures  employed.  It  has  often  been  stated  that  the 
fusibility  of  the  ash  served  as  an  indication  of  the  origin  of  the 
material,  the  ash  from  the  bone  glues  fusing,  since  they  contain 
phosphates,  and  that  from  the  hide  glues  not  fusing  as  they  did 
not  contain  phosphates.  At  the  present  time,  however,  phos- 
phoric acid  is  often  added  in  part  to  neutralize  the  lime  used  in 
swelling  hide  stock,  and  so  finds  its  way  into  this  product  in  as 
great  amount  as  it  exists  in  bone  glues.  This  means  of  distin- 
guishing between  the  two  types  is  therefore  not  permissible 
today.  On  the  other  hand,  both  hide  and  bone  glues  may  be 
incinerated  and  the  ash  obtained  in  an  unfused  condition  if 
especial  care  is  taken  in  keeping  the  temperature  low. 

The  ash  content  of  glues  varies  from  about  1  to  5  per  cent 


CHEMICAL  ANALYSIS  OF  GELATIN  431 

where  no  inorganic  material  has  been  added  directly  to  the  glue. 
Where  such  additions  have  been  made,  as  in  colored  glues,  the 
ash  content  may  rise  to  10  or  15  per  cent. 

Organic  Matter. — The  organic  matter  is  estimated  by  subtract- 
ing the  sum  of  the  percentages  of  the  moisture  and  the  ash 
present  from '100.  On  the  basis  of  dry  moisture-free  glue,  the 
organic  matter  does  not  vary  materially  on  passing  from  a  high 
to  a  low  grade  glue,  but  since  the  moisture  content  has  been  shown 
to  vary  with  the  grade,  the  organic  matter,  calculated  on  the 
basis  of  the  ordinary  glue,  is  found  to  be  actually  lower  in  the 
high  grades  than  it  is  in  the  low  grades.  The  total  content  of 
organic  matter  in  the  glue  is  obviously  not  of  critical  importance 
in  the  determination  of  grade. 

Nitrogen. — The  total  nitrogen  of  gelatin  or  glue  is  best  deter- 
mined by  a  modification  of  the  Kjeldahl  process.  One  gram 
samples  are  weighed  out  and  placed  in  Kjeldahl  digestion  flasks, 
of  Pyrex  or  other  acid-resistant  make,  of  800  c.c.  capacity. 
About  10  grams  of  potassium  sulphate  and  a  small  crystal  of 
copper  sulphate  the  size  of  a  pin  head  are  added .  Fifteen  to  twenty 
c.c.  of  pure  concentrated  sulphuric  acid  are  then  poured  down  the 
side  of  the  flask,  and  the  contents  shaken  gently  that  the  glue 
may  be  disseminated  throughout  the  acid.  If  a  lump  is  left 
adhering  to  the  glass  the  flask  may  crack  on  applying  heat.  It 
has  been  found  very  advantageous  also  to  introduce  into  the 
flask  a  few  small  crystals,  the  size  of  a  pin  head,  of  garnet  or 
carborundum.  These  prevent  a  bumping  of  the  flask  during  the 
digestion  and  distillation.  The  flask  is  then  placed  in  an  inclined 
position,  resting  upon  a  circular  opening  in  an  asbestos  board, 
with  the  neck  introduced  into  a  pipe  from  which  the  sulphur 
trioxide  fumes  are  exhausted  by  a  good  draught,  but  open  at  the 
lower  end  that  the  condensed  water  and  acid  may  drain  out  into  a 
receiver.  If  no  good  chimney  or  fan  draught  is  available,  very 
satisfactory  results  may  be  obtained  by  using  a  1J^  inch  lead 
pipe,  closed  at  its  upper  end,  and  emptying  into  water  at  the 
lower  end.  In  this  case  the  connections  with  the  flasks  are 
made  through  %  inch  lead  tubes  introduced  into  the  larger  tube 
at  convenient  intervals,  and  tight  joints  secured  with  rubber 
stoppers.1 

A  low  flame  is  introduced  below  the  flask  and  watched  care- 
fully until  the  first  excessive  foaming  has  subsided.  A  higher 

1  Cf.  F.  G.  MERKLE,  J.  Ind.  Eng.  Chem.,  8  (1916),  521. 


432  GELATIN  AND  GLUE 

temperature  is  then  applied,  and  continued  until  the  contents 
have  assumed  a  clear  light  blue  or  colorless  appearance.  No 
trace  of  brown  or  amber  color  should  remain.  This  will  take 
from  3  to  6  hours.  The  flame  is  then  extinguished  and  the  flask 
left  undisturbed  until  the  contents  have  cooled.  Not  until 
the  bottom  of  the  flask  may  be  held  in  the  hand  without  burning 
should  the  next  step  be  taken. 

About  400  c.c.  of  cold  water  are  added  to  the  contents  of  the 
cooled  flask,  followed  by  a  sufficient  amount  of  saturated  sodium 
hydroxide  to  render  the  contents  distinctly  alkaline.  This  may 
conveniently  be  ascertained  by  adding  a  drop  of  methyl-red 
indicator  solution,  which  is  yellow  in  alkaline  but  red  in  acid 
solution.  In  the  event  that  this  red  color  gives  place  to  yellow 
at  any  time  during  the  subsequent  distillation,  more  alkali  must 
be  added.  The  flask  is  at  once  connected  with  a  condenser,  the 
lower  end  of  which  dips  into  a  flask  containing  25  c.c.  of  a 
standard  solution  of  N/2  hydrochloric  acid,  to  which  2  or  3 
drops  of  methyl-red  indicator  have  been  added.  Heat  is  again 
applied  to  the  flask,  and  about  100  c.c.  distilled  over. 

The  distillate  is  titrated  against  a  standard  solution  of  N/2 
sodium  hydroxide.  The  difference  in  the  volume  of  standard 
acid  used  and  standard  alkali  necessary  to  complete  the  titration 
represents  the  volume  of  N/2  acid  necessary  to  neutralize  the 
nitrogen,  in  the  form  of  ammonia,  which  was  contained  in  the 
sample.  Since  1  c.c.  of  N/2  acid  will  neutralize  0.007  gram  of 
nitrogen  as  ammonia,  then  the  volume  of  acid  required,  multi- 
plied by  0.007,  and  the  product  divided  by  the  weight  of  sample 
taken,  will  give,  on  multiplying  by  100,  the  percentage  of  nitrogen 
in  the  sample,  or 

Vol.  N/2  HC1  X  0.007  X  100 

.  ,  .     ,  -  =  per  cent  N. 

weight  of  sample 

The  practice  of  multiplying  the  nitrogen  value  by  a  factor,  as 
6.25  or  6.56,  which  has  often  been  suggested  as  a  means  for  the 
estimation  of  the  protein  gelatin  in  the  sample  is  without  justi- 
fication except  the  sample  be  known  to  be  pure  gelatin.  By 
such  a  procedure  a  low  grade  glue  is  shown  to  possess  nearly  the 
same  percentage  of  protein  as  a  high  grade  gelatin,  and  this  has 
been  shown  by  the  author1  to  be  altogether  contrary  to  the  facts. 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  107. 


CHEMICAL  ANALYSIS  OF  GELATIN  433 

The  nitrogen  content  of  glues  and  gelatins  varies  but  little, 
but  on  a  moisture-free  basis  is  slightly  higher  in  the  better  grades 
than  in  the  lower  grades.  It  runs  from  14  to  15  per  cent  on  the 
moisture-present  basis,  and  16  to  18  per  cent  on  the  moisture- 
free  basis. 

On  account  of  the  constancy  of  the  nitrogen  content  of  all 
glues,  the  determination  is  not  of  great  value  in  ordinary  work. 
But  the  determination  has  been  the  best  means  for  detecting 
adulteration  of  glue  or  gelatin  with  certain  foreign  substances, 
as  dextrines,  starches,  dextrose,  glycerin,  etc.  These  substances, 
being  non-nitrogenous,  depress  greatly  the  nitrogen  content  of  the 
material.  Such  adulteration  is  easily  distinguished  from  the 
excessive  use  of  a  " filler,"  as  the  latter  will  be  evident  by  an 
ash  analysis.  But  if  the  nitrogen  content  of  the  organic  material 
of  the  glue  drops  below  its  customary  value,  adulteration  as 
above  is  indicated.  The  addition  of  nitrogenous  adulterants  as 
casein,  blood  albumin,  etc.,  could  not  pass  unobserved  on  account 
of  their  special  properties,  as  coagulation  on  heating,  etc. 

Several  determinations  for  the  nitrogen  content  of  water-free 
gelatin  are  given  below: 

MULDER,  Ann.,  45  (1843),  63 18.3 

SCHUTZENBERGER  and  BOURGEOIS,  Jahresber.  Tierchem.  (1876),  30..  .  18.3 

CHITTENDEN  and  SOLLEY,  J.  Physiol,  12  (1891),  33 18.0 

PAAL,  Ber.,  26  (1892),  1202 18. 12 

VAN  NAME,  /.  Exptl.  Med.,  2  (1897),  117 17.81 

SAKIDOFF,  Z.  physiol.  Chem.,  37  (1903),  397.     (Kjeldahl) 17.47 

(Dumas) 18.18 

HALLA,  Z.  angew.  Chem.,  20  (1907),  24 17.61 

BOGUE,  Chem.  Met.  Eng.,  23  (1920),  105.     (Kjeldahl) 17.50 

SMITH,  J.  Am.  Chem.  Soc.,  43  (1921),  1350 17.53 

3.  The  Ash  Analysis. — A  complete  ash  analysis  of  glues  and 
gelatins  should  include  determinations  for  the  following  inorganic 
components : 

Silica SiO2 

Ferric  oxide Fe2O3 

Lime CaO 

Magnesia MgO 

Potash  and  soda K2O  and  Na2O 

Sulphate SO3 

Phosphate 

Chloride Cl 

28 


434  GELATIN  AND  GLUE 

The  above  list  includes  all  of  the  inorganic  radicals  that  are  apt 
to  be  present  in  a  glue  or  gelatin  to  which  foreign  substances 
have  not  been  added.  Many  substances  may  be  added,  however, 
as  zinc  oxide,  calcium  carbonate,  or  lead  sulphate,  to  produce  a 
white  opaque  glue,  alum  for  clarifying  the  material  or  to  produce 
abnormal  viscosities,  barium  hydroxide  for  neutralizing  acidity, 
and  at  times  various  other  salts  or  pigments  for  special  purposes, 
as,  for  example,  potassium  dichromate  for  producing  insolubility. 
That  this  discussion  may  be  as  complete  as  is  consistently 
practicable,  the  following  radicals  will  be  added  to  those  given 
above  in  the  analytical  procedures  outlined  below: 

Zinc  oxide ZnO 

Alumina Al2Os 

Barium  oxide BaO 

Lead  oxide PbO 

The  analyses  in  table  48  are  typical  of  glues  and  gelatins. 

Silica.1 — Five  grams  of  the  dry  and  intimately  mixed  ash  are 
weighed  into  a  small  evaporating  dish,  10  c.c.  of  dilute  hydro- 
chloric acid  are  added,  and  the  mixture  evaporated  to  dry  ness 
and  heated  gently  to  render  the  silica  insoluble.  Ten  c.c. 
of  hydrochloric  acid  are  again  added  and  50  c.c.  of  water 
introduced.  The  dish  is  warmed  on  the  water  bath  for  a  few 
minutes  and  the  contents  filtered  through  an  ashless  filter  and 
washed  till  free  of  chloride.  The  combined  filtrate  and  washings 
are  made  up  to  250  c.c.  in  a  volumetric  flask  and  reserved  for 
further  determinations.  This  solution  will  be  designated  as 
" Solution  A."  The  residue  on  the  filter  paper  is  now  dried  and 
ignited,  and  after  cooling  in  a  desiccator,  weighed  as  SiO2. 

Weight  SiO2  X  100 


=  Per  cent  SiO 


Phosphoric  Acid.2 — Twenty-five  cubic  centimeters  of  solution 
A  are  pipetted  into  a  250  c.c.  beaker.  If  ferrous  iron  is  present, 
add  a  few  c.c.  of  concentrated  nitric  acid  or  hydrogen  peroxide 
and  boil  for  a  few  minutes.  Cool  and  add  ammonium  hydroxide 
until  a  precipitate  just  forms,  then  a  few  drops  of  nitric  acid  to 
clear,  and  2  to  3  c.c.  of  concentrated' nitric  acid  in  excess.  Now 
add  25  c.c.  of  50  per  cent  ammonium  nitrate  solution,  warm  to 

1  Association  of  Official  Agricultural  Chemists,  "Methods  of  Analysis" 
(1920),  15. 

2  A.  O.  A.  C.,  op.  tit.,  18. 


CHEMICAL  ANALYSIS  OF  GELATIN 


435 


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436  GELATIN  AND  GLUE 

40°C.,  and  add  slowly  with  constant  stirring  an  excess  of  molyb- 
date  solution  (prepared  as  directed  below),  and  allow  to  stand  for 
an  hour  at  a  warm  temperature.  Ascertain  if  an  excess  of  the 
molybdate  solution  has  been  added  by  adding  a  few  c.c.  of  the 
latter  to  a  clear  portion  of  the  solution.  In  case  a  precipitate 
forms  more  molybdate  solution  must  be  added.  The  solution  is 
allowed  to  stand  in  a  warm  place  over  night,  then  filtered,  and 
washed  with  about  75  c.c.  of  a  2.5  per  cent  ammonium  nitrate  sol- 
ution. The  combined  filtrate  and  washings  are  retained  for  other 
determinations,  and  designated  as  "  Solution  B."  The  precipitate 
is  dissolved  on  the  filter  by  pouring  through  it  successive  small 
portions  of  ammonium  hydroxide  and  hot  water.  The  solution 
which  should  not  exceed  100  c.c.  is  collected  in  a  beaker  and 
nearly  neutralized  with  hydrochloric  acid.  After  cooling,  mag- 
nesia mixture  (see  below)  is  added  very  slowly  from  a  burette, 
stirring  constantly.  After  some  minutes  12  c.c.  of  ammonium 
hydroxide  (sp.  gr.  0.90)  are  added.  The  beaker  is  set  aside  until 
the  supernatant  liquid  is  clear  (about  2  hours).  The  precipitate 
is  then  filtered  through  an  ashless  filter,  washed  with  dilute 
ammonium  hydroxide  till  free  of  chloride,  and  dried.  The 
residue  is  separated  from  the  filter  paper,  and  the  latter  igni  ted 
in  a  weighed  porcelain  crucible.  The  precipitate  is  then  added, 
and  the  crucible  heated  to  a  dull  redness.  It  is  cooled,  and  the 
phosphorus  weighed  as  Mg2P207.  This  is  calculated  to  P2O5. 

Weight  Mg2P207  X  0.638  X  100 

-~7i-£—  -  =  per  cent  P205. 

U.o 

The  volumetric  method  of  titrating  the  phospho-molybdate  precipitate 
with  standard  alkali  may  be  used  if  desired.  A  10  gram  sample  of  glue 
is  incinerated  in  a  porcelain  dish,  cooled,  and  2  c.c.  of  concentrated  hydro- 
chloric acid  and  5  c.c.  of  concentrated  nitric  acid  added.  This  is  evaporated 
to  dryness  and  the  residue  heated  to  a  dull  red  heat  or  until  perfectly  white. 
The  residue  is  dissolved  in  5  c.c.  of  hot  concentrated  nitric  acid,  boiled,  and 
filtered,  the  filter  being  washed  with  hot  water.  The  filtrate  is  treated 
with  ammonium  hydroxide  until  a  precipitate  just  forms,  this  is  dispelled 
with  a  few  drops  of  nitric  acid,  and  the  solution  warmed  to  about  85°C. 
Twenty-five  to  50  c.c.  of  ammonium  molybdate  solution  are  then  added, 
or  an  amount  sufficient  to  completely  precipitate  the  phosphate.  The  solu- 
tion is  allowed  to  stand  for  at  least  15  minutes,  and  is  then  filtered  through 
a  Gooch  crucible,  and  the  precipitate  washed  with  water  until  two  fillings 
of  the  filter  do  not  appreciably  decrease  the  color  produced  by  1  drop  of 
standard  sodium  hydroxide  and  3  drops  of  phenolphthalein.  The  filter  and 
precipitate  are  then  returned  to  the  precipitating  flask,  and  a  measured 
quantity  of  standard  solution  of  N/10  alkali  added  (at  least  enough  to  dis- 


CHEMICAL  ANALYSIS  OF  GELATIN  437 

charge  the  yellow  color  and  dissolve  the  precipitate)  .  A  few  drops  of  phenol- 
phthalein  are  added  and  the  mixture  titrated  back  with  standard  N/10 
acid.  The  reaction  involved  is  : 


2(NH4)3.12  MoO3.PO4  +  46  NaOH 

2(NH4)2  HPO4  +  (NH4)2  MoO4  +  23Na2MoO4  +  23H2O. 

Hence  46NaOH  are  equivalent  to  1P2O5,  or  1  c.c.  of  N/lO.NaOH  is  equiva- 
lent to  0.000309P2O5. 

C.c.  NaOH  X  0.000309  X  100 

—  —  r  —  =  per  cent  P2O5. 

Reagents  Used  in  Phosphate  Determinations.  Molybdate  Solu- 
tion. —  One  hundred  grams  of  molybdic  acid  are  dissolved  in 
dilute  ammonium  hydroxide  (144  c.c.  of  concentrated  ammonium 
hydroxide  +  271  c.c.  of  water).  This  solution  is  poured  slowly, 
stirring  constantly,  into  dilute  nitric  acid  (489  c.c.  of  concen- 
trated nitric  acid  +  1148  c.c.  of  water).  The  mixture  is  allowed 
to  stand  in  a  warm  place  for  several  days,  or  until  a  portion  heated 
to  40°C.  deposits  no  yellow  precipitate  of  ammonium  phos- 
phomolybdate.  The  solution  is  decanted  and  preserved  in 
glass-stoppered  vessels. 

Magnesia  Mixture.  —  Twenty-two  grams  of  recently  ignited 
magnesium  oxide  are  dissolved  in  dilute  hydrochloric  acid,  avoid- 
ing an  excess.  The  magnesia  is  added  in  slight  excess  and 
boiled.  Any  iron,  aluminium  or  phosphoric  acid  precipitated 
is  filtered  off.  Two  hundred  and  eighty  grams  of  ammonium 
chloride  and  261  c.c.  of  concentrated  ammonium  hydroxide  are 
added  and  the  whole  diluted  to  2  liters.  In  place  of  the  22 
grams  of  calcined  magnesia,  110  grams  of  crystallized  magnesium 
chloride  (MgCl2.6H2O)  dissolved  in  water  may  be  used,  and  the 
other  reagents  added  as  above. 

Phosphoric  Acid  in  Leach  Liquors.  —  A  very  convenient  method 
for  the  determination  of  the  form  of  combination  of  P2O5  in 
leach  liquors  has  been  developed  by  Shuey1  as  follows: 

As  the  titration  of  a  polybasic  weak  acid  does  not  follow  the  commonly 
accepted  rules  of  titration,  volumetric  determination  of  P2C>5  is  not  con- 
sidered possible.  Probably  to  obtain  extremely  accurate  results  by  a 
volumet  ic  method  would  require  more  care  and  time  than  a  gravimetric 
determination,  but  for  rough  factory  control  a  method  has  been  worked  out 
which  has  given  the  necessary  accuracy. 

If  a  known  H3PO4  solution  is  titrated  with  a  standard  NaOH  solution, 
using  methyl  orange  as  an  indicator,  the  neutral  point  of  the  indicator 
is  reached  at  the  equilibrium  H3PO4  +  NaOH  =  NaH2PO4  +  H2O. 

1  R.  C.  SHUEY,  personal  communication. 


438  GELATIN  AND  GLUE 

If  phenolphthalein  is  then  added  and  titration  continued  until  the 
development  of  a  rose'color  the  equilibrium  will  stand  H3PO4  +  NaOH  = 
Na2HPO4  +  2H2O. 

These  reactions  apply  to  the  soluble  salts  of  phosphoric  acid  either  direct 
or  dissolved  in  known  quantities  of  HC1.  If,  however,  a  calcium  salt  is 
dissolved  in  known  HC1  and  titrated  with  Ca(OH)2  for  example,  the  differ- 
ence in  titration  between  the  two  end  points  is  equivalent  to  two-thirds  of 
the  phosphate  present  in  place  of  one-third  as  above.  In  fact  the  titrations 
give  more  accurately  the  P2O6  content  of  the  solution  than  the  regular  titra- 
tions of  the  pure  acid  in  the  absence  of  calcium.  Ca(H2PO4)2  +  2Ca(OH)2 
=  Ca3(P04)2  +  4H20. 

Also,  in  the  presence  of  high  calcium,  it  was  observed  that  precipitation 
commences  at  the  exact  point  of  neutralization  of  the  first  third  of  the 
P2O6  and  is  a  sharper  indicator  than  the  methyl  orange  itself. .  That  is,  at 
this  point,  due  to  hydrolysis,  a  precipitate  of  CaHPO4  forms,  and  can  be 
used  as  a  check  indicator,  of  the  end  point  of  formation  of  Ca(H2PO4)2. 

If  to  a  known  phosphoric  acid  an  excess  of  carefully  neutralized  calcium 
chloride  is  added  and  titration  made  with  NaOH,  we  produce  a  combined 
condition.  In  the  first  case  it  was  titration  of  free  acid  with  sodium. 
In  the  second  case,  with  calcium.  In  this  case  it  is  titration  of  free  acid  with 
sodium  in  the  presence  of  calcium. 

The  methyl  orange  neutral  point  is  normal  and  coincident  with  the  above 
appearance  of  precipitate.  If  phenolphthalein  is  then  added  and  titration 
continued,  a  transitory  color  change  will  appear  at  the  two-thirds  point, 
and  will  not  become  permanent  until  sufficient  NaOH  has  been  added  to 
neutralize  all  the  acid.  This  indicates  that  although  the  precipitate  first 
formed  is  CaHPO4  it  rapidly  changes  to  Ca3(PO4)2,  and  is  completely 
changed  over  by  the  time  the  color  has  become  permanent. 

Experiments  based  upon  the  above  observations  were  used  in  working 
out  the  following  procedure,  which  is  applicable  to  the  determination  of 
soluble  sodium  and  calcium  salts  of  phosphoric  acid  either  alone  or  mixed 
with  each  other,  and  the  free  acid  or  the  corresponding  bases,  all  in  a  single 
sample  of  solution. 

A  10  c.c.  sample  is  taken  and  half  normal  NaOH  and  HC1  are  used  in 
the  titration.  A  50  per  cent  solution  of  calcium  chloride  is  made  up,  and 
adjusted  to  exact  neutrality  of  phenolphthalein. 

Phenolphthalein  is  added  to  the  sample;  if  it  does  not  turn  pink  methyl 
orange  is  added;  if  this  also  shows  acid,  titrate  with  standard  alkali  until  the 
orange  is  almost  gone.  Four  drops  more  should  show  a  pure  yellow. 
If  there  is  considerable  calcium  present  a  permanent  precipitate  will  form 
at  this  point,  and  the  change  to  yellow  will  not  be  as  sharp.  This  titration 
shows  one-third  of  the  free  acid.  H3PO4  +  NaOH  =  NaH2PO4  +  H2O. 

Titration  to  the  neutral  point  of  phenolphthalein  is  then  commenced, 
adding  about  50  c.c.  of  water  if  considerable  precipitate  is  formed.  The 
products  from  the  former  titration  again  react,  the  three  possible  reactions 
being : 

1.  If  there  was  free  acid:  NaH2PO4  +  NaOH  =  Na2HPO4  +  H2O,  again 
giving  }$  the  acid; 


CHEMICAL  ANALYSIS  OF  GELATIN  439 

2.  If  there  was  mono-sodium  phosphate:  NaH2PO4  +  NaOH  =  Na-r 
HPO4  +  H2O,  giving  M  the  mono-sodium  salt; 

3.  If  there  was  mono-calcium  phosphate:  3Ca(H2PO4)2  +  SNaOH   = 
4Na2HPO4  +  Ca3(PO4)2  +  8H2O,  giving  %  of  the  mono-calcium  salt. 

As  before  indicated,  this  is  made  possible  by  the  calcium  being  replaced  by 
sodium  which  again  is  titrated. 

When  an  excess  of  neutral  calcium  chloride  is  added,  all  three  of  the  above 
reactions  having  produced  Na2HPO4,  this  is  now  converted,  we  will  say: 
(1),  (2)  and  (3):  Na2HPO4  +  CaCl2  =  CaHPO4  +  2NaCl,  and  titration 
continued.  (1),  (2)  and  (3):  2CaHPO4  +  2NaOH  +  CaCl2  =  Ca3  (PO4)2 
+  2H2O  +  2NaCl,  again  giving  one-third  the  acid  and  one-third  the  mono- 
sodium  salt  originally  present  but  ^  of  the  mono-calcium  phosphate,  for 
we  have  titrated  the  final  %  of  the  P2O5  in  reactions  (3). 

Calculation. — Using  the  symbols  M.  O.  for  the  methyl  orange  titration, 
Ph  for  the  first  phenolphthalein  titration,  and  CaPh  for  the  final  titration 
in  excess  of  calcium,  and  having  in  mind  that  all  readings  are  taken  from 
zero: 

M.  O.  X  0.355  =  per  cent  P2O6  existing  as  free  acid. 

Ph  =  Free  Acid  +  Sodium  +  %  Calcium. 
CaPh  =  Free  Acid  +  Sodium  +  ^  Calcium. 
Solving  these  two  equations  for  sodium  we  get: 

(CaPh  -  3Ph)  X  0.355  =  P2O5  combined  with  Na. 
Solving  for  calcium  we  get: 

2i(2Ph  -  M.    O.  -  CaPh)  X  0.355  =  P2O6   combined   with    Ca. 
Also 

— - —  (all    from   monobasic   to  tribasic)  X  0.355  =  total  P2O6. 

If,  however,  the  original  sample  shows  acid  to  phenophthalein  and  alkaline 
to  methyl  orange  it  can  contain  only  Ca(H2PO4)2  and  NaH2PO4.  In  this 
case  titrate  back  to  the  neutral  point  of  methyl  orange  with  acid  and  M.  O. 
(acid)  X  0.355  =  P2O6  of  Na2HPO4.  We  now  have  all  monobasic  salts 
and  the  reasoning  of  the  former  example  applies,  except  that  the  alkali 
readings,  of  course,  start  from  the  M.  O.  neutrality  point,  therefore: 

2CaPh  -  3Ph  X  0.355  =  P2O5  combined  with  Na. 

%(2Ph  -  CaPh)  X  0.355  =  P2O6  combined  with  Ca. 

Also  (2CaPh  -  3Ph  -  M.    O.)  X  0.355  =  P2O6  combined  as  NaH2PO4, 

CaPh  X  0.355 

and  -  -    =  total  P2O6. 

2 

If  the  original  sample  shows  alkaline  to  phenolphthalein,  the  original 
sample,  if  sodium  is  absent,  will  contain  only  Ca(OH)2.  If  sodium  is 
present,  it  may  exist  as  Na2HPO4  or  NaOH  and  calcium  cannot  exist  in 
solution  to  any  appreciable  extent.  The  acid  required  to  reach  the  phenol- 
phthalein end  point  represents  Na3PO4  +  NaOH.  CaCl2  is  then  added 
in  excess  and  the  solution  titrated  with  alkali.  This  value,  if  smaller  than 
the  previous  titration,  represents  Na3PO4.  If  this  value  is  larger  than  the 
previous  titrations,  no  NaOH  is  present  and  it  represents  Na3PO4  + 
Na2HP04. 


440  GELATIN  AND  GLUE 

If  the  solution  is  titrated  back  to  neutrality  to  M.  O.,  the  value  should  be 
twice  that  of  the  CaPh  titration,  representing  the  total  P2O5.  For  calcu- 
lations, then  using  only  values  of  -  sign,  Ph  (acid)  -  CaPh  (alk.)  X  0.355  = 
NaOH  (ref.  as  P2O5  for  comparison). 

CaPh  (alk.)  X  0.355  =  P2O5  of  Na3PO4. 
Or  in  the  second  case, 

CaPh  (alk.)  -  Ph  (acid)  X  0.355  =  P2O6  of   Na2HPO4. 

Ph  acid  X  0.355  =  P2O6  of  Na3PO4. 
CaPh  (alk.)  X  0.355  =  total  P2O5. 

M'  °'  (aCid)  X  0.355  =  total  P20, 


In  this  calculation,  all  burette  readings  are  independent. 

An  insoluble  calcium  salt  of  phosphoric  acid  may  be  dissolved  in  HC1 
and  the  total  P2O5  determined,  but  failure  only  attended  attempts  to 
work  out  a  calcium  determination  which  would  show  the  partition  between 
the  two  insoluble  salts  Ca3(PO4)2  and  CaHPO4. 

The  accuracy  of  this  method  is  generally  within  5  per  cent  of  the  total 
P2O5,  but  the  error  in  the  sodium-calcium  position  is  multiplied  in  the 
calculations  and  is  somewhat  larger.  As  we  are  working  in  amphoteric 
solutions  the  end  points  are  reached  gradually  and  the  highest  accuracy  is 
attained  only  by  studying  the  exact  color  of  the  correct  end  points  in  solu- 
tions of  known  mixture.  In  working  this  out  the  titrations  were  made  on 
known  solutions  of  the  different  salts  and  the  reactions  are  an  attempt  to 
explain  the  observations.  Then  the  calculations  were  deduced  and  found 
to  hold  for  known  mixtures.  It  is  evident  that  a  method  lacking  accuracy 
has  only  limited  applications,  and  the  main  object  in  presenting  it,  is  that 
an  idea  of  the  composition  of  a  solution  can  be  gained  in  say  five  or  ten 
minutes,  and  in  the  factory  precipitation  of  phosphates,  the  supernatant 
liquor  varies  in  composition  so  much  more  widely  than  the  precipitate, 
that  the  composition  of  the  precipitate  from  that  liquor  may  be  known 
with  a  much  greater  degree  of  accuracy. 

Ferric  and  Aluminum  Oxide.1 — The  whole  of  solution  B  is 
cautiously  neutralized  with  ammonium  hydroxide  and  a  slight 
excess  of  the  alkali  added.  The  beaker  is  allowed  to  stand  at 
40°C.  until  the  precipitate,  which  contains  the  iron  and  aluminum 
oxides,  has  completely  settled.  The  clear  supernatant  liquid  is 
then  poured  through  a  filter  paper,  and  the  precipitate  washed  a 
few  times  with  hot  water  by  decantation  before  transferring  to 
the  filter,  and  a  few  times  on  the  filter.  The  precipitate  is 
dissolved  on  the  filter  with  hot  dilute  (1:5)  nitric  acid,  collected 
in  a  200  c.c.  beaker,  made  up  to  about  90  c.c.,  and  reprecipitated. 
It  is  filtered  through  the  same  filter  paper  and  washed  as  before. 

1  A.  O.  A.  C.,  op,  cit.,  16. 


CHEMICAL  ANALYSIS  OF  GELATIN  441 

The  combined  filtrates  and  washings  from  the  two  precipitations 
are  reserved  for  subsequent  determinations  and  designated  as 
"  solution  C."  The  filter  paper  with  its  contents  is  dried  and 
ignited,  and  weighed  as  Fe2O3  +  A12O3. 

Weight  Fe203  +  Al203X  100  =  p 

U.O 

It  is  usually  sufficient  to  determine  and  report  the  iron  and  aluminum 
oxides  together,  but  if  it  is  desired  to  separate  them,  the  following  procedure 
may  be  used.  The  residue  of  Fe2O3  +  A12O3  is  fused  in  a  platinum  crucible 
with  about  4  grams  of  fused  potassium  hydrogen  sulphate.  Only  a  few 
minutes  should  be  allowed  for  the  fusion.  After  the  crucible  is  cool,  5  c.c. 
of  concentrated  sulphuric  acid  are  added  and  heat  applied  until  sulphuric 
acid  fumes  are  given  off  copiously.  The  crucible  is  again  allowed  to  cool 
and  is  then  transferred  to  a  beaker  of  water  and  heated  until  the  solution 
is  clear.  The  iron  is  then  reduced  with  zinc,  the  solution  cooled,  and 
titrated  with  N/50  potassium  permanganate.  Since  2KMnC>4  =  5Fe2Os, 
1  c.c.  N/50  KMnO4  =  0.0016  Fe2O3.  Therefore 

C.c.  KMnO4  X  0.0016  X  100 

— — — —  -  =  per  cent  Fe2O3. 

0 .  o 

The  difference  between  the  percentage  of  Fe2O3  +  A12O3  and  of  Fe2O3 
gives  the  porcentage  of  A12O3. 

Calcium  Oxide.1 — For  this  determination  either  solution  C  or 
solution  A  may  be  used.  If  solution  A  is  used,  an  aliquot  of 
25  c.c.  (or  more  if  desired,  bearing  in  mind  the  amount  taken  for 
the  calculation)  is  taken,  and  pure  ferric  chloride  solution  added  in 
excess  of  that  required  to  combine  with  the  phosphoric  acid,  and 
ammonium  hydroxide  added  until  neutral.  The  precipitate  is 
dissolved  in  a  slight  excess  of  hydrochloric  acid,  and  1-2  grams 
of  sodium  acetate  added.  After  boiling  for  a  few  minutes  it  is 
filtered  and  washed  with  boiling  water.  The  precipitate  is 
dissolved  in  hydrochloric  acid  and  reprecipitated  as  before. 

The  combined  filtrates  and  washings  from  this  treatment,  or 
solution  C  taken  as  it  is,  are  evaporated  to  about  50  c.c.,  made 
slightly  alkaline  with  ammonium  hydroxide,  and  while  still  hot, 
ammonium  oxalate  solution  added  very  slowly  in  slight  exess 
of  the  amount  required  to  completely  precipitate  the  calcium. 
The  mixture  is  heated  to  boiling,  the  precipitate  allowed  to 
settle,  and  the  clear  solution  decanted  into  a  filter.  The  precipi- 
tate is  washed  in  the  beaker  with  20  c.c.  of  hot  water,  and  then 

1  A.  O.  A.  C.,  op.  cit.,  17. 


442  GELATIN  AND  GLUE 

dissolved  with  a  few  drops  of  hydrochloric  acid.  A  little  water 
is  added  and  the  precipitation  repeated  as  before,  filtering  through 
the  same  paper.  The  precipitate  is  transferred  completely  to  the 
filter,  and  washed  with  hot  water  till  free  of  chlorides.  It  is  then 
dried,  ignited  over  a  blast  lamp  to  constant  weight,  and  weighed 
as  CaO.  The  filtrates  and  washings  from  the  above  precipita- 
tions are  reserved  for  the  magnesia  determination,  and  designated 
as  " solution  D." 

Weight  CaO  X  100 

—fr£—          -  =  Per 
y.o 

The  calcium  may  also  be  precipitated  as  the  sulphate,  converted  into  the 
oxalate,  and  the  oxalic  acid  liberated  and  titrated  against  a  permanganate. 
Ten  grams  of  glue  are  incinerated  in  a  porcelain  dish,  allowed  to  cool,  and 
2  c.c.  of  concentrated  hydrochloric  acid  and  5  c.c.  of  concentrated  nitric 
acid  added.  The  solution  is  evaporated  to  dryness  and  again  incinerated 
at  a  low  red  heat,  or  until  white.  The  residue  is  dissolved  in  2  c.c.  of  con- 
centrated hydrochloric  acid,  filtered,  and  the  filter  thoroughly  washed  with 
hot  water.  The  nitrate  is  evaporated  to  about  40  c.c.  and  while  warm 
10  c.c.  of  dilute  (1: 1)  sulphuric  acid  added  and  then  150  c.c.  of  95  per  cent 
alcohol,  and  allowed  to  stand  for  12  hours.  The  precipitate  is  then  filtered 
off  and  washed  with  70  to  75  per  cent  alcohol  until  free  of  sulphates,  and 
dried.  The  precipitate  is  then  washed  from  the  filter  to  the  original  beaker, 
using  a  stream  of  hot  water,  and  the  paper  replaced  in  the  funnel,  and  thor- 
oughly washed  with  boiling  hydrochloric  acid  (1:5)  and  water.  The 
filtrate  is  made  slightly  alkaline  with  ammonia,  heated  to  boiling,  and 
ammonium  oxalate  solution  added  in  slight  excess.  The  mixture  is  boiled 
for  a  few  minutes,  allowed  to  stand  a  half  hour,  then  filtered  and  the  precipi- 
tate washed  with  hot  water.  The  paper  is  punctured  and  the  calcium  oxal- 
ate washed  through  into  the  precipitating  beaker  with  50  c.c.  of  boiling 
water.  Twenty  c.c.  of  dilute  (1:1)  sulphuric  acid  are  then  added 
and  the  oxalic  acid  liberated  is  titrated  against  a  standard  N/10  solu- 
tion of  potassium  permanganate.  When  the  end  point  is  nearly  reached 
the  filter  paper  is  also  thrown  into  the  beaker,  and  the  titration  finished. 
Since  2KMnO4  are  equivalent  to  5CaC2O4, 

C.c.  N/10  KMnO4  X  0.0028  XlOO 

— =  per  cent  CaO. 

Magnesium  Oxide.1 — Solution  D  is  evaporated  to  dryness  on  a 
water  bath  and  heated  gently  to  expel  ammonium  salts.  The 
residue  is  treated  with  about  25  c.c.  of  hot  water  and  5  c.c.  of 
hydrochloric  acid,  filtered,  and  washed.  The  filtrate  and  wash- 
ings are  concentrated  to  about  50  c.c.,  cooled,  and  a  sufficient 

1  A.  O.  A.  C.,  op.  ciL,  17. 


CHEMICAL  ANALYSIS  OF  GELATIN  443 

amount  of  disodium  hydrogen  phosphate  solution  added  to 
precipitate  the  magnesium,  after  which  ammonium  hydroxide  is 
added  slowly  and  with  constant  stirring  until  the  solution  is 
distinctly  alkaline.  It  is  ascertained  if  precipitation  is  complete 
by  the  addition  of  a  little  more  of  the  precipitant.  If  no  further 
precipitation  results,  the  mixture  is  allowed  to  stand  for  30 
minutes,  then  10  c.c.  of  strong  ammonium  hydroxide  slowly 
added,  the  beaker  covered,  and  placed  in  a  cool  place  for  12 
hours.  The  precipitate  is  then  filtered  through  an  ashless  paper, 
washed  with  dilute  (1 : 10)  ammonium  hydroxide  till  free  of 
chlorides,  and  dried.  The  residue  is  separated  from  the  filter 
paper,  the  latter  placed  in  a  weighed  porcelain  crucible  and 
ignited.  The  precipitate  is  then  added  and  the  whole  heated 
to  a  dull  red  heat  for  some  time,  finally  raising  the  temperature. 
The  magnesium  residue  is  allowed  to  cool,  and  weighed  as 
Mg2P2Oy.  It  is  calculated  to  MgO. 

Weight  Mg2P2O7  X  0.362  X  100 

—7^—  -   =  per  cent  MgO. 

U.o 

Zinc  Oxide.1 — A  25  c.c.  portion  of  Solution  A  or  0.5  gram  of 
ash  is  treated  with  10  c.c.  of  1  to  1  sulphuric  acid  in  an  evaporating 
dish  and  evaporated  until  dense  fumes  of  sulphuric  anhydride 
are  evolved.  After  cooling,  40  c.c.  of  water  are  added,  together 
with  about  a  gram  of  20  mesh  aluminum,  and  the  whole  boiled 
for  several  minutes.  The  residue  is  then  filtered  off  and  washed 
with  hot  water.  The  filtrates  and  washings  are  made  up  to  200 
c.c.  and  potassium  hydroxide  solution  added  until  nearly  neutral. 
A  drop  of  methyl  orange  is  introduced  into  the  solution,  and 
potassium  carbonate  added  to  within  an  acidity  of  a  couple 
drops  of  20  per  cent  sulphuric  acid.  Two  c.c  of  5  per  cent  sul- 
phuric acid  is  then  added,  the  solution  cooled,  and  a  rapid 
stream  of  hydrogen  sulphide  passed  through  the  solution  for  40 
minutes.  After  being  allowed  to  settle,  the  precipitate  is  filtered 
off  into  an  ashless  paper  and  washed  with  cold  water.  The 
paper  and  its  contents  are  then  ignited  in  a  weighed  crucible  and 
heated  to  800  to  900°C.,  in  a  muffle  furnace  for  an  hour  to  convert 
the  sulphide  completely  into  oxide.  It  is  weighed  as  ZnO. 

1  W.  W.  SCOTT,  "  Standard  Methods  of  Chemical  Analysis,"  2nd  ed. 
(1917),  484. 


444  GELATIN  AND  GLUE 

Weight    ZnO  X  100 

n  -  =  per  cent  ZnO. 

U.o 

Barium  Oxide.1 — A  25  c.c.  portion  of  Solution  A  is  diluted  in  a 
400  c.c.  beaker  to  250  c.c.  with  water,  and  a  slight  excess  of 
dilute  hot  sulphuric  acid  added  slowly  and  with  constant  stirring. 
The  beaker  is  then  allowed  to  stand  for  some  time  on  a  water 
bath,  and  after  the  precipitate  has  settled,  is  decanted  through 
an  ashless  filter  paper  or  Gooch  crucible.  The  precipitate  is 
washed  with  dilute  (0.5  per  cent)  sulphuric  acid,  transferred  to 
the  paper  or  crucible,  and  washed  with  hot  water  till  free  of 
acid.  It  is  then  dried  and  ignited  for  a  half  hour.  The  residue 
is  weighed  as  BaSO4  and  calculated  to  BaO. 

Weight  BaSO4  X  0.657  X  100 

n  , —  -  =  per  cent  BaO. 

U.O 

Lead  Oxide.2 — If  barium  is  present  a  25  c.c.  aliquot  of  Solution 
A  is  treated  with  50  c.c.  of  hot  slightly  ammoniacal  ammonium 
acetate  and  1  c.c.  of  a  very  dilute  solution  (1 : 10)  of  sulphuric  acid 
added.  The  barium  will  precipitate  out  completely.  This  is 
filtered  off,  and  a  few  drops  more  of  the  sulphuric  acid  added  to 
the  filtrate  to  insure  the  absence  of  barium.  Five  c.c.  of  con- 
centrated sulphuric  acid  are  then  added  and  the  mixture  evapo- 
rated until  dense  fumes  of  sulphuric  acid  are  given  off.  The  dish 
is  allowed  to  cool,  and  the  contents  then  rinsed  into  a  beaker 
containing  200  c.c.  of  water.  The  lead  sulphate  which  sepa- 
rates out  is  filtered  after  an  hour  and  washed  with  water  contain- 
ing 10  per  cent  sulphuric  acid.  It  is  then  dried,  ignited  at  a 
dull  red  heat,  and  weighed  as  PbS04.  This  is  calculated  to  PbO. 

Weight  PbS04  X  0.736  X  100 

A  ,  -  =  per  cent  PbO. 

U.o 

Potassium  and  Sodium  Oxides.3 — A  25  c.c.  aliquot  of  Solution 
A  is  treated  with  ammonium  hydroxide  slowly  until  the  pre- 
cipitate formed  just  dissolves.  It  is  heated  to  boiling  and  a  slight 
excess  of  the  alkali  added  to  precipitate  the  iron,  alumina,  etc. 
The  solution  is  boiled  for  a  minute,  poured  onto  a  filter,  and 

1  W.  W.  SCOTT,  lib.  cit,,  58. 

2  W.  W.  SCOTT,  lib.  cit.,  236. 

3  Association  of  Official  Agricultural  Chemists,  op.  cit.,  18. 


CHEMICAL  ANALYSIS  OF  GELATIN  445 

washed  with  hot  water.  The  precipitate  is  then  returned  to  the 
original  beaker,  dissolved  in  a  few  drops  of  hydrochloric  acid, 
and  reprecipitated  with  ammonium  hydroxide  as  above.  It  is 
washed  until  free  of  chlorides.  The  combined  filtrates  and 
washings  are  then  evaporated  to  dryness,  heated  until  the 
ammonium  salts  are  expelled,  and  dissolved  in  hot  water.  Five 
cubic  centimeters  of  barium  hydroxide  are  then  added,  the 
solution  heated  to  boiling,  and  the  precipitate  allowed  to  settle. 
If  precipitation  is  complete,  as  ascertained  by  the  addition  of 
more  of  the  barium  hydroxide  to  the  clear  solution,  the  precipi- 
tate is  filtered  and  washed  with  hot  water.  The  filtrate  is  heated 
to  boiling  and  ammonium  hydroxide  and  ammonium  carbonate 
added  to  precipitate  out  the  calcium  and  barium.  This  residue 
is  filtered,  washed  with  hot  water,  and  the  filtrate  and  washings 
evaporated  to  dryness,  and  heated  below  redness  to  expel  the 
ammonium  salts.  The  residue  is  taken  up  in  a  little  hot  water, 
and  a  few  drops  of  ammonium  hydroxide,  ammonium  carbonate, 
and  ammonium  oxalate  added.  The  mixture  is  warmed  a  few 
minutes  on  a  water  bath  and  allowed  to  stand  several  hours. 
It  is  then  filtered,  and  the  residue  again  evaporated  to  dryness  on 
the  water  bath,  and  heated  to  dull  redness  until  all  ammonium 
salts  are  expelled,  and  the  residue  is  white.  The  residue  is  taken 
up  in  a  small  amount  of  water,  filtered  into  a  weighed  platinum 
dish,  and  a  few  drops  of  hydrochloric  acid  added.  This  is  then 
evaporated  to  dryness  on  the  water  bath,  heated  to  dull  redness 
(not  higher)  and,  after  cooling  in  a  desiccator,  weighed  as 
KC1  +  NaCl. 

The  chlorides  may  now  be  converted  to  sulphates  by  dissolving 
in  a  little  water,  and  adding  a  few  cubic  centimeters  of  ammonium 
sulphate  (75  grams  to  the  liter),  and  digesting  for  several  hours 
on  a  water  bath.  The  solution  is  then  filtered  into  a  weighed 
platinum  dish,  evaporated  to  dryness,  and  ignited  at  a  dull 
redness.  A  gram  of  powdered  ammonium  carbonate  is  added, 
and  the  ignition  continued  slowly  until  all  ammonium  salts  are 
expelled.  The  residue  is  cooled  and  weighed  as  K2SO4  + 
Na2S04. 

From  the  weight  of  the  chlorides  and  of  the  sulphates  the 
amount  of  potassium  and  of  sodium  may  be  calculated. 

Let  the  combined  weight  KC1  +  NaCl  =  a,  and  K2SO4  + 
Na2  S04  =  b. 

Let  also  the  weight  of  K  =  x,  and  of  Na  =  y. 


446  GELATIN  AND  GLUE 


KOI      .  i    n.1  n    r  A  /-i\ 

Then     x  -r=-  +  V~    =  a,  or  l.9lx  +  2.54i/  =  a,  (1) 


and    x  4  +  y  =  b,  or  2.23*  +  3.09s/  =  6  (2) 


By  multiplying  (1)  by  1.168  and  subtracting  from  (2)  we  get 

Q.lZy  =  b  -  1.168a, 
from  which 

6  -  1.168a 

11  =  -  > 
0.13 

and  by  substitution  in  (2) 

_  b  -  3.09?/ 

2.23 
K  is  then  calculated  to  K2O: 

x  X  1.21   X  100 


0.5 
and  Na  is  calculated  to  Na2O : 

y  X  1.35  X  100 
0.5 


=  per  cent  K2O, 
=  per  cent  Na2O. 


If  desired  the  K2O  may  be  determined  in  the  residue  of  chlorides  or  sul- 
phates as  follows :  The  residue  is  dissolved  in  100  c.c.  of  water  and  a  platinum 
solution  (containing  the  equivalent  of  1  gram  of  metallic  platinum,  or  2.1 
grams  of  H2PtCl6  in  each  10  c.c.)  added  in  slight  excess.  The  mixture  is 
evaporated  on  a  water  bath  to  a  thick  paste,  and  the  residue  washed  by 
means  of  a  little  80  per  cent  alcohol  into  a  weighed  Gooch  crucible,  and 
washed  again  with  the  alcohol.  The  crucible  is  dried  for  30  minutes  at 
100°  to  130°C.3  and  weighed.  The  warming  is  repeated  until  the  weight  is 
constant.  The  potassium  is  in  the  form  of  K2PtCl6,  and  is  calculated 
to  K2O. 

Weight  K2PtCl6X  0.194  X  100 

-  =  per  cent  K2O. 


0.5 

To  obtain  the  percentage  of  Na2O,  the  potassium  is  calculated  to  KC1 
(weight  K2PtCl6  X  0.307  =  weight  KC1),  or  to  K2SO4  (weight  K2Pt- 
C16  X  0.359  =  weight  K2SO4),  depending  on  whether  the  combined  sodium 
and  potassium  salts  were  weighed  as  chlorides  or  sulphates,  and  the  value 
obtained  subtracted  from  the  weight  of  the  two  salts.  The  difference 
represents  the  weight  of  NaCl  or  Na2SO4  present.  This  is  calculated  to 
Na2O. 

Weight  NaQX  0.531X100  = 

Weight  Na2SQ4  X  0.436  X  100  = 


CHEMICAL  ANALYSIS  OF  GELATIN  447 

Sulphate.1 — A  25  c.c.  portion  of  Solution  A  is  diluted  in  a  300 
c.c.  beaker  to  100  c.c.  with  water,  heated  to  boiling,  and  drop  by 
drop  a  10  per  cent  solution  of  barium  chloride  added  until  no 
further  precipitation  occurs.  The  mixture  is  allowed  to  boil 
for  5  minutes  and  then  set  in  a  warm  place  for  several  hours. 
The  precipitate  is  then  decanted  onto  an  ashless  filter  paper, 
washed  with  20  c.c.  of  boiling  water  in  the  beaker,  transferred  to 
the  filter,  and  the  washing  continued  until  free  of  chlorides.  The 
precipitate  is  then  dried,  ignited,  and  weighed  as  BaSO4.  It  is 
calculated  to  S03. 

Weight  BaS04  X  0.343  X  100 

-~  —  -  =  per  cent  S03. 

u.o 

Chloride.2 — A  half  gram  sample  of  the  ash  is  dissolved  in  dilute 
(1 : 10)  nitric  acid,  and  any  residue  is  filtered  off  and  washed  with 
water.  An  accurately  measured  volume  of  N/10  silver  nitrate 
solution  in  slight  excess  of  the  amount  required  to  precipitate  the 
chloride  is  added  to  the  combined  filtrate  and  washings.  The 
precipitate  of  silver  chloride  is  allowed  to  stand  at  a  warm 
temperature  until  settled,  and  is  then  filtered  off  and  washed 
thoroughly  with  water.  To  the  filtrate  and  washings  5  c.c.  of  a 
saturated  solution  of  ferric  alum  to  serve  as  an  indicator,  and  a 
few  c.c.  of  nitric  acid,  are  added.  The  excess  of  silver  is  then 
titrated  with  N/10  potassium  sulphocyanate  solution  until  a 
permanent  light  brown  color  appears.  The  c.c.  of  the  sulpho- 
cyanate solution  used  are  equivalent  to  the  c.c.  of  silver  nitrate 
solution  added  in  excess  of  that  required  for  the  precipitation  of 
the  chloride.  This  figure  subtracted  from  the  total  c.c.  of  silver 
nitrate  solution  used  gives,  therefore,  the  c.c.  of  the  latter 
required  to  precipitate  the  chloride.  Since  1  c.c.  of  N/10 
AgNO3  is  equivalent  to  0.00355  gram  Cl, 

C.c.  AgNO3  X  0.00355  X  100 

^-rr-r—  -  =  percentCl. 

U.o 

A  method  for  determining  the  presence  of  traces  of  chloride  in  gelatin 
was  suggested  by  Luppo-Cramer. 3  He  places  a  small  amount  of  a  10  per 
cent  gelatin  solution  on  a  glass  plate  and  sets  it  aside  until  it  has  solidified. 
A  drop  of  a  10  per  cent  solution  of  silver  nitrate  is  then  added.  In  the 

1  A.  O.  A.  C.,  op.  tit.,  20. 

2  A.  O.  A.  C.,  op.  tit.,  19. 

3  LUPPO-CRAMER,  Z,  Chem.  Ind,  Kolloide,  5  (1909),  249. 


448  GELATIN  AND  GLUE 

presence  of  the  slightest  trace  of  chloride  (0.001  per  cent)  an  opalescent 
ring  will  be  formed  around  the  outside  of  the  drop,  and  the  breadth  and 
turbidity  of  the  area  will  increase  after  a  few  hours.  An  approximation 
of  the  quantity  of  chloride  present  may  be  made  by  comparing  this  test 
with  other  similar  ones  made  by  using  known  amounts  of  chloride. 

Shuey1  employed  the  precipitation  method  with  silver  nitrate  obtaining 
excellent  results.  Ten  grams  of  the  glue  are  heated  on  a  hot  plate  with  about 
2  grams  of  sodium  carbonate  in  a  porcelain  dish  until  carbonized.  The  dish 
is  then  transferred  to  a  muffle  furnace  at  a  very  low  red  heat  and  allowed  to 
remain. until  the  carbon  ceases  to  burn.  The  dish  is  then  removed,  cooled, 
and  25  c.c.  of  hot  water  added.  Dilute  nitric  acid  is  then  added  until  no 
further  effervescence  results,  and  the  mixture  decanted  through  a  filter. 
The  remaining  charcoal  is  washed  once  with  hot  water,  and  returned  in  the 
original  dish  to  the  furnace  where  it  is  allowed  to  remain  until  the  ash 
assumes  a  gray  color.  The  dish  is  again  cooled  and  the  residue  dissolved 
as  before,  filtered,  and  washed  thoroughly  on  the  filter.  (A  slight  residue 
of  carbon  is  often  left  on  the  filter,  but  the  amount  of  chloride  contained 
therein  has  been  shown  by  several  determinations  to  be  only  about  0.0003  per 
cent  of  the  weight  of  the  sample,  and  as  the  method  is  accurate  to  only 
0.001  per  cent,  the  loss  is  negligible.)  The  filtrate  is  heated  to  boiling, 
an  excess  of  silver  nitrate  solution  added,  and  the  flask  well  shaken  and  heated 
to  boiling  until  the  precipitate  settles  clear.  The  precipitate  is  then  col- 
lected on  a  weighed  Gooch  filter,  dried  at  200  to  400°C.,  and  weighed.  The 
silver  chloride  is  calculated  to  chloride: 

Weight  AgCl  X  0.2475  X  100 

_ — — —  -   =  per  cent  Cl. 

4.  The  Organic  Analysis  of  Gelatin.  Protein,  Proteose,  Pep- 
tone, and  Amino-acid. — There  are  several  ways  by  which  gelatin 
and  glue  may  be  examined  for  their  organic  constituents.  The 
material  may  be  separated  into  protein,  proteose,  peptone  and 
amino  acids,  and  the  distribution  of  the  total  nitrogen  among 
these  four  groups  determined.  Such  a  separation  is  of  consider- 
able value  in  any  study  requiring  information  upon  the  degree 
to  which  the  gelatin  molecule  has  been  hydrolyzed,  and  the  exact 
form  which  such  hydrolysis  has  taken.  The  procedure  as  finally 
adopted  by  the  author  after  an  extended  study  of  the  methods 
proposed  is  described  on  pages  25  to  28. 

It  should  be  emphasized,  however,  that  this  separation  is 
entirely  empirical.  In  a  given  material,  as  for  example,  a  high 
grade  glue,  a  slight  precipitation  begins  at  a  comparatively  low 
concentration  of  magnesium  sulphate,  e.g.,  at  18  to  20  per  cent 
of  saturation,  and  with  each  increase  in  the  concentration  of  the 

1  R.  SHUEY,  Unpublished  Report  to  Mellon  Institute  (1912). 


CHEMICAL  ANALYSIS  OF  GELATIN  449 

salt  a  greater  amount  of  nitrogenous  material  is  thrown  down. 
The  line  of  distinction  between  proteins  and  proteose  has  arbi- 
trarily been  placed  at  the  50  per  cent  of  saturation  point.  That 
is,  all  nitrogenous  material  precipitated  at  concentrations  of 
magnesium,  zinc,  or  ammonium  sulphate  (to  which  a  little 
sulphuric  acid  has  been  added)  up  to  50  per  cent  of  saturation 
has  been  designated  as  protein,  while  all  further  precipitation 
up  to  complete  saturation  has  been  designated  as  proteose. 
While  these  distinctions  are  very  useful  when  understood  and 
intelligently  applied,  yet  the  arbitrariness  of  the  differentiation 
should  not  be  lost  sight  of.  By  an  application  of  this  procedure 
the  author1  has  demonstrated  the  great  dependence  of  viscosity, 
jelly  consistency,  and  melting  point  upon  the  ratio  of  the  protein 
nitrogen  to  the  products  of  protein  hydrolysis. 

"Hausmann"  Numbers  and  Van  Slyke  Analysis. — A  some- 
what more  extended  and  in  some  respects  a  more  comprehensive 
insight  into  the  nature  of  the  gelatin  molecule  may  be  obtained 
by  a  determination  of  the  "Hausmann"  numbers,  or  of  the 
distribution  of  the  nitrogen  in  groups  by  the  Van  Slyke  method. 
These  are  described  on  pages  37  and  38  to  46  respectively.  This 
method  is  obviously  not  suited  to  the  study  of  hydrolysis,  as 
before  beginning  the  separations  the  sample  must  be  completely 
hydrolyzed  by  an  extended  digestion  with  hydrochloric  acid. 
But  it  does  permit  of  a  more  searching  inspection  of  the  con- 
stitution of  the  gelatin  molecule  itself.  The  method  has  been 
applied  not  only  to  pure  chemistry,  the  sole  object  of  which  was 
to  learn  of  the  molecular  make-up  of  proteins,  but  to  an  estima- 
tion of  the  purity  of  proteins  or  to  the  relative  percentages  of 
two  proteins  present  in  a  mixture.2  The  author3  has  applied 
the  method  to  a  study  of  glues  with  the  intent  of  determining 
the  relative  amount  of  chondrin,  mucin,  keratin,  etc.,  that  were 
intermixed  with  the  gelatin  in  the  lower  grades,  and  to  distinguish 
between  glues  of  marine  and  of  animal  origin. 

Fischer's  Esterification  Method. — The  individual  amino-acid 
constituents  of  a  protein  may  be  most  conclusively  recognized 
by  isolating  them  by  the  esterification  method  of  Fischer,  and 
examining  these  several  fractions  by  tests  specific  for  the  given 
amino-acids  in  question.  By  this  procedure  the  presence  or 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  105.     See  pages  28  and  29. 

2  See  M.  J.  BLISH,  /.  Ind.  Eng.  Chem.,  8  (1916),  138. 

3  R.  H.  BOGUE,  loc.  cit.,  155. 

29 


450  GELATIN  AND  GLUE 

absence  of  any  given  amino-acid  may  usually  be  demonstrated, 
and  approximations  to  quantitative  data  have  been  attempted. 
But  with  the  exception  of  the  analyses  of  Dakin  the  most  pains- 
taking work  by  the  greatest  chemists  of  the  day  has  failed  to 
account  for  more  than  a  half  to  three  quarters  of  the  original 
material  by  this  method.  By  no  other  method  may  the  indi- 
vidual components  of  the  molecule  be  so  well  demonstrated, 
but  its  defects  are  so  great  that  its  applications  in  research  are 
strictly  limited.  A  brief  description  of  the  process  is  given  on 
page  35. 

5.  Further  Analytical  Tests.  Total  Acidity  or  Alkalinity. — 
For  the  estimation  of  the  total  acidity  or  alkalinity,  the  older 
method  is  as  follows.  A  5  gram  sample  is  weighed  into  a  500  c.c. 
Erlenmeyer  flask,  allowed  to  soak  in  cold  water  until  thoroughly 
swollen,  brought  into  solution  by  warming,  and  recently-boiled 
distilled  water  added  to  bring  the  volume  to  about  300  c.c. 
Five  drops  of  phenolphthalein  indicator  solution  (0.1  per  cent 
alcoholic  solution)  are  added,  and  the  color  of  the  solution 
observed.  If  no  change  in  color  occurs  on  adding  the  indicator 
the  solution  was  acid,  while  if  a  pink  color  develops,  the  solution 
was  alkaline.  If  acid,  the  solution  is  then  titrated  with  N/10 
sodium  hydroxide  until  the  pink  color  just  becomes  permanent, 
and  if  alkaline,  it  is  titrated  with  N/10  hydrochloric  acid  until 
the  pink  color  just  disappears.  The  results  are  usually  expressed 
as  the  equivalent  of  a  calculated  amount  of  hydrochloric  acid  or 
of  sodium  hydroxide.  That  is: 

C.c.  N/10  HC1  used  X100 

— —  —  =  alkalinity  in  terms  of  N/10  NaOH 

per*  100  grams. 

C.c.  N/10  NaOH  usedX  100 

—=—  -  =  acidity  in  terms  of  N/10  HC1  per 

5 

100  grams. 

Hydrogen  Ion  Concentration. — Whenever  the  equipment  is  at 
hand,  much  more  comprehensive  data  may  be  obtained,  how- 
ever, by  a  direct  determination  of  the  hydrogen  ion  concentra- 
tion of  the  glue  or  gelatin  solution.  For  this  purpose  a  1  per 
cent  solution  is  used,  and  the  determination  may  be  made  by 
either  the  electrometric  or  the  colorimetric  methods.  These  pro- 
cedures are  described  in  detail  in  the  Appendix,  pages  579  to  606. 
The  results  are  best  expressed  in  terms  of  S0rensen's  logarithmic 


CHEMICAL  ANALYSIS  OF  GELATIN  451 

symbol  pH  (see  Appendix,  page  581),  and  indicate  the  exact 
concentration  of  hydrogen  or  hydroxyl  ions  in  the  solution. 

Free  Mineral  Acids. — The  free  mineral  acids  may  be  determined 
by  the  Hehner  method.  A  measured  excess  of  standard  alkali  is 
added  to  a  weighed  portion  of  the  sample  in  solution,  the  mixture 
placed  in  a  porcelain  crucible,  evaporated  to  dryness,  and  ignited 
at  a  dull  red  heat.  The  ash  is  brought  into  solution  with  water, 
and  titrated  with  standard  acid,  using  methyl  red  as  an  indicator. 
The  difference  between  the  number  of  c.c.  of  alkali  first  added, 
and  the  number  of  c.c.  of  acid-  used  in  titrating  the  excess  of 
alkali  (the  acid  and  base  being  of  the  same  normality)  rep- 
resents the  free  mineral  acid  present.  It  is  expressed  as  Free 
Mineral  Acidity  in  terms  of  N/10  HC1  per  100  grams. 

Free  Organic  Acids. — The  free  organic  acids  are  estimated  by 
subtracting  the  free  mineral  acids  from  the  total  acidity.  They 
are  expressed  in  terms  of  N/10  HC1  per  100  grams. 

Volatile  Acids. — The  volatile  acids  may  be  determined  by  pass- 
ing steam  through  a  solution  of  the  gelatin  and  collecting  the 
distillate.  Ten  grams  are  dissolved  in  500  c.c.  of  water  that  has 
recently  been  boiled  to  expel  carbon  dioxide.  This  is  placed  in  a 
flask,  connected  to  a  condenser,  and  heat  applied  to  incipient 
boiling.  Steam  obtained  from  water  that  has  been  boiled  for 
several  minutes  before  using  in  the  determination  is  then  passed 
through  the  solution  and  the  distillate  allowed  to  collect  in  a  flask 
containing  an  excess  of  N/10  alkali  and  3  drops  of  phenolph- 
thalein  indicator  solution.  About  150  c.c.  are  distilled  over, 
and  then  the  distillate  titrated  to  neutrality  with  N/10  acid. 
The  difference  between  the  amounts  of  alkali  and  acid  used 
indicates  the  amount  of  N/10  alkali  required  to  neutralize  the 
volatile  acids  from  the  10  gram  sample. 

C.c.  alkali  —  c.c.  acid  X  100  .  ,.,  ,., 

— — —  =  acidity  due  to  volatile 

acids  in  terms  of  N/10  HC1  per  100  grams.  The  volatile  acids 
in  glues  and  gelatins  are  usually  confined  to  hydrochloric  acid 
and  sulphur  dioxide. 

Fixed  Acids. — The  fixed  acids  are  estimated  by  subtracting  the 
volatile  acids  from  the  total  acids.  They  are  expressed  in  terms 
of  N/10  HC1  per  100  grams. 

Amino-acids. — The  free  amino-acids,  together  with  the  free 
carboxyl  or  amino  groups  of  the  protein,  proteose,  and  peptone 


452  GELATIN  AND  GLUE 

molecules,  may  be  accurately  estimated  by  the  Van  Slyke  method 
(described  on  pages  30  to  34)  which  determines  the  free  amino 
groups,  or  by  the  S0rensen  method  (described  on  page  30)  which 
determines  the  free  carboxyl  groups.  The  samples  should  be 
brought  into  solution  in  the  usual  way,  and  should  not  be  more 
concentrated  than  1  or  2  grams  per  100  c.c. 

Tague1  has  pointed  out  that  amino-acids  may  be  titrated  by 
means  of  the  hydrogen  electrode2  with  distinct  advantages,  and 
with  greater  accuracy  than  by  the  use  of  the  older  methods.  He 
finds  that  an  hydroxyl  ion  concentration  of  about  2  X  10~2 
(pH  12.5)  is  necessary  to  suppress  completely  the  basic  ionization 
of  the  sodium  salts  of  the  amino-acids,  and  thus  make  possible 
the  titration.  Sufficient  standard  alkali  is  added  to  a  definite 
volume  of  the  aqueous  solution  of  the  amino-acid  to  give  it  a  pH 
of  about  12.5.  Standard  alkali  is  also  added  to  an  equal  volume 
of  water  so  that  it  may  have  the  same  pH  value  and  the  same 
volume  as  the  amino-acid  solution.  By  subtracting  the  c.c.  used 
in  the  blank  from  that  required  in  the  original,  one  obtains  the 
c.c.  of  standard  alkali  necessary  to  neutralize  the  amino-acid 
alone. 

The  study  of  amino-acid  titration  has  been  carried  further  by 
Eckweiler,  Noyes  and  Falk,3  who  present  titration  curves  for  a 
number  of  amino-acids,  and  discuss  the  relations  found  from 
the  standpoint  of  the  chemical  structure  of  the  substance. 

Sulphur  Dioxide. — The  Association  of  Official  Agricultural 
Chemists4  employs  the  iodine  titration  method  for  the  estimation 
of  sulphur  dioxide  in  foods.  A  10  gram  sample  is  dissolved  in  500 
c.c.  of  water  and  5  c.c.  of  a  20  per  cent  solution  of  glacial  phos- 
phoric acid  added.  The  mixture  is  placed  in  a  flask  and  distilled 
in  a  current  of  carbon  dioxide  into  a  known  excess  of  an  N/20 
solution  of  iodine.  After  150  c.c.  have  distilled  over,  the  distillate 
is  titrated  with  an  N/20  solution  of  sodium  thiosulphate  until 
the  brown  color  of  the  iodine  just  disappears.  The  difference 
between  the  volume  of  iodine  taken  and  the  volume  of  sodium 
thiosulphate  used  represents  the  volume  of  N/20  iodine  solution 
necessary  to  react  with  the  sulphur  dioxide  distilled  over. 
Iodine  reacts  with  sulphur  dioxide  according  to  the  equation: 

1  E.  L.  TAGUE,  /.  Am.  Chem.  Soc.,  42  (1920),  173. 

2  See  Appendix  page  587,  for  a  discussion  of  electrometric  titrations. 

3  H.  ECKWEILER,  H.  NOYES,  and  K.  FALK,  /.  Gen.  Physiol,  3  (1921),  291. 

4  Association  Official  Agricultural  Chemists,  op.  tit.,  127. 


CHEMICAL  ANALYSIS  OF  GELATIN 


453 


21  +  S02  +  2H20  -*  H2S04  +  2HI. 

One  c.c.  of  N/20  iodine  solution  is  therefore  equivalent  to  0.0016 
gram  S02. 

C.c.  iodine  -  c.c.  sodium  thiosulphate  X  0.0016  X  100  _ 

10 

per  cent  SO2. 

Fade1  employs  the  above  procedure  in  a  slightly  modified 
form,  but  Gudeman2  has  pointed  out  that  many  animal  and 
vegetable  food  products  normally  contain  sulphur  compounds 
which  on  distillation  with  acids  give  off  volatile  sulphur  com- 
pounds which  react  with  the  iodine  solution  in  the  same  manner 
as  added  sulphur  dioxide,  so  that  when  the  method  is  used  to 
detect  added  sulphur  dioxide  positive  results  may  be  obtained 
when  none  was  added,  and  high  results  when  small  amounts  were 
added.  Gudeman  accordingly  modifies  the  method  by  employ- 
ing steam  distillation,  a  rather  heavy  flow  of  low  pressure  steam 
being  introduced  to  the  distilling  flask  so  that  concentration 
cannot  take  place,  and  in  large  measure  preventing  the  decompo- 
sition of  the  material  which  would  result  from  such  concentration. 
He  finds  the  following  amounts  of  sulphur  dioxide  in  gelatins: 


TABLE  49. — SULPHUR  DIOXIDE  GELATIN 


Official 

method 

Modified 

method 

Direct  titra- 
tion,  per  cent 

After  oxida- 
tion, per  cent 

Direct  titra- 
tion,  per  cent 

After  oxida- 
tion, per  cent 

Home  made  gelatin..  .  . 
French  gelatin  
American  gelatin 

0.0002 
0.0270 
0  0130 

0.0006 
0.0276 
0  0142 

0.0000 
0.0260 
0.0120 

0.0000 
0.0260 
0.0121 

Home  and  Winton3  have  suggested  passing  the  distillate 
through  solutions  of  metallic  salts,  such  as  cadmium,  copper, 
silver,  or  lead  salts,  in  which  the  hydrogen  sulphide  would  be 
precipitated,  before  collecting  in  the  iodine  solution,  or  even 
adding  the  salts  directly  to  the  distilling  flask.  Any  volatile 

1  FADE,  Ann.  Chim.  anal.  chim.  appl,  13  (1908),  299. 

2E.  GUDEMAN,  /.  Ind.  Eng.  Chem.,  1  (1909),  81. 

3  W.  HORNE  and  A.  WINTON,  Cf.  E.  GUDEMAN,  loc.  cit. 


454  GELATIN  AND  GLUE 

sulphur  compounds  which  did  not  appear  as  hydrogen  sulphide 
would,  however,  pass  over  and  react  with  the  iodine,  but  a  large 
portion  of  the  error  would  be  eliminated. 

Leffman  and  La  Wall1  favor  precipitating  the  sulphuric  acid, 
produced  by  the  oxidation  of  the  sulphur  dioxide  by  the  iodine, 
with  barium  chloride,  and  weighing  the  precipitate  as  barium 
sulphate.  They  found  many  samples  of  gelatin  containing  as 
much  as  300  or  more  parts  per  million  of  sulphur  dioxide,  but 
affirm  that  by  proper  methods  of  manufacture  it  is  easily  possible 
for  this  to  be  kept  down  to  less  than  10  parts  per  million. 

Poetschke2  also  found  the  gravimetric  method,  by  precipitating 
the  sulphur  as  barium  sulphate,  more  satisfactory  than  the  iodine 
titration  method.  He  found  20  per  cent  of  all  samples  tested 
by  him  to  contain  less  than  10  parts  of  sulphur  dioxide  per  million ; 
48  per  cent  contained  less  than  100;  43  per  cent  contained  from 
100  to  500;  and  3.3  per  cent  contained  over  1000  parts  per  million. 

The  method  of  Poetschke  has  been  found  most  satisfactory  in 
the  author's  laboratory.  The  procedure  is  as  follows:  27.5 
grams  of  gelatin  are  weighed  into  a  distilling  flask,  and  300  c.c.  of 
water  and  a  quantity  of  phosphoric  acid  weighing  about  5  grams 
added.  Carbon  dioxide  is  passed  through  the  flask  until  the  air 
has  been  completely  expelled,  and  the  gelatin  dissolved  by  placing 
in  a  bath  of  hot  water.  After  solution  of  the  gelatin,  the  flask  is 
heated  over  a  flame,  a  constant  flow  of  C02  being  maintained, 
until  175-200  c.c.  of  distillate  have  been  obtained.  This  dis- 
tillate is  collected  in  a  flask  containing  25  c.c.  of  N/20  iodine 
solution,  the  flask  being  connected  with  the  condenser,  at  the 
beginning  of  the  operation,  so  that  the  end  of  the  condenser 
reaches  below  the  surface  of  the  solution.  Five  c.c.  of  con- 
centrated HC1  are  then  added  to  the  distillate,  and  the 
latter  concentrated  to  about  75  c.c.  It  is  then  filtered,  brought 
to  boiling,  and  10  c.c.  of  BaCl2  solution  added.  The  BaSO4  is 
treated  in  the  customary  way  and  the  amount  determined  gravi- 
metrically.  The  weight  of  BaSO4  gives  directly  the  per  cent  of 
S02  if  27.5  grams  of  gelatin  were  used.  A  blank  test  should  be 
made  with  the  reagents. 

Formaldehyde. — A  sample  weighing  not  less  than  10  grams  is 
dissolved  in  water  and  distilled  by  heating  and  passing  steam 
through  the  solution.  About  30  c.c.  of  the  first  distillate  are 

1  LEFFMAN  and  LA  WALL,  Analyst,  36  (1911),  271. 

2  POETSCHKE,  J.  Ind.  Eng.  Chem.,  6  (1913),  980. 


CHEMICAL  ANALYSIS  OF  GELATIN  455 

collected  and  divided  into  two  portions.  To  one  of  these  an  equal 
volume  of  pure  milk  is  added,  followed  by  a  little  concentrated 
sulphuric  acid.  The  presence  of  formaldehyde  is  noted  by  the 
development  of  a  rose-purple  color.  One  part  of  formaldehyde 
in  10,000  may  be  detected  by  this  means.  As  a  confirmatory 
test,  the  second  portion  is  treated  with  a  drop  of  dilute  carbolic 
acid  followed  by  concentrated  sulphuric  acid.  A  pink  color 
indicates  formaldehyde.  This  test  also  is  very  delicate. 

Fat  or  Grease. — The  material  should  be  ground  to  a  very  fine 
powder,  and  10  grams  taken  for  the  determination.  If  it  is  not 
possible  to  pulverize  the  material,  the  sample  may  be  soaked  in 
the  minimum  of  water  in  an  evaporating  dish,  warmed  to  bring 
into  solution,  and  an  absorbent  material  as  infusorial  earth 
added  to  absorb  the  moisture.  The  dried  residue  is  ground 
finely  in  a  mortar,  care  being  taken  that  no  loss  occurs  during 
these  operations.  The  finely  ground  material,  or  the  pulverized 
sample  without  such  preliminary  treatment,  is  then  placed  in  a 
Soxhlet  extraction  thimble,  a  layer  of  fat-free  cotton  being  placed 
above  and  below  the  sample.  It  is  then  extracted  for  6  to  8 
hours  with  petroleum  ether  which  distills  between  50  and  80°C. 
The  receiving  flask  which  should  previously  have  been  weighed 
is  then  removed,  and  the  solvent  evaporated  off  on  a  steam  bath. 
The  flask  is  then  placed  in  an  oven  at  100°C.  for  an  hour,  cooled 
in  a  desiccator,  and  weighed.  The  increase  in  weight  represents 
the  amount  of  fat  or  grease  in  the  10  gram  sample. 

Weight  fat  X  100 

-— yr —          -  =  per  cent  fat. 

If  desired,  the  Roese-Gotlieb  method  may  be  substituted  for 
the  above  to  some  advantage,  the  procedure  being  exactly  as 
outlined  for  casein  on  pages  329-330. 

An  optional  method  has  been  suggested  by  Fahrion.1  He  digests  a  10 
gram  sample  on  the  water  bath  with  40  c.c.  of  alcoholic  soda  until  the  alcohol 
has  volatilized.  If  solution  is  not  then  complete  more  alcoholic  soda  is 
added  and  the  mixture  again  digested.  The  residue  is  taken  up  in  hot  water 
and  if  any  insoluble  inorganic  matter  is  present  this  is  dissolved  by  the 
addition  of  hydrochloric  acid.  The  whole  is  then  heated  nearly  to  boiling 
for  a  half  hour,  rinsed  into  a  separatory  funnel,  and  when  cold  shaken  with 
ether  and  left  to  stand  over  night.  The  two  layers  of  solution  are  then  run 
off  separately  and  the  insoluble  oxy-fatty  acids  left  in. the  funnel  dissolved 

1  FAHRION,  Chem.  Ztg.,  23  j(1899),  452. 


456  GELATIN  AND  GLUE 

in  warm  alcohol  and  mixed  with  the  ethereal  solution  which  contains  the 
rest  of  the  fatty  matter.  This  is  then  evaporated  on  a  water  bath,  dried 
and  weighed.  Some  inorganic  material  may  also  have  been  dissolved  in  the 
ethereal  solution,  and  to  determine  this  the  residue  is  ignited  and  again 
weighed.  The  weight  of  ash  found  is  deducted  from  the  fat  as  weighed,  the 
difference  being  recorded  as  fat.  There  are  two  sources  of  error  in  the  pro- 
cess: the  glycerine  of  the  glycerides  is  not  determined,  and  the  oxy-fatty 
acids  are  slightly  soluble  in  the  acid.  The  actual  error  introduced,  however 
is  small  and  usually  insignificant. 

Insoluble  Residue. — A  5  gram  sample  is  put  into  solution  in 
500  c.c.  of  water,  and  allowed  to  stand  at  room  temperature  for 
2  hours.  It  is  then  decanted  through  a  weighed  Gooch  crucible 
with  a  thin  asbestos  mat,  the  residue  washed  into  the  crucible 
with  a  little  water  and  the  precipitate  washed  twice  with  water. 
The  crucible  is  then  dried  at  120°C.,  and  weighed.  The  increase 
in  weight  is  recorded  as  insoluble  material. 

Weight  insoluble  material  X  100  ,  .       ,   ,  .  .  , 

— - —  -  =  per  cent  insoluble  material, 

o 

6.  Traces  of  Metals  in  Gelatin. — In  the  manufacture  of  gelatin 
for  food  or  medicinal  purposes  it  is  necessary  that  the  product  be 
practically  free  from  metallic  impurities  that  would  impart 
poisonous  properties,  or  that  would  render  the  material  in  the 
least  questionable  for  human  consumption.  Unfortunately 
there  are  in  the  course  of  the  manufacture  several  possible 
sources  of  such  contamination.  Arsenic  may  be  introduced 
either  through  the  use  of  impure  acids  used  in  the  treatment. of 
the  bones,  or  as  an  impurity  in  the  sulphur  dioxide  used  in 
bleaching.  If  leather  scraps  are  employed  in  the  stock,  arsenic 
may  be  further  introduced  through  the  arsenical  preparations 
which  may  have  been  used  in  treating  the  hides  for  leather. 
In  testing  for  arsenic  in  gelatin,  Kopke1  found  8  out  of  12  samples 
of  commercial  gelatin  to  contain  from  0.05  to  0.30  mg.  of  arsenic 
per  10  gram  sample  (0.0005  to  0.003  per  cent).  The  remaining 
4  samples  also  contained  traces. 

Copper  may  find  access  to  the  gelatin  through  the  use  of  impure 
acids  or  bleaching  agents,  or  by  the  use  of  copper  kettles  or 
containers.  Hart2  has  examined  a  number  of  gelatins  and  gelatin 
containing  preparations  used  for  food  purposes  and  found  as 

1  KOPKE,  Arb.  kais.  Gesund.,  38  (1911),  290. 

2  HART,  7th  Internal.  Congress  Appl.  Chem. 


CHEMICAL  ANALYSIS  OF  GELATIN 


457 


much  as  104  mg.  of  copper  per  1,000  grams  of  gelatin  (a  little 
over  0.01  per  cent). 

Zinc,  lead,  and  tin  may  also  be  present  in  small  amounts,  their 
origin  being  the  use  of  impure  chemicals  in  the  manufacture, 
the  addition  of  unlawful  chemicals  to  produce  certain  desired 
properties,  or  the  use  of  containers,  boiling  kettles  or  wire  screens 
from  which  the  metals  in  question  may  have  been  dissolved. 
Iron  is  always  present  in  small  amounts,  but  its 
presence  is  not  injurious.  Small  amounts  of 
alkali  and  alkaline  earth  metals  are  likewise 
present,  but  they  are,  of  course,  harmless  in  the 
form  in  which  they  are  present  in  gelatin. 

Arsenic.1 — About  10  grams  of  the  finely  divided 
sample  are  weighed  into  a  porcelain  casserole,  10 
to  15  c.c.  of  nitric  acid  are  added,  and  after  cover- 
ing with  an  inverted  watchglass,  warmed  until 
vigorous  action  is  over.  After  cooling,  10  c.c. 
of  concentrated  sulphuric  acid  are  added,  and 
the  mixture  heated  on  a  wire  gauze  over  a  flame 
until  it  turns  brown  or  black,  then  more  nitric 
acid  added  in  5  c.c.  portions,  heating  after  each 
addition,  until  the  liquid  remains  colorless  or 
yellow  when  evaporated  until  fumes  of  sulphur 
trioxide  are  evolved.  All  nitric  and  nitrous  acid 
is  removed  by  continuing  the  evaporation  to 
about  5  c.c.  It  is  then  cooled,  diluted  with 
10  to  15  c.c.  of  water,  and  again  evaporated 
until  white  fumes  are  given  off.  It  is  again 
cooled,  diluted  with  water,  and  made  up  with 
water  to  25  or  50  c.c.  in  a  volumetric  flask,  the 
volume  being  adjusted  at  room  temperature. 

The  apparatus  (see  Fig.  98)  and  solutions 
for  the  determination  should  previously  have 
been  prepared.  A  2  ounce  glass  bottle  is 
used  as  a  generator.  A  glass  tube  1  cm.  in  diameter  and  6  cm. 
long  and  containing  a  piece  of  lead  acetate  paper  rolled  into  a 
cylinder,  is  fitted  to  the  bottle  by  means  of  a  perforated  rubber 
stopper  with  a  similar  glass  tube  filled  with  absorbent  cotton 
that  has  been  soaked  in  5  per  cent  lead  acetate  solution,  and 
squeezed  to  remove  excess  of  solution.  The  cotton  in  all  sample 

1  A.  O.  A.  C.,  op.  tit.,  147.    ' 


FIG.  98.— The 
apparatus  for  the 
determination  of 
arsenic. 


458  GELATIN  AND  GLUE 

tubes  should  be  uniformly  moist  to  obtain  comparative  stains. 
This  tube  is  connected  by  a  perforated  rubber  stopper  with  a 
narrow  glass  tube,  3  mm.  in  internal  diameter  and  12  mm.  long, 
containing  a  strip  of  mercuric  bromide  paper.  This  paper  is 
prepared  by  cutting  heavy,  close-textured  drafting  paper  into 
strips  exactly  2.5  mm.  wide  and  about  12  cm.  long.  They  are 
soaked  for  an  hour  in  a  5  per  cent  solution  of  mercuric  bromide 
in  95  per  cent  alcohol,  the  excess  of  solution  then  squeezed  out, 
and  dried  on  glass  rods.  The  ends  of  the  strips  are  cut  off  before 
using.  All  rubber  stoppers  used  should  be  free  from  any  white 
coating. 

Twenty  c.c.  of  the  solution  prepared  as  above  directed  from  the 
gelatin  is  introduced  into  the  2  ounce  bottle  and  20  c.c.  of  dilute 
(1:2)  arsenic-free  sulphuric  acid  added,  followed  by  4  c.c.  of  a 
20  per  cent  solution  of  potassium  iodide.  The  generator  is 
warmed  to  about  90°C.,  3  drops  of  stannous  chloride  solution 
(40  grams  of  stannous  chloride  crystals  made  up  to  100  c.c.  with 
concentrated  hydrochloric  acid)  added,  and  heated  for  10 
minutes.  The  generator  is  then  cooled  by  placing  in  a  pan  con- 
taining water  and  ice,  and  when  cold  about  15  grams  of  arsenic- 
free  zinc,  broken  into  small  sticks,  are  added,  and  the  tubes  as 
above  described  set  in  position  into  the  generator.  The  bottle 
is  allowed  to  remain  in  ice  water  for  15  minutes,  is  then  removed 
and  the  evolution  of  gas  permitted  to  proceed  for  an  hour  longer. 
The  sensitized  paper  is  then  removed  and  compared  with  stains 
produced  similarly  with  known  amounts  of  arsenic,  using  por- 
tions of  standard  arsenic  solution  containing  0.001,  0.002,  0.005, 
0.010,  0.015,  0.025,  and  0.030  mg.  of  arsenious  oxide  (As203), 
and  quantities  of  water  and  suphuric  acid  used  in  the  test  such 
that  the  same  volume  and  acid  strength  are  maintained  as  above. 

The  standard  arsenic  solution  is  prepared  by  dissolving  1  gram 
of  arsenious  oxide  in  25  c.c.  of  20  per  cent  sodium  hydroxide 
solution,  neutralizing  with  dilute  sulphuric  acid,  adding  10  c.c. 
of  concentrated  arsenic-free  sulphuric  acid,  and  diluting  to  1 
liter  with  recently  boiled  water.  One  c.c.  of  this  solution  con- 
tains 1  mg.  of  arsenious  oxide  (As2O3).  Twenty  c.c.  of  this  solu- 
tion are  then  diluted  to  1  liter.  Fifty  c.c.  of  the  latter  solution 
when  diluted  to  1  liter  give  a  dilute  standard  solution  containing 
0.001  mg.  of  arsenious  oxide  (As203)  per  c.c.  which  is  used  to 
prepare  the  standard  stains.  The  dilute  solutions  must  be 
prepared  immediately  before  use. 


CHEMICAL  ANALYSIS  OF  GELATIN  459 

Blank  tests  must  be  conducted  on  all  reagents,  and  results 
corrected  accordingly. 

The  difficulty  of  digesting  the  sample  with  nitric  and  sulphuric 
acids  until  no  brown  coloration  appears  upon  evaporating  to 
fumes  of  sulphur  trioxide  will  be  obvious  to  anyone  who  has 
attempted  it.  Very  satisfactory  results  have  been  obtained  by 
merely  hydrolyzing  the  gelatin  with  a  dilute  sulphuric  acid  for 
2  hours,  and  proceeding  as  in  the  Official  Methods,  after  making 
up  to  a  standard  volume. 

Copper. 1 — Twenty  grams  of  the  powdered  gelatin  are  weighed 
into  an  800  c.c.  Kjeldahl  flask  and  100  c.c.  of  concentrated 
nitric  acid  added.  This  is  heated  on  a  wire  gauze  over  a  free 
flame  until  the  contents  boil  quietly.  After  cooling,  25  c.c.  of 
concentrated  sulphuric  acid  are  added  and  the  heating  continued 
until  white  fumes  are  evolved.  The  flask  is  cooled,  5  c.c.  of 
concentrated  nitric  acid  added,  and  the  heating  continued. 
This  procedure  is  repeated  until  the  solution  remains  clear  after 
boiling  off  the  nitric  acid  and  fumes  of  sulphur  trioxide  appear. 

The  solution  is  then  concentrated  by  continued  digestion 
until  only  10  to  15  c.c.  remain,  then  cooled  and  after  diluting 
with  water,  transferred  to  a  400  c.c.  beaker.  The- flask  is  rinsed 
out  with  water,  and  this  added  to  the  solution  -in  the  beaker. 
Water  is  added  to  a  volume  of  about  200  c.c.,  and  the  solution 
boiled.  After  cooling,  the  solution  is  made  slightly  alkaline 
with  ammonium  hydroxide  and  boiled  to  expel  the  excess  of 
ammonia,  5  c.c.  of  concentrated  hydrochloric  acid  are  then  added 
for  each  100  c.c.  of  solution,  and  the  mixture  heated  to  incipient 
boiling,  and  saturated  with  hydrogen  sulphide.  After  standing 
on  the  steam  bath  for  a  few  minutes  until  the  precipitate  floc- 
culates, the  latter  is  filtered  off  and  washed  with  hydrogen 
sulphide  water.  The  precipitate  must  be  protected  from  the  air 
as  much  as  possible,  and  the  operation  carried  on  without 
interruption.  The  filtrate  is  reserved  for  the  determination  of  zinc, 
if  necessary.  The  filter  containing  the  copper  sulphide  precipi- 
tate is  placed  in  a  small  flask,  4.5  c.c.  of  concentrated  sulphuric 
acid  and  the  same  volume  of  concentrated  nitric  acid  are  added, 
and  heated  until  sulphur  trioxide  is  evolved.  The  oxidation 
is  continued  with  small  additions  of  nitric  acid  until  the  liquid 
remains  colorless  upon  heating  to  the  appearance  of  the  white 
fumes.  The  solution  is  then  cooled,  diluted  with  about  30  c.c.  of 

1  A.  O.  A.  C.,  op.  cit.,  151. 


460  GELATIN  AND  GLUE 

water,  and  an  excess  of  bromine  water  added.  It  is  then  boiled 
until  the  bromine  is  completely  expelled,  removed  from  the  heat, 
and  a  slight  excess  of  strong  ammonium  hydroxide  added.  The 
excess  of  ammonia  is  expelled  by  boiling,  as  shown  by  a  change 
of  color  of  the  liquid,  and  a  partial  precipitation.  A  slight  excess 
of  strong  acetic  acid  (3  to  4  c.c.  of  80  per  cent  acid)  is  added  and 
boiled  for  a  minute.  It  is  cooled  to  room  temperature  and  10  c.c. 
of  30  per  cent  potassium  iodide  added.  The  copper  will  oxidize 
this  to  iodine,  and  the  latter  is  titrated  at  once  with  N/100  sodium 
thiosulphate  until  the  brown  tinge  has  become  weak,  then  suffi- 
cient starch  indicator  (made  by  mixing  0.5  gram  of  finely  pow- 
dered potato  starch  with  cold  water  to  a  thin  paste,  then  pouring 
into  about  100  c.c.  of  boiling  water,  stirring  constantly)  added  to 
give  a  blue  coloration,  and  the  titration  continued  until  the  color 
due  to  the  iodine  has  entirely  vanished. 

The  reactions  involved  are: 

2CuS04  +  4KI  -»  2CuI  +  2K2SO4  +  I2, 

and  2  Na2S2O3  +  I2  -»  Na2S4O6  +  2  Nal. 

The  thiosulphate  solution  must  first  have  been  standardized 
against  pure  metallic  copper  treated  in  the  same  manner  as  above . 
Two  and  five-tenth  grams  of  Na2S2O3.5H2O  are  dissolved  in 
water  and  made  up  to  1  liter.  In  the  standardization: 

Weight  of  copper  taken 

^ — ~ r- r1-         — : — -j  =  value  of  1  c.c.  thiosulphate  solution, 

C.c.  thiosulphate  required 

and  in  the  determination : 

C.c.  thiosulphate  used  X  value  of  1  c.c.  X  100 

-ITT  •  i+    e —  r~i —  ~  =  Per  cent  Cu. 

Weight  of  sample  taken 

A  5  hour  digestion  with  dilute  hydrochloric  acid  may  be 
substituted  to  advantage  for  the  nitric-sulphuric  acid  digestion 
described  above. 

Zinc.1 — The  filtrate  from  the  copper  sulphide  precipitation, 
indicated  above  as  to  be  reserved  for  the  zinc  determination,  is 
boiled  to  expel  hydrogen  sulphide,  and  evaporated  to  a  volume 
of  250  to  300  c.c.  A  drop  of  methyl  orange  and  5  grams  of 
ammonium  chloride  are  added  and  the  solution  made  alkaline 
with  ammonium  hydroxide.  Hydrochloric  acid  is  then  added, 
drop  by  drop,  until  faintly  acid,  and  10  to  15  c.c.  of  50  per  cent 
sodium  or  ammonium  acetate  solution  added.  Hydrogen 

1  A.  O.  A.  C.,  op.  cit.,  151. 


CHEMICAL  ANALYSIS  OF  GELATIN  461 

sulphide  is  then  passed  into  the  solution  for  a  few  minutes  until 
precipitation  is  complete.  The  precipitate  is  allowed  to  settle, 
filtered  (repeating  the  filtration  if  necessary  until  the  filtrate  is 
clear),  and  washed  twice  with  hydrogen  sulphide  water.  The 
precipitate  is  dissolved  on  the  filter  with  a  little  dilute  (1  to  3) 
hydrochloric  acid,  the  filter  washed  with  water,  and  the  filtrate 
and  washings  boiled  to  expel  hydrogen  sulphide.  It  is  then 
cooled,  an  excess  of  bromine  water  added,  5  grams  of  ammonium 
chloride  stirred  in,  and  ammonium  hydroxide  introduced  until 
the  color  caused  by  the  bromine  disappears.  Dilute  (1  to  3) 
hydrochloric  acid  is  added,  drop  by  drop,  until  the  bromine 
color  just  reappears,  then  10  to  15  c.c.  of  sodium  or  ammonium 
acetate  solution  (50  per  cent  by  weight),  and  0.5  c.c.  of  ferric 
chloride  solution  (10  grams  per  100  c.c.)  or  enough  to  precipitate 
all  of  the  phosphates.  The  solution  is  boiled  until  the  iron  is  all 
precipitated,  then  filtered  hot,  and  the  precipitate  washed  with 
water  containing  a  little  sodium  acetate.  Hydrogen  sulphide 
is  passed  into  the  combined  filtrate  and  washings  until  all  of  the 
zinc  sulphide,  which  should  be  pure  white,  is  precipitated.  This 
is  filtered  through  a  tared  Gooch  crucible  and  washed  with  hydro- 
gen sulphide  water  containing  a  little  ammonium  nitrate.  The 
crucible  is  dried  in  an  oven,  ignited  at  a  bright  red  heat,  cooled, 
and  weighed  as  ZnO.  This  should  be  calculated  to  Zn. 

Weight  ZnO  X  0.8034  X  100 

— \\r   •    CT~~g —      ~~i —          "   =  Per  cent  Zn. 

Weight  of  sample 

Tin.1 — The  material  is  brought  into  solution,  and  the  organic 
matter  destroyed  exactly  as  described  under  "copper"  in  the 
first  paragraph  only.  Two-hundred  c.c.  of  water  are  added  to  the 
digested  sample,  and  poured  into  a  600  c.c.  beaker,  rinsing  out 
the  Kjeldahl  flask  with  three  portions  of  boiling  water,  making 
the  volume  of  solution  about  400  c.c.  It  is  cooled,  and  concen- 
trated ammonium  hydroxide  added  until  just  alkaline,  followed 
by  hydrochloric  or  sulphuric  acid  until  the  acidity  is  about  2 
per  cent.  The  beaker  is  covered,  placed  on  a  hot-plate,  and 
warmed  to  about  95°C.,  when  a  slow  stream  of  hydrogen  sulphide 
is  passed  through  the  solution  for  an  hour.  The  mixture  is 
allowed  to  digest  for  another  hour  on  the  hot-plate  and  let 
stand  1  or  2  hours  longer.  The  tin  sulphide  is  then  filtered 
through  an  11  cm.  filter  (similar  in  quality  to  No.  590,  white 

1  A.  O.  A.  C.,  op.  tit.,  149. 


462  GELATIN  AND  GLUE 

+ 

ribbon,  S.  &  S.),  and  washed  alternately  with  three  portions 
each  of  wash  solution  and  hot  water.  (The  wash  solution  is 
made  by  combining  100  c.c.  of  saturated  ammonium  acetate 
solution,  50  c.c.  of  glacial  acetic  acid,  and  850  c.c.  of  water.)  The 
filter  and  precipitate  are  digested  together  in  a  50  c.c.  beaker 
with  three  successive  portions  of  ammonium  polysulphide,  heat- 
ing to  boiling  each  time  and  filtering  through  a  9  cm.  filter. 
The  precipitate  on  the  filter  is  washed  with  hot  water.  The 
filtrate  and  washings  are  acidified  with  acetic  acid,  digested  on 
the  hot-plate  for  an  hour,  allowed  to  stand  over  night,  and  filtered 
through  a  double  11  cm.  filter.  The  precipitate  is  washed 
alternately  with  two  portions  each  of  the  wash  solution  and  hot 
water,  and  dried  thoroughly  in  a  weighed  porcelain  crucible.  It 
is  then  ignited  over  a  Bunsen  flame,  gently  at  first,  but  finally 
at  full  heat.  The  crucible,  partly  covered,  is  then  heated 
strongly  over  a  Meker  burner.  The  stannic  sulphide  is  thus  con- 
verted to  the  oxide  and  is  weighed  as  Sn02.  It  is  calculated  to  Sn. 

Weight  Sn02  X  0.7881  X  100 

-  ^T  .     *    ,  -  =  per  cent  Sn. 

Weight  of  sample 

Lead.1 — In  small  amounts,  lead  is  best  determined  colorimetri- 
cally.  If  present  between  10  and  50  parts  per  million  a  1  gram 
sample  is  taken.  If  above  or  below  these  values,  the  amount  of 
sample  is  regulated  accordingly.  The  gelatin  is  brought  into 
solution  and  digested  with  concentrated  sulphuric  acid  and  potas- 
sium hydrogen  sulphate  until  colorless.  It  is  then  cooled  and 
diluted  with  water.  Ten  c.c.  of  tartrate  solution  (25  grams  of 
sodium  potassium  tartrate,  NaKC4H4O6.4H2O,  is  dissolved  in 
50  c.c.  of  water.  A  little  ammonia  is  added  followed  by  a  solution 
of  sodium  sulphide.  After  settling  it  is  filtered  and  the  filtrate 
acidified  with  hydrochloric  acid,  boiled  free  of  hydrogen  sulphide, 
again  made  ammoniacal,  and  diluted  to  100  c.c.),  and  10  c.c.  of 
hydrochloric  acid  are  added,  and  the  mixture  brought  to  boiling. 
Any  ferric  iron  present  is  reduced  by  adding  0.5  gram  of  sodium 
metabisulphite.  Ammonium  hydroxide  is  added  to  neutralize 
the  free  acid,  and  5  c.c.  in  excess,  then  3  c.c.  of  potassium  cyanide 
solution  (10  per  cent,  lead  free)  to  repress  any  color  due  to 
copper,  and  the  entire  solution,  or  an  aliquot  portion,  placed 
in  a  cylinder  of  a  colorimeter  and  diluted  to  nearly  100  c.c.  A 
standard  lead  solution  made  as  described  below  is  placed  in  the 

1  W.  W.  SCOTT,  "  Standard  Methods  of  Chemical  Analysis,"  2nd  ed. 
(1917),  243. 


CHEMICAL  ANALYSIS  OF  GELATIN  463 

opposite  cylinder  and  the  readings  taken.     Any  standard  colori- 
meter may  be  used. 

The  standard  lead  solution  may  be  prepared  by  dissolving 
0.1831  gram  of  lead  acetate,  Pb(C2H302)2.3H2O,  in  100  c.c.  of 
water,  clearing  any  cloudiness  with  a  few  drops  of  acetic  acid, 
and  diluting  to  1  liter.  Ten  c.c.  of  this  solution  diluted  to  1 
liter  will  contain  in  each  c.c.  an  equivalent  of  0.000001  gram  Pb. 
The  standard  solution  is  treated  with  sulphide  solution  at  the  same 
time  as  the  gelatin  solution,  and  treated  similarly  throughout. 

II.  THE  ESTIMATION  OF  GELATIN  IN  COMMERCIAL  GELATIN  AND 

GLUE 

1.  Alcoholic  Precipitation. — A  number  of  procedures  have  been 
proposed  intending  to  differentiate  between  the  "  pure  glue  "  and  all 
other  substances  nitrogenous  or  otherwise  that  may  be  present,  and 
designed  as  "non-glues."  Stelling1  states  that  as  the  value  of  a 
glue  depends  simply  on  its  adhesive  power,  the  only  reasonable 
method  of  testing  it  is  to  attempt  to  determine  the  amount  of 
substances  other  than  gelatin  that  it  contains.  He  points  out 
that  tannin  precipitates  the  cleavage  products  of  gelatin  as  well 
as  the  unhydrolyzed  protein,  and  therefore  discards  the  use  of 
tannin.  Physical  tests  he  considers  as  variable  with  the  process 
of  manufacture.  He  accordingly  treats  the  glue  with  alcohol. 
Fifteen  grams  are  dissolved  in  60  c.c.  of  water  and  96  per  cent  al- 
cohol added  to  make  a  volume  of  250  c.c.  After  standing  6 
hours  25  to  50  c.c.  of  the  clear  supernatent  liquid  are  drawn  off, 
evaporated  to  dryness,  heated  in  an  oven  at  100°C.,  and  weighed. 
The  residue  is  termed  non-glues,  it  being  assumed  that  the  true 
glue  is  completely  precipitated  under  the  above  conditions,  and 
to  the  exclusion  of  any  non-glue  material.  He  obtains  percent 
ages  of  non-glues  for  the  several  types  of  glues  as  follows :  Gelatin, 
3.39;  hide  glues,  5.73;  bone  glues  10  to  16;  pressed  glues,  14  to 
32;  whole  glue,  22;  material  used  for  clarification  of  wine,  33-59. 

Miiller2  favors  the  tannin  precipitation  of  gelatin  for  the  higher 
grades  and  alcoholic  precipitation  for  the  lower  grades  of  glues. 
He  precipitates  the  gelatin  with  96  per  cent  alcohol  and  deter- 
mines the  nitrogen  in  an  aliquot  of  the  filtrate.  This  value  is 
deducted  from  the  total  nitrogen  of  the  sample,  the  difference 
representing  the  nitrogen  of  the  gelatin  content. 
•  1  C.  STELLING,  Chem.  Ztg.,  20  (1896),  461. 

2  A.  MULLER,  Z.  angew.  CTiem.,  16  (1902),  482 


464  GELATIN  AND  GLUE 

The  author  has  found  that  alcoholic  precipitation  throws  out 
the  proteose  as  well  as  the  protein,  and  as  it  is  especially  desirable 
that  these  components  be  separated,  this  method  has  not  the 
advantages  of  the  salt  precipitation  described  later. 

2.  Tannin  Precipitation. — Jean1  suggested  the  use  of  tannin 
for  precipitation  of  the  gelatinous  material,  the  purity  of  the 
glue  being  proportional  to  the  amount  of  tannin  required.  One 
gram  of  the  sample  is  dissolved  in  water  and  made  up  to  100  c.c. 
A  10  c.c.  portion  is  mixed  with  10  c.c.  of  a  1  per  cent  solution  of 
pure  tannin,  and  agitated  with  5  grams  of  sodium  chloride  and 

I  gram  of  sodium  bicarbonate,  to  render  the  gelatin  tannate 
insoluble.     The  mixture  is  poured  onto  a  filter  paper  and  the 
filtrate  collected  in  a  100  c.c.  graduated  cylinder.     A  solution  of 
sodium  chloride  (sp.  gr.  1.184)  is  added  to  make  a  volume  of 
45  c.c.,  and  a  0.4  per  cent  solution  of  iodine  added  drop  by  drop 
until  the  starch  test  reveals  the  presence  of  free  iodine,  where- 
upon the  solution  is  made  up  to  60  c.c.  with  water,  and  more 
iodine  solution  added  until  a  faint  blue  test  is  obtained  with 
starch.     The  excess  of  tannin  is  thus  estimated  by  the  iodine 
titration   (the  equivalent  of  tannin  solution  per  c.c.  of  iodine 
solution  should  previously  have  been  determined)  and  by  deduc- 
tion from  the  original  quantity  of  tannin  added,  the  amount 
required  to  precipitate  the  gelatinous  material  is  found. 

Carles2  takes  exception  to  the  above  method  by  pointing  out 
that  the  precipitating  power  of  different  commercial  gelatins  is 
very  variable.  For  example  10  grams  of  Russian  fish  glue  were 
found  to  throw  down  4  grams  of  tannin,  (at  40°C.),  granulated 
gelatin  8  grams,  cut  fish  glue  3  to  10  grams,  and  bone  gelatin 

II  grams. 

A  somewhat  different  procedure  of  evaluation  was  suggested 
by  Gantter.3  100  grams  of  glue  were  heated  in  water  with  a 
few  drops  of  sodium  hydroxide  until  solution  was  effected,  then 
made  up  to  2  liters.  After  standing  10  hours  20  c.c.  of  the  liquid 
were  evaporated  to  dryness,  heated  in  an  oven  at  105°C.,  and 
weighed.  This  was  then  ashed  and  the  weight  of  ash-free  dry 
glue  ascertained.  Another  20  c.c.  aliquot  was  treated  with  30 
c.c.  of  water,  neutralized  with  acetic  acid,  and  tannin  solution 
added  in  excess.  The  mixture  was  shaken,  made  to  100  c.c., 

1  F.  JEAN,  Ann.  Chim.  Analyt.,  2,  85. 

2  P.  CARLES,  Ann.  Chim.  Analyt.,  2,  181. 
3F.  GANTTER,  Z.  anal.  Chem.,  32  (1893),  413 


CHEMICAL  ANALYSIS  OF  GELATIN 


465 


and  filtered.  The  filtrate  was  then  shaken  with  hide  powder 
and  allowed  to  stand  10  hours  to  ensure  complete  elimination  of 
the  tannin.  It  was  then  filtered,  and  50  c.c.  of  the  filtrate 
evaporated,  dried,  and  weighed.  This  was  deducted  from  the 
weight  of  the  residue,  and  this  weight  of  residue  subtracted  from 
the  weight  of  ash-free  dry  glue  previously  obtained.  The 
difference  is  taken  as  pure  glue  substance. 

Many  modifications  of  the  tannin  precipitation  method  of  glue 
evaluation  have  been  suggested.  Miiller1  treated  10  c.c.  of  a 
2  per  cent  glue  solution  with  25  to  30  c.c.  of  a  0.5  per  cent  tannin 
solution,  and  20  c.c.  of  a  5  per  cent  potassium  alum  solution 
were  added.  This  was  warmed  for  a  minute,  filtered,  and  washed 
with  water  at  30°C.  The  filtrate  was  then  titrated  with  standard 
potassium  permanganate  solution.  To  correct  for  reducing 
substances  in  the  glue  not  precipitated  by  tannin  a  second  deter- 
mination was  made  in  which  the  filtrate  was  treated  with  hide 
powder  and  titrated.  The  difference  between  the  two  titrations 
gives  a  measure  of  the  gelatin  present.  One  hundred  grams  of 
tannin  were  found  to  precipitate  139.1  grams  of  gelatin  under  the 
above  conditions,  and  the  results  were  unaffected  by  the  presence 
of  other  nitrogenous  impurities.  In  very  impure  glues,  however, 
the  alcoholic  precipitation  was  preferred.2 

By  applying  the  method  of  Miiller,  Halla3  determined  the 
gelatin  in  a  number  of  gelatinous  substances.  He  found  pure 
gelatin  to  contain  17.615  per  cent  of  nitrogen,  and  so  deduced  the 
factor  100.00/17.615  =  5,677  for  use  in  calculating  gelatin  from 
nitrogen.  Halla  reports  the  following  data  on  his  determinations : 

TABLE  50. — ESTIMATION  OF  GELATIN  IN  GLUES 


Gelatin 

M.  P.  of 
jelly 

Nin 
gelatin 

Total  N 

Gelatin  

82.73 

36.5 

14.57 

14.83 

Gelatin-glue  

80.74 

31.0 

14.22 

14.13 

Glue  powder  

78.09 

26.5 

13.75 

14.33 

Size  ,  -....:  

74.44 

25.0 

13.11 

13.80 

Bone  glue  .....'.    

74.11 

25.0 

13.05 

14.59 

Bone  glue                                    .... 

69.72 

24.5 

12.28 

14.21 

Gilder's  size 

68.97 

24.0 

12.15 

14.30 

1  A.  MULLER,  Z.  angew.  Chem.,  15  (1902),  482. 

2  A.  MULLER,  idem,  1237. 

»  HALLA,  ibid.,  20  (1907),  24. 

30 


466  GELATIN  AND  GLUE 

A  study  of  the  gelatin-tannin  reaction  was  made  by  Trunkel1  in 
1910.  He  found  that  1  gram  of  gelatin  required  0.7  gram  of 
tannin  for  complete  precipitation  when  used  in  a  freshly  pre- 
pared condition,  but  after  the  gelatin  solution  had  stood  for  24 
hours  only  0.4  gram  of  tannin  was  required.  If  the  latter  solution 
is  warmed,  however,  it  regains  its  former  power  to  precipitate 
tannin.  Where  the  gelatin  and  the  tannin  are  both  quantita- 
tively precipitated,  the  precipitate  resists  decomposition  by 
water,  but  with  an  excess  of  tannin  a  precipitate  may  be  obtained, 
containing  3  tannin  to  1  gelatin  which,  however,  yields  up  tannin 
on  treatment  with  water.  By  repeated  extraction  of  the  pre- 
cipitate with  alcohol,  up  to  97  per  cent  of  the  tannin  may  be 
removed,  but  in  no  case  can  the  precipitate  be  entirely  resolved 
into  its  constitutents.  Only  about  6  per  cent  of  unaltered  gela- 
tinizable  gelatin  may  be  extracted  from  the  residue.  The  action 
of  alcohol  leads  Trunkel  to  believe  that  the  precipitation  of 
gelatin  by  tannin  is  an  adsorption  process. 

3.  Picric  Acid  Precipitation. — A  study  was  made  by  Berrar2 
in  1912,  upon  the  gelatin-picric  acid  reaction.  He  found  that 
gelatin  was  precipitated  quantitatively  by  the  addition  of  an 
equal  volume  of  picric  acid  at  8°C.,  or  below.  At  higher  tem- 
peratures even  an  excess  of  the  picric  acid  failed  to  precipitate 
the  gelatin  completely.  The  nitrogen  in  the  precipitate  was 
estimated  by  a  Kjeldahl  determination  after  reduction  with  iron 
turnings  and  acetic  acid.  The  gelatin-picric  acid  compound  dis- 
solved in  a  2  per  cent  solution  of  urea  containing  sodium  chloride, 
which  makes  the  method  useless  for  the  estimation  of  gelatin  in 
urine.  The  precipitate  dissolved  readily  in  alcohol,  and  this 
fact  provides  for  the  separation  of  gelatin  from  albumin,  albu- 
moses,  peptones,  mucin  and  casein,  as  the  picric  acid  compounds 
of  these  substances  are  insoluble.  For  the  estimation  of  gelatin 
in  admixture  with  other  proteins  Berrar  recommends  that  an 
aqueous  mixture  be  treated  with  a  solution  containing  1  part  of 
saturated  picric  acid  solution  in  4  parts  of  alcohol.  The  above 
mentioned  proteins  are  precipitated  and  removed  by  filtration. 
The  gelatin  is  then  precipitated  by  cooling  to  8°  and  adding  an 
excess  of  picric  acid.  The  picric  acid  method  is  also  recom- 
mended as  a  test  for  gelatin  in  the  presence  of  other  proteins  as 

1  TRUNKEL,  Biochem.  Z.y  26  (1910),  458. 

2  BERRAR,  Biochem.  Z.,  47  (1912),  189. 


CHEMICAL  ANALYSIS  OF  GELATIN 


467 


1  part  of  gelatin  in  100,000  parts  of  solution  yields  a  distinct 
turbidity  on  the  addition  of  an  excess  of  picric  acid. 

4.  Indirect  Methods. — A  more  ambitious  attempt  to  obtain 
the  evaluation  of  a  glue  by  chemical  examination  was  made  by 
Fahrion.1  He  determined  the  water,  ash,  unsaponifiable  matter, 
fatty  acids,  fluid  oxy-acids,  and  solid  oxy-acids,  and  estimated 
the  "proteid"  substance  by  difference.  Two  portions  of  3  to  5 
grams  were  weighed  out,  and  one  of  these  used  for  the  moisture 
and  ash  determinations.  The  other  was  mixed  with  15  to  25  c.c. 
of  8  per  cent  alcoholic  soda,  and  evaporated  to  dryness  on  a  water 
bath.  The  residue  was  taken  up  with  alcohol  and  again  evapo- 
rated to  dryness.  This  was  washed  into  a  separatory  funnel 
with  hot  water,  hydrochloric  acid  added  to  acidify,  and  on  cooling 
shaken  with  ether.  The  solid  oxy-acids  are  left  undissolved,  and 
were  estimated  by  dissolving  in  warm  alcohol,  evaporating  by 
dryness,  and  weighing.  The  etherial  extract  was  evaporated 
and  the  residue  weighed.  This  was  then  treated  with  petroleum 
spirit,  and  shaken  in  a  separatory  funnel.  The  fluid  oxy-acids 
are  insoluble  in  this  solvent,  and  were  separated  out  and  weighed. 
An  alcoholic  soda  solution  was  then  added  and  the  alkaline  layer 
containing  the  fatty  acids  separated  from  the  petroleum  spirit 
layer.  The  latter  was  evaporated  to  dryness  and  weighed,  giving 
unsaponifiable  matter.  The  alkaline  solution  was  heated  on  a 
water  bath  to  remove  the  alcohol,  the  residue  diluted  with  water, 
decomposed  in  a  separatory  funnel  with  hydrochloric  acid,  and 
shaken  out  with  petroleum  spirit.  The  fatty  acids  were  then 
obtained  by  evaporation  and  drying.  The  proteid  substance 
was  calculated  by  subtracting  the  sum  of  the  other  constituents 
from  100.  The  following  table  gives  some  of  the  results  obtained : 

TABLE  51. — GLUE  EVALUATION  BY  FAHRION'S  METHOD 


Water 

Ash 

Unsaponi- 
fiable 
matter 

Fatty 
acids 

Fluid 
oxy- 
acids 

Solid 
oxy- 
acids 

Proteid 
substance 

Glue 

13  74 

1  80 

0  49 

0  08 

0  04 

0  27 

83  58 

Hide  powder  
Purified  leather  
Horn  

19.15 
11.23 
9.09 

0.25 
10.06 
1.00 

0.72 
9.74 
0  68 

0.18 
0.99 
1.03 

0.08 
0.46 
0.29 

0.37 
1.01 
1.49 

79.25 
66.51 

87  62 

Bone  

10.00 

53.87 

4.81 

4.23 

0.19 

1.52 

25.38 

1  FAHRION,  Z.  angew.  Chem.,  8  (1895)529. 


468  GELATIN  AND  GLUE 

Mavrojannis1  has  proposed  the  use  of  formaldehyde  for  the 
separation  of  protein  and  proteose  from  the  more  completely 
hydrolyzed  peptones  and  amino-acids.  The  former  two  frac- 
tions are  rendered  insoluble  by  the  formaldehyde,  while  the 
latter  are  unaffected. 

Greifenhagen,  Konig,  and  Scholl2  have  examined  the  various 
methods  proposed  and  find  none  of  them  entirely  satisfactory. 
The  formaldehyde  method  they  find  to  be  useless.  Precipitation 
with  Nessler's  reagent  in  the  presence  of  a  tartrate  was  found  to 
yield  quantitative  results,  but  the  proteoses  were  also  precipi- 
tated. Trichloracetic  acid  in  large  excess  yielded  a  turbidity 
with  very  dilute  solutions  of  gelatin,  and  did  not  precipitate  the 
proteoses  completely.  Mercuric  chloride  was  found  to  pre- 
cipitate proteose  from  neutral  solutions,  but  not  gelatin.  The 
proteoses  were  not,  however,  precipitated  completely.  If  the 
gelatin  and  proteoses  were  precipitated  together  with  zinc 
sulphate  and  the  nitrogen  determined,  and  in  another  aliquot 
the  zinc  sulphate  precipitate  dissolved  and  the  proteoses  thrown 
down  with  mercuric  chloride,  the  nitrogen  content  was  found  to 
be  about  the  same  if  no  gelatin  was  present,  but  greater  in  the 
former  precipitation  if  gelatin  was  present.  Mercuric  iodide  in 
alcohol  or  acetone  was  also  found  to  be  a  precipitant  for  gelatin. 

In  the  author's  experience  mercuric  chloride  cannot  be  used  to 
separate  gelatin  from  proteose,  as  the  former  is  also  thrown  down 
to  a  considerable  extent  by  that  reagent. 

5.  Salt  Precipitation. — Trotman  and  Hackford3  in  1904  called 
attention  to  the  existing  disparity  in  the  methods  proposed  or  in 
use  for  the  determination  of  the  purity  of  gelatins  and  glues,  and 
proposed  a  method  based  upon  the  precipitation  of  the  "albu- 
moses,"  which  in  fact  included  the  proteins  and  proteoses,  by 
saturating  their  solution  with  zinc  sulphate.  They  first  deter- 
mined the  total  nitrogen  by  KjeldahFs  method,  then  the  "albu- 
moses"  by  precipitating  with  zinc  sulphate  and  subjecting  the 
precipitate  to  a  Kjeldahl  determination.  The  nitrogen  obtained 
by  both  of  these  determinations  was  multiplied  by  the  factor 
5.33.  The  difference  between  the  total  nitrogen  X  5.33  and  the 
proteoses  was  taken  as  peptones. 


1  MAVROJANNIS,  Z.  Hygiene,  45  (1904),  108. 

2  GREIFENHAGEN,  KONIG,  and  SCHOLL,  Biochem.  Z.,  35  (1911),  217. 

3  S.  TROTMAN  and  J.  HACKFORD,  J.  Soc.  Chem.  Ind.,  23  (1904),  1072. 


CHEMICAL  ANALYSIS  OF  GELATIN 


469 


The  procedure  was  as  follows :  1  gram  of  finely  powdered  glue 
was  dissolved  in  20  c.c.  of  water.  While  still  hot,  zinc  sulphate 
crystals  were  added  in  excess  to  saturate  the  solution.  The 
coagulated  "albumoses"  were  then  either  filtered  off,  or  caused  to 
cling  to  the  rod  and  beaker,  and  the  clear  liquid  and  crystals 
decanted  off.  The  precipitate  was  washed  with  saturated  zinc 
sulphate  solution,  dissolved  in  concentrated  sulphuric  acid,  and 
subjected  to  the  Kjeldahl  determination,  the  nitrogen  found 
being  multiplied  by  5.33. 

The  results  obtained  are  reported  in  the  following  table : 

TABLE  52. — ZINC  SULPHATE  PRECIPITATION  OF  "ALBUMOSES" 


N  pre- 

cipitated 

Jelly 

Total  N 

by   Zn- 

Peptones 

consis- 

X 5.33 

S04  X 

by 

tency 

"gelatin" 

5.33 

difference 

"Albu- 

moses" 

Best  gelatin  

150 

74.03 

72.22  • 

1.81 

Same  boiled  2  hours 

143 

74  03 

71  36 

2  67 

Glue 

135 

71  64 

69  54 

2  10 

Glue  

120 

74.62 

68.05 

6.57 

Glue 

110 

74  30 

67  0 

7  3 

Glue 

90 

71  04 

64  18 

7  86 

Overboiled  glue  

40 

73.02 

57.99 

15.03 

These  data  show  an  unmistakable  relationship  between  the 
jelly  consistency  and  the  "albumoses."  The  results  would  be  of 
somewhat  greater  value,  however,  if  the  precipitations  had  been 
conducted  at  a  fixed  and  not  too  high  temperature,  and  allowed 
to  come  to  complete  equilibrium  at  that  temperature  by  standing 
for  a  number  of  hours. 

The  author1  has  shown  that  considerable  differences  in  the 
degree  of  precipitation  are  experienced,  in  the  case  of  magnesium 
sulphate,  by  a  few  degrees  variation  in  temperature,  and  an 
enormous  difference  by  very  slight  variations  in  the  acidity  of  the 
solution.  From  3  to  8  per  cent  more  nitrogen  was  thrown  down 
at  17°  than  at  25°  as  shown  by  the  following  table: 


R.  H.  BOGUE,  Chem.  Met.  Eng^  23  (1920),  105. 


470 


GELATIN  AND  GLUE 


TABLE    53. — EFFECT   OF   TEMPERATURE    ON    MAGNESIUM   SULPHATE 
PRECIPITATIONS 


Precipitated  and  filtered 
at  25°C.,  per  cent  of 
total  N 

Precipitated  and  filtered 
at  17°C.,  per  cent  of 
total  N 

Precipitated  at  17°C., 
filtered  at  30°C.,  per 
cent  of  total  N 

41.9 

50.1 

41.3 

44.8 

52.0 

46.0 

50.9 

57.7 

49.7 

58.3 

50.3 

55.5 

58.7 

51.7 

56.2 

59.4 

52.2 

53.7 

56.2 

50.9 

The  effects  of  acidity  on  the  precipitation  was  much  greater. 
With  no  acid  added  only  2.95  per  cent  of  the  total  nitrogen  was 
thrown  down,  while  by  the  addition  of  0.5  c.c.  of  dilute  (1:4) 
sulphuric  acid  41.3  per  cent  of  the  total  nitrogen  was  precipi- 
tated. Further  additions  of  acid  resulted  in  decreased  precipita- 
tions. These  results  are  shown  in  graph  form  in  Fig.  4  (page 
26). 

By  an  application  of  the  magnesium  sulphate  precipitation 
method,  the  author1  has  thrown  out  the  protein  by  half  satura- 
tion and  the  proteose  and  protein  together  by  full  saturation  of 
the  salt.  Peptone  and  amino-acid  were  also  determined. 
Results  of  the  greatest  significance  and  value  may  be  obtained  by 
this  procedure. 

III.  THE  DETECTION  AND  ESTIMATION  OF  GELATIN  AND  GLUE  IN 
FOODS  AND  MISCELLANEOUS  PRODUCTS 

1.  Gelatin  in  Meat  and  Meat  Products. — The  presence 
of  gelatin  in  meat  extracts  is  due  not  to  the  occurrence  of  that 
protein  in  the  juices  of  the  meat,  for  these  have  been  shown  to  be 
practically  free  of  gelatin,  but  rather  to  the  hydrolytic  action  of 
the  hot  water  or  steam  upon  collagenous  tissues  that  are  present 
in  the  meat  fiber,  as  connective  tissue,  tendons,  cartilage,  etc. 
In  small  amounts  gelatin  is  not  regarded  as  an  impurity  in  meat 
extracts,  but  if  present  in  large  amounts  the  indication  is  that 
either  inferior  material  was  used  in  the  preparation,  or  that 


lVide  pages  25  to  29. 


CHEMICAL  ANALYSIS  OF  GELATIN 


471 


gelatin  has  been  added  deliberately  as  an  adulterant.  The 
percentage  of  gelatin  in  various  meat  extracts  is  shown  in  the 
following  table  by  Hehner,  cited  after  Richardson.1 


TABLE  54. — GELATIN  CONTENT  OF  MEAT  EXTRACTS 


Description 

Gelatin 

Description 

Gelatin 

Liebig's  Extractum  Carnis.  . 

5.18 

Vitalia  Meat  Juice  

0  45 

Armour's  Extract  of  Meat.  . 
Brand's  Extractum  Carnis 

3.31 
4  56 

Brand's  Essence  of  Beef.  . 
BorviPs  Fluid  Beef     . 

5.12 
3  81 

Liebig's  Extract  (Borvil  & 
Co.) 

5.50 

Borvil's  Fluid    Beef,    un- 
seasoned   

1  06 

Brand's  Meat  Juice  
Valentine's  Meat  Juice 

0.69 
0  75 

Borvil  for  Invalids  
Borvil  for  Invalids 

4.56 
2  56 

Wyeth's  Meat  Juice  
Borthwick's  Bouillon 

1.12 
1.37 

Caffyn's  Liquor  Carnis.  .  .  . 
Extract  of  Meat  with  Vege- 

0.25 

table  Extract 

1  69 

Alcoholic  Extraction. — An  ingenious  method  for  separating  the 
gelatin  from  a  meat  extract  was  suggested  by  Stutzer2  in  1895. 
The  procedure  depends  upon  the  insolubility  of  gelatin  in  alcohol 
and  cold  water,  it  being  assumed  that  all  other  ingredients  of  the 
extract  are  soluble.  Five  to  seven  grams  of  the  dry,  or  20  to  25 
grams  of  the  fluid  extract  are  weighed  into  a  tinfoil  dish  and 
sufficient  hot  water  added  to  dissolve  the  extract.  Ignited  sand, 
free  of  dust,  is  then  added  in  sufficient  quantity  to  absorb  the 
whole  of  the  liquid,  and  placed  in  an  oven  at  100°C.,  till  the 
weight  is  constant.  The  sand  and  extract  are  then  ground  in  a 
mortar,  the  tinfoil  cut  into  strips,  and  the  whole  placed  in  a 
beaker  and  extracted  four  times  with  absolute  alcohol,  the  super- 
natant liquid  being  decanted  through  an  asbestos  filter.  The 
residue  is  then  treated  as  follows:  The  beaker  containing  the 
residue  is  placed  in  ice  water  and  100  c.c.  of  a  mixture  of  alcohol 
and  ice  water  (100  grams  of  alcohol  +  300  grams  of  ice  +  600 
grams  of  water)  added.  After  stirring  for  2  minutes  the  super- 
natant liquid  is  poured  into  a  beaker.  This  is  repeated  four 
times,  saving  the  wash  water  each  time  in  a  different  beaker. 
The  residue  is  filtered  through  an  asbestos  filter,  and  the  several 

1  W.  D.  RICHARDSON,  "Allen's  Commercial  Organic  Analysis"  (1913), 
vol.  8,  p.  398. 

2  STUTZER,  Z.  anal.  Chem.,  34  (1895),  568. 


472  GELATIN  AND  GLUE 

wash  waters  filtered  through  separate  filters.  After  washing 
with  the  alcohol-ice-water,  all  the  filters,  the  sand,  and  the  first 
filter  from  the  absolute  alcohol  treatment  are  boiled  with  water 
in  a  porcelain  dish,  filtered,  and  the  filtrate  concentrated  by 
evaporation.  The  concentrated  residue  is  then  subjected  to  a 
Kjeldahl  nitrogen  determination.  The  nitrogen  value  5.55  was 
used  in  calculating  gelatin.  From  95  to  98  per  cent  of  the  total 
gelatin  is  obtained  in  this  way,  but  any  proteoses  that  are  present 
are  also  insoluble  in  the  alcohol  ice-water  and  are  thus  deter- 
mined with  the  gelatin. 

Practically  the  same  procedure  was  used  by  Kutscher1  in  1905. 

Chlorine  Precipitation. — Rideal  and  Stewart2  in  1897  suggested 
the  separation  of  gelatin  from  the  other  constituents  of  meat 
extract  by  precipitation  with  chlorine.  One  hundred  c.c.  of  the 
liquid  containing  not  more  than  0.2  per  cent  of  "proteids"  are  used. 
Chlorine  gas  is  allowed  to  bubble  through  the  solution  for  some 
time.  The  solution  remains  clear  for  a  while  and  then  begins  to 
froth  strongly,  the  bubbles  being  enclosed  in  a  white  film.  The 
frothing  ceases  shortly,  and  a  precipitate  forms  which  easily 
settles  leaving  a  clear  supernatant  liquid.  As  soon  as  this  liquid 
shows  a  yellow  color  the  supply  of  chlorine  is  shut  off.  After 
standing  for  some  hours  the  precipitate  which  is  granular  is 
filtered  on  a  hardened  filter  paper  with  suction,  and  washed  with 
cold  water  till  free  of  chlorine.  It  is  then  dried  first  in  warm  air, 
and  later  in  vacuo  over  sulphuric  acid.  If  heated  above  70  to  80°C. 
decomposition  ensues.  The  dried  precipitate,  which  is  a  pale, 
yellowish  white,  inodorous  powder,  is  weighed,  and  gelatin 
calculated. 

The  results  obtained  by  the  originators  were  found  to  compare 
favorably  with  the  tannin  precipitation,  while  the  process  was 
much  easier  of  manipulation.  The  precipitate  was  very  stable 
at  ordinary  temperatures,  but  when  heated  readily  decomposed, 
becoming  nearly  black,  and  rotting  the  filter  paper.  In  the 
precipitation,  hypochlorous  acid  seemed  to  be  the  principal 
product. 

Bromine  Precipitation. — Allen  and  Searle3  investigated  the 
action  of  bromine  on  gelatin  and  found  that  it  could  be  substituted 
for  chlorine  in  the  above  process  to  advantage.  The  results 

1  KUTSCHER,  Z.  Nahr.  Genussm.,  10  (1905),  528;  11  (1906),  582. 

2  S.  RIDEAL  and  C.  STEWART,  Analyst,  22  (1897),  228. 

3  A.  ALLEN  and  A.  SEARLE,  Analyst,  22  (1897),  259. 


CHEMICAL  ANALYSIS  OF  GELATIN 


473 


obtained  and  the  reactions  involved  were  practically  identical 
with  those  produced  by  chlorine  but  the  procedure  was  easier  of 
manipulation.  In  either  case  the  proteoses,  peptones,  albumin, 
and  syntonin  were  thrown  down  with  the  gelatin,  but  creatine, 
creatinine,  asparagine  and  aspartic  acid  were  not  precipitated 
in  an  acidified  solution. 

One  hundred  c.c.  of  the  solution  containing  about  1  gram  of 
gelatinous  or  albuminous  matter  are  treated  with  dilute  hydro- 
chloric acid  until  distinctly  acid  to  litmus.  Bromine  water  is 
then  added  in  large  excess,  and  stirred  vigorously  for  some 
time.  The  precipitate  is  at  first  flocculent,  but  soon  becomes 
viscous  and  adheres  to  the  rod  and  beaker.  It  is  allowed  to 
stand  for  a  half  hour,  then  filtered  through  an  asbestos  filter 
and  washed  with  cold  water.  The  filter  and  rod  are  then 
returned  to  the  original  beaker,  20  c.c.  of  concentrated  sulphuric 
acid  added,  and  the  covered  beaker  warmed  over  a  wire  gauze. 
When  the  frothing  has  ceased  10  grams  of  potassium  sulphate 
are  added,  and  the  heating  continued  until  colorless.  The 
determination  is  finished  by  the  usual  Kjeldahl  distillation. 

The  nitrogen  X  5.5  gives  the  gelatin  precipitated  by  the 
bromine.  The  bromine  precipitate  is  insoluble  in  water  and 
dilute  hydrochloric  acid,  and  is  less  easily  handled  than  the 
chlorine  compound  as  it  is  not  granular  like  the  latter. 

Some  results  obtained  by  Allen  and  Searle  by  the  bromine 
method  are  tabulated  below: 

TABLE   55. — BROMINE   PRECIPITATION  OF  GELATIN 


N  per  cent 

N  X  factor 

Total  in 
original 

Precipi- 
tated by 

Total  in 
original 

Precipi- 
tated by 

Factor 
employed 

substance 

bromine 

substance 

bromine 

Commercial  gelatin  

14.10 

14.00 

76.42 

76.14 

Gelatin  peptone  

14.10 

13.90 

76.42 

75.44 

5.5 

Commercial  scale  albumin  ... 

8.80 

8.72 

55.8 

55.2 

Syntonin  from  scale  albumin  .  . 

9.86 

9.76 

62.41 

61.78 

Digested  scale  albumin  

8.89 

8.81 

56.3 

55.8 

Fresh  white  of  egg  

1.89 

1.88 

11.96 

11.90 

Syntonin  from  white  of  egg  .  .  . 

1.89 

1.89 

11.96 

11.96 

6.33 

Peptone  from  white  of  egg  ... 

0.70 

0.69 

4.43 

4.37 

Beef  extractives  

0.33 

0.004 

2.11 

0.03 

Formaldehyde  Precipitation. — Beckmann1  has  proposed  the  use 
of  formaldehyde  for  the  estimation  of  gelatin  in  meat  extracts. 
1  E.  BECKMANN,  Kept.  13th  Assembly  Bavarian  Chemists  (1894),  18. 


474  GELATIN  AND  GLUE 

The  method  is  based  upon  the  fact  that  gelatin  combines  with 
formaldehyde  to  form  a  non-fusible  and  insoluble  compound, 
formo-gelatin.  The  coagulable  albumins  are  also  precipitated, 
but  these  may  easily  be  determined  in  an  aliquot  of  a  water 
solution  by  throwing  out  with  acid.  Another  aliquot  is  then 
treated  with  formalin,  steamed  on  a  water  bath,  and  after 
boiling  for  a  short  time  with  water  the  residue  is  collected  in  a 
Gooch  filter,  dried  at  100°C.,  and  weighed.  By  deducting  the 
amount  of  albumins  previously  found,  the  gelatin  is  obtained. 
Peptones  were  found  not  to  be  affected  by  the  formaldehyde 
treatment. 

Tannin  Precipitation. — The  Association  of  Official  Agricultural 
Chemists1  has  adopted  the  tannic  acid  precipitation  method  for 
the  estimation  of  gelatin,  proteoses,  and  peptones  in  meats.  A 
7  to  25  gram  sample  of  the  finely  macerated  meat  is  weighed  into  a 
150  c.c.  beaker,  5  to  10  c.c.  of  cold  (15°C.)  ammonia-free  water 
added,  and  stirred  to  a  paste.  Fifty  c.c.  of  cold  water 
are  then  added  and  stirred  every  3  minutes  for  15  minutes, 
then  decanted  onto  a  filter,  and  drained  by  pressing  with  a 
glass  rod.  Fifty  c.c.  more  of  water  are  added  to  the 
meat  in  the  beaker,  this  stirred  for  5  minutes  and  decanted  as 
before.  The  extraction  as  above  is  repeated  using  the  following 
additional  amounts  of  cold  water:  50,  50,  25,  25,  25,  and  25  c.c. 
After  the  last  extraction  the  meat  is  transferred  to  the  filter  and 
washed  three  times  with  10  c.c.  portions  of  water.  The  extract 
is  collected  in  a  500  c.c.  volumetric  flask,  and  made  up  to  the 
mark  with  water.  One  hundred  and  fifty  c.c.  are  placed  in 
a  250  c.c.  beaker  and  evaporated  to  40  c.c.  on  a  steam  bath 
with  occasional  stirring.  It  is  then  neutralized  to  phenolphthalein, 
1  c.c.  of  normal  acetic  acid  added,  and  boiled  gently  for  5 
minutes.  The  coagulum  is  filtered  off,  and  washed  four 
times  in  the  beaker  and  three  times  on  the  filter  with  hot 
water.  The  filtrate  and  washings  are  made  up  to  200  c.c.  and 
designated  as  Solution  A.  An  aliquot  of  50  c.c.  is  transferred  to  a 
100  c.c.  graduated  flask,  and  15  grams  of  sodium  chloride  and  10 
c.c.  of  cold  water  added.  The  flask  is  shaken  until  the  sodium 
chloride  has  dissolved,  then  cooled  to  12°C.,  and  30  c.c.  of  a  24 
per  cent  tannin  solution  added.  The  flask  is  filled  to  the  mark 
with  water  at  12°,  the  mixture  shaken  well,  and  allowed  to  stand 
at  that  temperature  for  12  hours  or  more.  The  precipitate  is 

1  A.  O.  A.  C.,  "Methods  of  Analysis"  (1920),  215. 


CHEMICAL  ANALYSIS  OF  GELATIN  475 

then  filtered  off,  and  50  c.c.  of  the  filtrate  treated  with  a  few 
drops  of  sulphuric  acid  and  evaporated  in  vacuo  to  dryness. 
The  residue  is  subjected  to  a  Kjeldahl  nitrogen  determination 
(page  431).  A  50  c.c.  aliquot  of  Solution  A  is  also  subjected  to  a 
Kjeldahl  nitrogen  determination,  and,  from  the  value  obtained 
is  deducted  twice  the  value  of  the  first  mentioned  nitrogen  deter- 
mination. The  difference  obtained  is  multiplied  by  6.25  to  give 
the  gelatin,  proteose  and  peptone.  This  figure  divided  by  the 
weight  of  meat  represented  by  the  aliquot  taken,  and  multi- 
plied by  100  gives  the  percentage  of  these  constituents  in  the 
meat. 

The  peptones  and  most  of  the  proteoses  will  undoubtedly  be 
determined  by  the  above  (tentative)  method,  but  one  is  at 
difficulty  to  understand  how  unhydrolyzed  gelatin  may  be 
extracted  with  water  at  15°C. 

Salt  Precipitation. — The  method  for  the  determination  of  gela- 
tin and  proteoses  in  meat  extracts  adopted  by  the  Association  of 
Official  Agricultural  Chemists,1  is  the  zinc  sulphate  precipitation 
method.  A  sample  of  5  grams  of  powdered  preparations,  8  to 
10  grams  of  pasty  extracts,  or  20  to  25  grams  of  fluid  extracts  is 
dissolved  in  cold  water,  and  filtered.  Coagulable  proteins  are 
removed  by  neutralizing  to  phenolphthalein,  adding  1  c.c.  of 
normal  acetic  acid,  boiling  3  minutes,  cooling,  diluting  to  500 
c.c.,  and  filtering  through  a  folded  filter.  The  filtrate  is  evap- 
orated to  a  small  volume  and  saturated  with  zinc  sulphate  (85 
grams  to  50  c.c.  solution).  After  standing  several  hours  the 
residue  is  filtered  and  washed  with  a  saturated  solution  of  zinc 
sulphate.  The  filter  paper  with  its  contents  is  then  placed  in  an 
800  c.c.  Kjeldahl  flask,  and  the  nitrogen  determined.  The 
figure  obtained  multiplied  by  6.25,  divided  by  the  weight  of 
sample  taken,  and  multiplied  by  100,  is  taken  as  the  percentage 
of  gelatin  and  proteose  in  the  meat  extract. 

The  above  procedure  somewhat  modified  was  employed  by 
Emery  and  Henley2  in  an  exhaustive  study  of  meat  extracts  in 
1919.  Twenty-five  c.c.  of  a  10  per  cent  solution  of  the 
solid  extract  or  a  20  per  cent  solution  of  the  liquid  extract 
were  placed  in  a  50  c.c.  graduated  flask,  1  c.c.  of  a  50  per  cent 
sulphuric  acid  solution  added,  with  zinc  sulphate  sufficient  to 
saturate  the  solution,  and  then  a  saturated  solution  of  the  same 

1  A.  O.  A.  C.,  op  cit.,  222. 

2  J.  EMERY  and  R.  HENLEY,  J.  Agr.  Res.,  17  (1919),  1. 


476 


GELATIN  AND  GLUE 


salt  added  to  make  50  c.c.  After  standing  18  hours  it  was  filtered 
and  20  c.c.  of  the  filtrate  employed  for  a  nitrogen  determination 
by  the  Gunning  process.  The  total  nitrogen  of  the  extract,  less 
the  sum  of  the  coagulable  and  insoluble  nitrogen,  which  had 
previously  been  determined,  and  the  zinc  sulphate  filtrate  nitro- 
gen, represented  the  nitrogen  of  the  zinc  sulphate  precipitate. 
This  value  includes  both  the  gelatin  and  proteoses. 

In  order  to  obtain  comparative  data,  a  tannic  acid  precipitation 
was  also  employed.  For  this  20  c.c.  of  the  solution  as  above  were 
placed  in  a  100  c.c.  graduated  flask,  50  c.c.  of  a  saturated  solution 
of  sodium  chloride  added,  and  the  flask  filled  to  the  mark  with  a 
24  per  cent  solution  of  tannic  acid.  After  mixing  thoroughly  it 
was  placed  in  the  ice  box  and  allowed  to  stand  over  night,  any 
loss  in  volume  being  made  good  with  the  tannic  acid  solution. 
The  mixture  was  filtered  in  the  ice  box  on  the  following  day,  and 
50  c.c.  of  the  filtrate  transferred  to  a  Kjeldahl  flask,  evaporated 
to  dryness  on  a  water  bath,  and  the  nitrogen  in  the  residue  deter- 
mined by  the  Gunning  method.  The  nitrogen  of  the  tannic  acid 
precipitate  was  then  calculated  by  deducting  the  sum  of  the  tan- 
nic acid  filtrate,  the  coagulable,  and  the  insoluble  nitrogen  from 
the  total  nitrogen. 

The  following  table  gives  some  of  the  data  obtained.  It  will  be 
observed  that  the  nitrogen  by  the  tannic  acid  precipitation  is 

TABLE  56. — DISTRIBUTION  OF  NITROGEN  IN  MEAT  EXTRACTS 


Commercial  extracts 

Laboratory    extracts 

Per  cent  of  total  N 

Per  cent  of  total  N 

Source  of  extract 

Total  N 

Total  N 

per  cent 

ZnSO4 

Tannic 

per  cent 

ZnSO4 

Tannic 

precipi- 

acid pre- 

precipi- 

acid pre- 

tate 

cipitate 

tate 

cipitate 

Chuck  and  plate  

10.08 

17.75 

44.13 

11.67 

21.57 

48.87 

Beef  hearts  

9.02 

17.17 

39.99 

8.77 

11.17 

30.76 

Hog  spleens  

9.38 

26.53 

52.45 

9.98 

23.74 

56.10 

Hog  liver  

6.00 

18.66 

53.33 

8.14 

24.19 

58.37 

Hog  liver  

6.00 

18.70 

53.25 

6.52 

9.35 

52.30 

Roast  beef  soak  water  .  .  . 

8.99 

10  34 

44  05 

Corn  beef  cook  liquor.  .  .  . 

9.23 

17.21 

41.04 

Beef  bones  

9.47 

13.58 

47.96 

8.26 

15.25 

20.1 

Beef  spleens  

9.98 

30.07 

51.81 

9.98 

23.7 

56.0 

Beef  liver  

6.42 

23.23 

57.29 

CHEMICAL  ANALYSIS  OF  GELATIN  477 

much  greater  than  that  by  the  zinc  sulphate  method.  This  is 
due  to  the  fact  that  the  former  method  precipitates  peptones  in 
addition  to  the  gelatin  and  proteoses  thrown  down  by  the  zinc 
sulphate.  This  shows,  incidentally,  that  the  two  methods  may 
not  be  used  indiscriminately. 

2.  Gelatin  in  Milk,  Cream  and  Ice  Cream.  Picric  Acid 
Precipitation. — The  picric  acid  test  for  gelatin  in  milk  and  cream 
was  suggested  by  Stokes1  in  1897,  and  has  been  adopted  by  the 
Association  of  Official  Agricultural  Chemists.2 

To  a  10  c.c.  sample  is  added  an  equal  volume  of  acid  mercuric 
nitrate  solution  (mercury  dissolved  in  twice  its  weight  of  con- 
centrated nitric  acid,  and  this  solution  diluted  to  25  times  its 
volume  with  water).  The  mixture  is  shaken,  20  c.c.  of  water 
added,  and  again  shaken.  After  standing  for  5  minutes  it  is 
filtered.  If  much  gelatin  is  present  the  nitrate  will  be  opalescent 
and  cannot  be  obtained  altogether  clear.  A  portion  of  the  filtrate 
is  added  in  a  test  tube  to  an  equal  volume  of  saturated  aqueous 
picric  acid  solution.  If  gelatin  is  present  in  any  considerable 
amount  a  yellow  precipitate  will  be  produced,  while  smaller 
amounts  will  produce  a  cloudiness.  If  no  gelatin  is  present  the 
solution  will  remain  clear.  Stokes  affirms  that  this  test  will 
show  the  presence  of  1  part  of  gelatin  in  10,000  parts  of  solution. 

Patrick3  has  pointed  out  that  if  a  milk  or  cream  showing  no 
test  for  gelatin  by  the  picric  acid  test  when  sweet,  is  allowed  to 
sour,  a  very  perceptible  turbidity  is  often  obtained  upon  the 
addition  of  the  picric  acid  that  would  lead  to  the  assumption 
that  gelatin  was  present.  He  believes  this  is  due  to  the  formation 
by  bacteria  of  a  " pseudo-gelatin"  decomposition  product  that 
is  not  distinguishable  by  any  test  from  true  gelatin. 

Seidenberg"  has  shown,  however,  that  the  turbidity  or  precipi- 
tation in  the  above  cases  may  be  distinguished  by  differences  in 
their  solubility  in  hot  neutral  water.  While  both  are  soluble  on 
heating  in  slightly  acid  solutions,  only  the  gelatin  picrate  is 
soluble  in  hot  neutral  water  alone.  The  picric  acid  precipitate 
from  the  sour  cream  is  apparently  quite  insoluble  in  hot  water 
after  the  complete  removal  of  all  of  the  picric  or  other  acid.  The 
precipitate  obtained  by  the  regular  procedure  is  filtered,  after 

1  A.  W.  STOKES,  Analyst,  22  (1897),  320. 

2  A.  O.  A.  C.,  op.  cit.,  229. 

3  G.  E.  PATRICK,  U.  S.  Dept.  Agr.  Bull  116  (1908),  24. 

4  A.  SEIDENBERG,  J.  Ind.  Eng.  Chem.,  5  (1913),  927. 


47B  GELATIN  AND  GLUE 

shaking  to  produce  coalescence,  and  washed  with  water,  contain- 
ing 2  drops  of  ammonium  hydroxide  per  100  c.c.  of  solution,  until 
the  washings  are  slightly  alkaline  to  litmus,  and  then  with  neutral 
water  until  the  washings  are  neutral  to  litmus.  The  precipitate, 
or  the  entire  filter  paper,  are  then  transferred  to  a  small  beaker 
and  10  to  20  c.c.  of  water  added  and  heated  to  boiling.  This  is 
then  filtered.  The  filtrate  will  contain  the  gelatin  picrate  but 
not  the  precipitate  derived  from  the  decomposition  products  in 
the  sour  cream.  The  filtrate  is  cooled  and  an  equal  volume  of 
picric  acid  added.  If  gelatin  is  present  in  the  original  cream  a 
decided  precipitate  will  be  formed. 

Qualitative  Identification  of  Thickeners  and  Gelatinizing  Agents. 
A  variety  of  substances  are  in  use  as  thickeners  and  gelatinizing 
agents  in  ice  cream,  jellies,  jams,  and  other  food  materials. 
These  sometimes  are  of  advantage,  and  perhaps  not  less  fre- 
quently are  added  with  the  intent  of  concealing  the  inferiority  of 
the  product.  The  detection  of  these  substances  is  often  difficult 
and  uncertain.  Congdon1  has  suggested  a  scheme  by  which  all 
of  the  common  materials  employed  for  this  purpose  may  be 
detected  either  when  present  alone,  or  when  several  or  all  are 
present  together.  The  materials  included  in  the  scheme  are 
starch,  dextrin,  gelatin,  acacia,  agar-agar,  tragacanth,  albumin, 
and  pectin  bodies  of  the  fruit  juices.  In  order  that  the  treatment 
used  may  be  comprehensively  presented,  the  complete  procedure 
of  Congdon  is  shown  in  the  table  on  the  following  page. 

3.  Gelatin  in  Other  Food  Products.  Gelatin  in  Fruits  and 
Fruit  Products. — Henzold2  has  proposed  a  method  for  the  esti- 
mation of  gelatin  in  fruit  juices  by  precipitating  with  potassium 
dichromate.  He  dilutes  the  sample  with  water,  boils,  and  filters 
off  any  insoluble  material.  To  the  solution  is  then  added  an 
excess  of  10  per  cent  potassium  dichromate  solution,  the  mixture 
again  brought  to  a  boil,  and  cooled  at  once  by  placing  the  flask 
in  cold  water.  When  cold,  2  to  3  drops  of  concentrated  sulphuric 
acid  are  added.  If  gelatin  is  present  a  precipitate  appears  which 
is  at  first  white  and  flocculent,  but  soon  coheres  into  lumps  and 
sticky  masses  which  separate  out.  This  precipitate  may  be 
filtered  off,  if  quantitative  results  are  desired,  and  subjected  to  a 
nitrogen  determination  by  the  Kjeldahl  process. 

1  L.  A.  CONGDON,  J.  Ind.  Eng.  Chem.,  7  (1915),  606. 

2  HENZOLD,  Z.  offent.  Chem.,  6  (1900),  292. 


CHEMICAL  ANALYSIS  OF  GELATIN 
IDENTIFICATION  OF  GELATINIZING  AGENTS 


479 


Group  reagent 


Reactions  with  water-soluble  solutions 


Group  I 
Iodine  Solution. 


Blue  coloration  indicates  starch. 
Purple  coloration  indicates  amylo-dextrin. 
Red  coloration  indicates  erythro-dextrin. 
No  coloration  may  indicate  neither  starch 
dextrin,  but  may  be  achro-dextrin. 


nor 


Group  II 

Millon's  or  Stoke's  re- 
agent (acid  nitrate  of 
mercury) . 


Mixture,  after  shaking  substance  in  solution  with 
reagent,  is  cloudy.  Yellow  precipitate  with 
picric  acid  solution  indicates  gelatin. 

Drop  of  this  reagent:  Gelatinous  precipitate, 
soluble  in  excess  of  this  reagent,  indicates  acacia. 

A  slight  white  cloudy  precipitate  may  indicate 
either  agar-agar  or  tragacanth,  or  both. 


Group  III 

Concentrated  solution  of 
sodium  borate. 


A  white  gelatinous  precipitate  indicates  either 
agar-agar  or  acacia  or  both.  Acacia  will  give  a 
gelatinous,  opaque  white  precipitate  with  solu- 
tion of  basic  lead  acetate.  Acacia  further  tested 
for  as  in  Group  II  or  IV,  or  by  adding  a  solu- 
tion of  tannin  which  gives  a  bluish  black  colora- 
tion. 


Group  IV 

Solution  of  sodium  hy- 
droxide. 


A  brownish   yellow   color   on  heating   indicates 

tragacanth. 
A  white  cloudy  precipitate  indicates  acacia. 


Group  V 

Solution      of      mercuric 
chloride. 


A  slight  turbidity  may  indicate  dextrin. 
A   white   precipitate    may   indicate   albumin 
gelatin. 


or 


Group  VI 

Schweitzer's  reagent  (so- 
lution  of  cupra- 
ammonia). 


If  a  concentrated  water  solution  of  the  unknown  is 
treated  with  this  reagent  and  placed  on  a  glass 
slide  under  a  microscope,  a  delicate  framework  of 
cupric  pectate  is  evident,  showing  pectin  of  fruit 
or  vegetable  origin  present. 


The  Association  of  Official  Agricultural  Chemists1  has 
adopted  a  method  dependent  upon  precipitation  with  alcohol. 
A  concentrated  solution  of  the  jelly,  jam,  or  fruit  juice  is  pre- 
cipitated with  10  volumes  of  absolute  alcohol,  and  the  residue 
filtered  off.  This  is  dried  and  subjected  to  the  Kjeldahl  nitrogen 
determination. 

1  A.  O.  A.  C.,  op.  cit.,  156. 


480  GELATIN  AND  GLUE 

An  optional  method  is  to  determine  the  nitrogen  by  KjeldahFs 
method  in  the  original  product,  when  the  presence  of  gelatin  is 
indicated  by  the  greater  percentage  of  nitrogen  which  it 
contains. 

Gelatin  in  Chocolate. — Onfrey1  uses  a  modification  of  the  picric 
acid  method  for  the  estimation  of  gelatin  in  chocolate.  Five 
grams  of  chocolate  are  treated  with  50  c.c.  of  boiling  water,  and 
5  c.c.  of  10  per  cent  lead  acetate  solution  are  added.  The  liquid 
is  filtered  and  several  drops  of  a  saturated  aqueous  solution  of 
picric  acid  added  to  the  filtrate.  If  gelatin  is  present  in  appreci- 
able amounts  a  yellow  precipitate  will  be  formed,  but  if  only 
traces  of  gelatin  are  present  the  tannin  of  the  cocoa  combines 
with  it  forming  an  insoluble  precipitate.  In  such  cases  10  grams 
of  the  chocolate  are  extracted  with  ether  to  remove  the  fat,  and 
the  residue  treated  with  100  c.c.  of  hot  water,  5  to  10  c.c.  of  a 
10  per  cent  solution  of  potassium  hydroxide,,  and  10  c.c.  of  lead 
acetate  solution,  and  the  mixture  filtered.  The  filtrate  is  then 
treated  with  picric  acid  as  above  and  gelatin  even  in  traces  is 
revealed  by  a  turbidity  or  precipitation  of  gelatin  picrate. 

Gelatin  in  Gums. — Trillat2  employed  the  formaldehyde  process 
of  Beckmann3  for  the  estimation  of  gelatin  in  gums  and  other 
food  substances.  The  material  to  be  tested  was  dissolved  in 
water,  filtered,  and  evaporated  to  the  consistency  of  a  syrup. 
One  c.c.  of  formalin  was  then  added,  and  the  evaporation  con- 
tinued until  a  thick  pasty  mass  was  produced.  This  residue  was 
then  taken  up  in  hot  water  which  dissolved  out  the  gum  but  left 
an  insoluble  residue  of  formo-gelatin.  After  standing  for  24 
hours  it  was  decanted,  washed  with  boiling  water,  dried  on  the 
water  bath,  and  weighed. 

Vamvakas4  employes  Nessler's  reagent  for  the  detection  of 
gelatin  in  gums.  To  20  c.c.  of  the  sample  are  added  4  c.c.  of 
Nessler's  reagent.  In  the  absence  of  gelatin  a  gelatinous  pre- 
cipitate will  be  produced  which  is  brownish  gray  in  color,  and 
remains  in  suspension  for  several  days.  If  gelatin  is  present  the 
precipitate  will  be  dull  gray  in  color,  and  subsides  much  more 
rapidly,  especially  when  the  gelatin  is  present  to  the  extent  of 
20  per  cent  or  more. 

1  P.  ONFREY,  /.  pharm.  chim.,  8  (1898),  7. 

2  A.  TRILLAT,  Ann.  chim.  anal.  chim.  appl.,  3  (1898),  401. 

3  See  pages  473-474. 

4  VAMVAKAS,  Ann.  chim.  anal.  chim.  appl.,  12  (1907),  139. 


CHEMICAL  ANALYSIS  OF  GELATIN  481 

Gelatin  in  Feeding  Stuffs. — Wagner  and  Scholer1  have  applied 
the  tannin  precipitation  method  to  the  estimation  of  gelatin  in 
feeding-stuffs.  Five  grams  of  the  material  are  boiled  with  200 
c.c.  of  water  for  5  hours.  The  mixture  is  transferred  to  a  500 
c.c.  volumetric  flask  and  when  cool  made  up  to  volume  with 
water  and  filtered.  One  hundred  c.c.  of  the  filtrate  are  treated 
by  the  Kjeldahl  method  for  nitrogen.  Another  100  c.c.  portion 
is  treated  in  a  250  c.c.  volumetric  flask  with  40  c.c.  of  a  10  per 
cent  solution  of  tannin  (of  known  nitrogen  content),  made  to 
volume,  allowed  to  settle  over  night,  and  filtered.  The  nitrogen 
in  150  or  200  c.c.  of  the  filtrate  is  then  determined  by  the  Kjeldahl 
method.  The  result,  corrected  for  the  nitrogen  in  the  tannin 
solution,  gives  the  amide  nitrogen,  and  by  deducting  this  from 
the  total  nitrogen  of  the  filtrate  the  nitrogen  as  gelatin  and  other 
proteins  is  estimated. 

The  presence  or  absence  of  albumins  may  be  ascertained  by 
testing  a  portion  of  the  filtrate  in  the  500  c.c.  volumetric  flask 
with  acetic  acid  and  potassium  ferrocyanide.  Gelatin  gives  no 
reaction  with  these  reagents,  but  the  albumins  are  precipitated. 
The  xanthoproteic  reaction  may  also  be  applied,  when  the 
presence  of  albumins  is  shown  by  a  deep  yellow  or  orange  colora- 
tion. Gelatin  will  react  negative  or  only  very  faintly  to  the  test. 

Gelatin  as  a  Glaze  or  Coating  on  Coffee. — The  Association  of 
Official  Agricultural  Chemists2  employs  the  tannin  precipitation 
method  in  testing  for  gelatin  used  as  a  glaze  or  a  coating  on  coffee. 
Such  use  is  intended  to  improve  the  appearance  of  the  coffee 
bean. 

One  hundred  grams  of  the  whole  coffee  are  treated  with  500 
c.c.  of  water  and  allowed  to  stand  with  frequent  stirring  for  5 
minutes.  It  is  then  filtered  and  one  portion  of  the  filtrate 
treated  with  a  strong  solution  of  tannic  acid,  another  portion 
with  Millon's  reagent  (see  page  46),  and  a  third  portion  boiled. 
In  the  presence  of  gelatin  a  precipitate  will  be  formed  in  the  first 
two  tests,  but  not  in  the  portion  boiled.  Egg  albumin  also  gives 
positive  tests  in  the  first  two  cases,  but  differs  from  gelatin  in 
producing  also  a  coagulation  upon  boiling. 

A  further  confirmatory  test  may  be  made  by  adding  an  excess 
of  tannic  acid  to  an  aliquot  of  the  filtrate,  adding  salt  if  necessary 
to  secure  flocculation  of  the  precipitate,  and  without  washing 

1  WAGNER  and  SCHOLER,  Landw.  Ver.  Stat.,  92  (1918),  171 

2  A.  O.  A.  C.,  op.  cit.,  272. 

31 


482  GELATIN  AND  GLUE 

subjecting  the  paper  and  its  contents  to  a  Kjeldahl  nitrogen 
determination.  If  no  albumin  or  gelatin  is  present  this  will 
yield  less  than  10  mg.  of  nitrogen  per  100  gram  sample. 

An  Emulsification  Test  for  Gelatin. — The  determination  of 
gelatin  upon  an  entirely  new  principle  was  suggested  by  Winkel- 
blech1  in  1906.  He  observed  that  when  gelatin  was  shaken  with 
benzine  there  was  formed  a  stiff  emulsion  consisting  of  gelatin, 
benzine  and  water,  which  separated  from  the  water  on  standing, 
partly  as  a  result  of  entangled  air.  If  much  gelatin  is  present 
the  emulsion  is  very  voluminous  and  lumpy,  but  when  only  a 
small  amount  of  gelatin  is  present  a  number  of  bubbles  of  all 
sizes  are  observed  resting  on  the  surface  of  the  water  for  a 
considerable  time.  Upon  breaking,  a  permanent  whitish  ring 
of  very  small  bubbles  is  left  adhering  to  the  walls  of  the  tube. 

Winkelblech  applied  this  observation  to  the  quantitative 
estimation  of  gelatin.  He  found  that  a  heavy  precipitate  was 
obtained  upon  shaking  with  benzine  10  c.c.  of  a  solution  contain- 
ing 0.234  gram  of  gelatin  per  liter.  Even  upon  diluting  this 
solution  10,  20,  and  40  times,  precipitates  were  likewise  obtained. 
The  limit  of  dilution  at  which  the  presence  of  the  gelatin  could  be 
definitely  established  was  by  the  use  of  10  c.c.  of  a  solution 
containing  0.06  gram  per  liter,  or  0.06  milligram  per  10  c.c.  This 
was  accomplished  only  by  very  energetic  shaking,  and  by  using  a 
test  tube  or  other  container  in  which  the  area  of  contact  between 
the  two  liquids  would  be  small. 

It  became  necessary  therefore,  in  making  use  of  the  test  with 
unknown  solutions,  only  to  dilute  the  samples  until  the  same 
slight  amount  of  precipitation  occurred,  and  beyond  which  point 
no  positive  results  could  be  obtained.  If  a  small  amount  of  acid 
is  present  the  precipitate  seems  to  be  slightly  diminished,  while 
in  the  presence  of  a  little  alkali  the  reverse  is  true.  Large  amounts 
of  either  acid  or  alkali  make  the  tests  unreliable.  Also  if  the 
dilute  gelatin  solution  is  allowed  to  boil  for  a  few  minutes  the 
delicacy  of  the  test  is  seriously  impaired. 

Besides  benzine  other  hydrocarbons  including  kerosene,  ben- 
zene, chloroform,  and  carbon  disulphide  may  be  used.  It  does 
not  matter  if  the  hydrocarbon  is  lighter  or  heavier  than  the 
water.  If  the  former,  the  precipitate  floats  on  the  water;  if 
the  latter,  the  precipitate  floats  on  the  hydrocarbon.  The  test 
should  not  be  relied  upon  in  the  presence  of  other  colloids,  as 

1  WINKELBLECH,  Z.  angew.  Chem.,  19  (1906),  1953. 


CHEMICAL  ANALYSIS  OF  GELATIN  483 

albumin,  soap,  water  soluble  starch,  rosin  dissolved  in  dilute 
alkali,  etc.,  also  give  somewhat  similar  tests.  The  purity  of  the 
hydrocarbon  used  should  be  assured,  as  it  also  may  contain 
impurities  which  give  a  faint  test  when  shaken  alone.1 

Winkelblech  explains  the  action  observed  by  assuming  that  the 
violent  shaking  breaks  the  hydrocarbon  into  a  large  number  of 
droplets  which  have  the  power  of  condensing  the  finely  divided 
colloid  particles  upon  their  surface.  These  particles  then 
coalesce  to  form  aggregates  and  a  rigid  emulsion  is  formed.  At 
the  lowest  dilutions  a  few  large  transparent  fairly  stable  bubbles 
appeared  which  were  filled  with  hydrocarbon  except  for  a  small 
air  bubble.  This  appears  as  evidence  that  the  wet  colloid 
particles  are  able  in  some  way  to  form  surface  films.  In  its 
essential  aspects  this  theory  is  in  complete  harmony  with  Ban- 
croft's theory  of  film  formation  in  emulsification.2 

Bancroft3  has  suggested  that  Winkelblech's  method  is  in 
reality  a  test  for  interfacial  substances  (substances  which  adsorb- 
the  two  immiscible  liquids  simultaneously,  and  therefore  tend  to 
pass  into  the  dineric  interface),  and  as  such,  is  capable  of  much 
wider  application  than  has  yet  been  made  in  the  detection  of 
colloidal  solutions. 

4.  Glue  in  Size  and  Miscellaneous  Preparations. — Gelatin  in 
the  form  of  an  inferior  grade  of  glue  is  commonly  used  as  a  size 
in  the  manufacture  of  paper,  textiles,  hats,  etc.  As  other 
materials  such  as  casein  glue,  vegetable  glue,  rosin  glue  and  other 
preparations  are  similarly  used  it  is  sometimes  necessary  to 
distinguish  between  them.  For  differentiating  animal  glue  and 
casein  from  the  others  in  paper,  Levi4  recommends  the  biuret  test. 
The  paper  is  steeped  for  a  few  minutes  in  2  per  cent  copper 
sulphate  solution  and  then  treated  with  a  5  per  cent  solution  of 
sodium  hydroxide,  the  latter  being  added  by  dropping  directly 
upon  the  paper.  If  gelatin  or  casein  are  present  a  violet  colora- 
tion is  at  once  produced.  In  order  to  distinguish  between  gelatin 
and  casein,  Levi  recommends  the  xanthoproteic  reaction  which 
reacts  positive  to  casein  but  negative  to  gelatin.  The  test  is 
carried  out  by  merely  moistening  the  paper  with  a  drop  of  con- 
centrated nitric  acid,  when,  if  casein  is  present,  an  intense  yellow 


1  W.  D.  BANCROFT,  J.  Phys.  Chem.,  19  (1915),  330. 

2  See  page  214. 

3  W.  D.  BANCROFT,  loc.  cit.,  308. 

4  LEVI,  Papierfabr.,  9  (1911),  365. 


484  GELATIN  AND  GLUE 

stain  is  produced.  On  adding  sodium  hydroxide  the  stain  turns 
brown,  or  if  ammonium  hydroxide  is  added,  the  stain  becomes 
orange.  The  xanthoproteic  reaction  cannot  be  used,  however, 
in  the  presence  of  wood  fiber,  as  the  lignin  in  the  latter  also 
reacts  with  nitric  acid  giving  the  yellow  stain.  This  may  be 
avoided  however,  by  scraping  off  the  size,  or  extracting  it  with  a 
solution  of  borax  or  an  alkali,  precipitating  with  acetic  acid,  and 
testing  the  dried  precipitate  with  nitric  acid  as  above. 

Herzberg1  identifies  rosin  in  sizing  material  by  the  Raspail 
reaction,  a  rose  or  violet  coloration  being  produced  upon  adding 
sugar  and  concentrated  sulphuric  acid.  Methods  of  extraction 
with  alcohol  or  ether  he  declares  useless.  Herzberg  identifies 
an  animal  size  by  precipitation  of  its  solution  with  tannin;  by 
the  yielding  of  a  yellow  or  brown  coloration  with  iodine  in  solution 
with  potassium  iodide;  by  the  xanthoproteic  reaction;  by  Millon's 
test;  or  by  the  biuret  test.  Of  these  the  biuret  test  is  preferred, 
as  it  gives  an  absolutely  negative  test  with  rosin.  The  test  is 
best  carried  out  under  the  microscope.  Casein  he  distinguishes 
from  gelatin  in  that  the  former  reacts  to  the  Adamkiewicz 
test,  giving  a  red  or  violet  coloration  with  glacial  acetic  acid  and 
sulphuric  acid. 

Schmidt,2  in  testing  for  gelatin  in  textile  dressings,  regards  the 
ammonium  molybdate  and  the  Nessler  tests  when  taken  together 
to  be  best  suited  for  the  purpose.  In  the  presence  of  gelatin, 
ammonium  molybdate  yields  a  flocculent  white  precipitate  which 
partially  dissolves  on  heating  but  reappears  on  cooling.  Nessler's 
reagent  should  be  rendered  feebly  acid  with  sulphuric  acid,  the 
red  precipitate  filtered  off,  and  the  clear  filtrate  used  for  the  test. 
This  when  added  to  a  gelatin  solution  yields  a  white  turbidity 
whether  the  solution  is^hot  or  cold.  Ammonium  salts  do  not 
interfere,  but  protein  other  than  gelatin  must  not  be  present. 
These  two  tests  Schmidt  claims  will  detect  0.01  mg.  of  gelatin  in 
5  c.c.  of  solution,  a  quantity  not  detectable  by  the  biuret  or  the 
tannin  tests.  The  solution  should  be  freed  from  fat  and  albumin 
by  means  of  nitric  acid,  and  alkaline  solutions  must  be  neutralized 
before  applying  the  tests. 

Herz  and  Barraclough3  have  pointed  out  that  when  wool  or 
hair  is  boiled  with  water  a  substance  is  dissolved  which  gives  the 

1  HERZBERG,  Chem.  News  (England),  110  (1914),  19. 

2  SCHMIDT,  Farben-Ztg.,  24  (1913),  97. 

3  HERZ  and  BARRACLOUGH,  /.  Soc.  Dyers  and  Color,  25  (1909),  274. 


CHEMICAL  ANALYSIS  OF  GELATIN  485 

same  reactions  with  tannin  and  the  biuret  test  as  gelatin.  For 
this  reason  these  tests  cannot  be  used  in  testing  for  gelatin  as  a 
sizing  material  on  woolen  goods,  fur,  or  any  other  animal  fiber. 
Gelatin  may  ordinarily  be  distinguished  from  the  wool  product 
by  the  fact  that  the  latter  is  precipitated  from  solution  by  normal 
or  basic  lead  acetate  solutions  Basic  dyestuffs  form  insoluble 
lakes  with  wool  products  but  acid  or  direct  dye-stuffs  do  not  form 
such  compounds.  The  wool  product  may  also  be  fractionated 
into  three  apparently  distinct  constituents.  The  first  of  these 
gives  no  precipitate  with  Night  Blue,  but  is  precipitated  by  a 
solution  of  2  per  cent  tannin  mixed  with  an  equal  volume  of 
saturated  sodium  chloride  solution.  The  second  gives  a  pre- 
cipitate with  Night  Blue  which  is  dissolved  by  treating  with 
barium  hydroxide  solution,  but  is  reprecipitated  upon  adding 
either  Night  Blue  or  tannin  salt  solution,  after  the  removal  of  the 
excess  of  the  alkali.  The  third  constituent  also  gives  a  precipi- 
tate with  Night  Blue,  but  this  remains  insoluble  when  decom- 
posed with  barium  hydroxide. 

The  Technical  Association  of  the  Pulp  and  Paper  Industry1 
does  not  attempt  to  distinguish  gelatin  from  casein  quantitatively 
but  determines  the  total  nitrogen,  and  makes  a  qualitative  test 
for  gelatin.  A  small  portion  of  the  paper  is  boiled  with  10  c.c.  of 
water  in  a  test  tube.  The  solution  is  decanted  to  another  tube 
and  cooled.  Five  c.c.  of  ammonium  molybdate  solution  are 
added,  followed  by  a  few  drops  of  nitric  acid.  The  formation  of 
a  white  amorphous  precipitate  indicates  the  presence  of  glue. 

A  quantitative  as  well  as  qualitative  test  for  glue  in  papers  was 
proposed  by  Cross,  Bevan  and  Briggs,2  based  upon  the  monochlor- 
amine  reaction  which  ammonia  and  amino  groups  of  proteins 
were  found  by  them  to  undergo  in  the  presence  of  chlorine  or  a 
hypochlorite : 

NH3  +  MOC1  -»  NH2C1  +  MOH. 

The  chloramine  reacts  with  iodides  similarly  to  the  hypochlorites : 
NH2C1  -f  2HI  ->  NH4C1  +  I2. 

These  reactions  were  utilized  in  the  detection  and  estimation 
of  gelatin  in  tub-sized  papers  as  follows :  The  moistened  paper  is 
treated  with  chlorine  gas  and  placed  in  a  bath  of  2  per  cent  solu- 

1F.  C.  CLARK,  "Paper  Testing  Methods,"  New  York  (1920),  21. 

2  C.  CROSS,  E.  BEVAN,  and  J.  BRIGGS,  J.  Soc.  Chem.  Ind.,  27  (1908),  260. 


486 


GELATIN  AND  GLUE 


tion  of  sodium  phosphate,  previously  heated  to  45°C.,  for  exactly 
5  minutes.  This  treatment  eliminates  any  interfering  action 
that  might  result  from  iron  compounds.  The  fragments  are 
then  removed  from  the  bath  and  tested  with  potassium  iodide 
and  starch.  A  strong  coloration  will  be  shown  when  less  than 
0.5  per  cent  of  gelatin  is  present.  Large  quantities  of  mechanical 
wood  pulp  in  a  paper  will  produce  a  faint  chloramine  reaction 
owing  to  traces  of  protein  matter  in  the  raw  wood.  For  a 
quantitative  determination  the  sheets  of  chlorinated  paper  are 
suspended  in  front  of  a  fan  and  exposed  to  a  full  blast  of  air  for  at 
least  \y%  hours.  The  paper  is  then  torn  up,  placed  in  N/100 
arsenite  for  at  least  one  hour,  and  the  excess  of  arsenite  deter- 
mined by  titration  with  N/10  iodine.  The  proportion  of  chlorine 
taken  up  by  gelatin  under  these  conditions  was  found  to  be 
15.4  per  cent  of  its  air-dry  weight,  therefore  the  weight  of  chlorine 
found  divided  by  15.4  and  multiplied  by  100  will  give  the  per- 
centage of  gelatin  per  gram  of  sample  taken.  Comparisons  of 
this  method  with  the  Kjeldahl  determination  of  nitrogen  show 
satisfactory  agreement,  as  shown  in  the  following  table : 


TABLE    57. — GELATIN    IN    PAPER 


Paper 

Gelatin  by  Kjeldahl 
method  N  X  6.53 

Gelatin  by  chlorine 
method 

Note  paper 

7  1 

717  25  7473 

Typewriting  paper  

2  62 

2  8,  3  0 

Ledger  paper.    .  .  . 

8  15 

7  7,  7  7 

Glue  is  occasionally  found  in  mineral  oils  from  carelessly  glued 
casks.  It  may  be  detected  as  follows:  100  grams  of  the  oil  are 
shaken  with  boiling  water  in  a  separatory  funnel  and  the  aqueous 
layer  run  off  into  a  measuring  cylinder.  An  aliquot  portion, 
50  c.c.,  is  filtered  through  a  paper  filter  and  evaporated  to 
dryness  on  a  water  bath.  If  there  is  a  residue  it  is  extracted 
with  three  portions  of  8  c.c.  each  of  hot  absolute  alcohol  which 
will  remove  any  soap  that  may  be  present.  If  the  residue  con- 
tains glue  the  characteristic  odor  will  be  perceptible,  and  a 
confirmatory  test  may  be  made  by  adding  tannic  acid  to  its 
aqueous  solutions.  A  heavy  precipitate  indicates  glue. 


CHAPTER  X 
THE  EVALUATION  OF  GLUE  AND  GELATIN 

I  often  say  that  if  you  can  measure  that  of 
which  you  speak,  and  can  express  it  by  a 
number,  you  know  something  of  your  subject; 
but  if  you  cannot  measure  it,  your  knowledge 
is  meagre  and  unsatisfactory. 

Lord   Kelvin    (1880) 

The  day  is  not  far  distant  when  glue  and 
gelatin  will  be  purchased  on  specification, 
and  such  a  trade  condition  will  necessitate 
the  adoption  of  a  uniform  system  of  tests 
throughout  the  United  States. 

Fernbach    (1906) 

PAGE 

1.  Present  Methods  of  Evaluation 488 

2.  Primary  and  Secondary  Tests 491 

3.  Diversity  of  the  Proposed  Tests 492 

4.  A  Scientific  Basis  for  Evaluation .   495 

5.  A  Rational  System  of  Evaluation ! %. 499 

6.  Differentiation  between  Edible  Gelatin  and  Glue 502 

7.  The  Designation  of  Grade 502 

8.  Advantages  of  the  Proposed  System 505 

There  are  very  few  substances  which  are  extensively  employed 
in  the  trades  and  the  arts  that  offer  so  much  embarrassment  to 
either  the  layman  or  the  chemist  in  defining,  or  specifying,  or 
testing  the  quality  or  the  purity  as  do  gelatin  and  glue.  Many 
factors  contribute  to  this  situation.  Commercial  gelatin  and 
glue  are  by  no  means  definite  substances,  and  simple  tests  for 
purity  are  not  available.  Tests  which  are  believed  by  one  set  of 
individuals  or  manufacturers  to  be  comprehensive  and  indicative 
of  certain  fundamental  properties  are  regarded  as  inadequate  or 
untrustworthy  by  another  set,  and  different  tests  are  accordingly 
used  by  the  latter  which  they  believe  to  be  more  satisfactory. 
And  the  uses  for  which  the  various  gelatins  and  glues  are  em- 
ployed are  so  divergent  and  dissimilar  that  tests  or  analyses 
which  may  be  entirely  adequate  for  some  given  service  would  be 
quite  useless  as  an  indication  of  the  value  of  the  material  for 
some  other  service. 

The  United  States  Bureau  of  Chemistry1  in  1910  accepted 

1  U.  S.  Bureau  of  Chem.,  Bull  109  revised  (1910),  54. 

487 


488  GELATIN  AND  GLUE 

FernbachV  statement  that  chemical  analysis  gives  little  infor- 
mation in  regard  to  the  value  of  glue  except  in  a  few  isolated  and 
unimportant  instances.  The  tests  described  by  the  Bureau  of 
Chemistry  Bulletin  are  moisture,  ash,  reaction,  gelatin  (nitro- 
gen X  5.56),  water  absorption,  viscosity  (Engler);  jelly  strength 
(Lipowitz  method),  and  melting  point  (by  Cambon's  fusiometer). 
Several  of  these  tests  are  certainly  obsolete.  Nitrogen  times  a 
factor  means  nothing  and  is  misleading,  unless  the  sample  be  pure 
unhydrolyzed  gelatin,  which  is  almost  never  the  case.  Water 
adsorption  varies  with  too  many  incidental  factors,  and  has  little 
significance.  The  Engler  Viscosity  test,  the  Lipowitz  jelly 
strength  test,  and  the  Cambon  melting  point  test  have  largely 
been  replaced  in  most  laboratories  by  more  recent  methods  or 
apparatus. 

1.  PRESENT  METHODS  OF  EVALUATION 

The  procedures  in  current  use  for  the  evaluation  of  gelatin  and 
glue  are  based  primarily  upon  the  jelly  consistency  at  low  tem- 
peratures or  the  viscosity  at  high  temperatures,  and  secondarily 
upon  other  incidental  characteristics  which  depend  upon  the 
service  for  which  they  are  designed.  The  Peter  Cooper  system  of 
grading  may  be  taken  as  typical  of  the  American  practice. 
Because  this  system  was  the  earliest  recognized  attempt  at  glue 
grading  in  this  country,  and  has  been  in  continuous  service  since 
its  inception  in  1844,  it  is  recognized  as  an  American  Standard 
to  which  other  glues  produced  by  other  houses  may  be  referred  in 
terms  that  will  be  at  least  partially  intelligible  to  the  professional 
glue  buyer.  The  various  manufacturing  houses  use  symbols, 
however,  expressive  of  their  several  grades,  which  are  more  or 
less  zealously  guarded  as  a  kind  or  relic  of  alchemic  mystery,  but 
the  consumer  is  enlightened  upon  their  meaning  only  to  the  extent 
of  learning  that  the  glue  in  question  is  the  equivalent  in  jelly 
strength,  for  example,  to  the  Peter  Cooper  grade  1%,  or  the 
equivalent  in  viscosity  to  the  Peter  Cooper  grade  \Y±.  Thus  the 
Peter  Cooper  system  may  be  looked  upon  as  a  standard  for 
reference. 

Alexander2  has  proposed  the  substitution  of  figures  varying  by 
ten  points  each  for  the  symbols  of  Peter  Cooper,  and  defined  the 

1  R.  L.  FERNBACH,  "Glues  and  Gelatins,"  New  York  (1907),  4. 

2  J.  ALEXANDER,  J.  Soc.  Chem.  Ind.,  25  (1906),  158. 


EVALUATION  OF  GLUE  AND  GELATIN 


489 


jelly  strength  in  ounces  and  in  grams  at  10°C.  (measured  by  his 
jelly  strength  tester),  to  which  the  numbers  correspond.  The 
viscosity  test  (as  measured  by  his  instrument)  is  also  used  by 
him. 

The  following  table  expresses  his  system  of  grading. 


TABLE  58. — ALEXANDER'S  GLUE  STANDARDS 


Standard 

Peter 
Cooper 
grade 

Viscosity 
80°C. 

Allowable 
variation 
in 
viscosity 

Jelly 
strength 
in  ounces 
10°C. 

Jelly 
strength 
in    gm. 
10°C. 

10  . 

15^ 

±M 

20 

16 

±M 

30 

2 

16^ 

±K 

40 

1% 

17 

±M 

60 

1701 

50 

m 

18 

±x 

82 

2324 

60 

IK 

19 

±y2 

104 

2948 

70 

iK 

20 

±y2 

126 

3572 

80 

IH 

21 

±l/2 

•148 

4196 

90 

1H 

22 

±K 

170 

4820 

100 

IX 

23 

±% 

192 

5443 

110 

i 

24 

±H 

214 

6067 

120 

1  extra 

25 

±1 

236 

6691 

130 

A  extra 

26 

±3 

258 

7314 

140 

28 

±5 

150 

34 

±8 

160 

40 

±12 

Kahrs1  in  1898  proposed  a  rather  radical  departure  from  the 
then  existing  (and  still  existing)  indisposition  on  the  part  of  glue 
manufacturers  to  grade  their  product  by  tests  which  were  intel- 
ligible and  valuable  to  the  buyer.  He  urged  the  adoption  of  four 
tests,  i.e.,  adhesion,  economic  value,  cohesion,  and  congealing 
point.  By  adhesion  he  meant  the  viscosity,  which  was  expressed 
in  terms  of  the  weight  of  dry  glue  necessary  to  make  up  100  unit 
weights  of  the  liquid  material  of  the  proper  viscosity  for  applica- 
tion in  joint  work.  This  figure  Kahrs  found  to  vary  from  29  to 
60.  That  is,  in  the  highest  grade  of  glue  a  29  per  cent  solution, 
and  in  the  lowest  grade  a  60  per  cent  solution,  was  required 
to  give  a  liquid  of  the  correct  consistency,  at  about  60°C.,  for 
service  in  joining  work. 

1  F.  KAHRS,  International  Fisheries'  Congress,  Bergen,  Norway,  1898. 


490 


GELATIN  AND  GLUE 


The  economic  value  was  obtained  by  multiplying  the  adhesion 
value  by  the  price  per  pound  of  the  glue  in  question,  and  repre- 
sented therefore  the  price  of  100  pounds  of  the  liquid  glue  ready 
for  application.  Thus  at  18  cents  for  the  highest  and  5^  cents 
for  the  lowest  grades  (Kahrs'  figures)  the  cost  per  100  pounds 
of  the  liquid  would  be  18  X  29  =  $5.22  for  the  highest  grade  and 
5J£  X  60  =  $3.30  for  the  lowest  grade. 

The  cohesion  or  strength  was  measured  by  the  crushing  strength 
of  the  glue  jelly  made  up  at  the  concentration  found  under 
adhesion,  and  at  a  temperature  of  65°F.  (about  18°C.).  The 
high  temperature  was  used  as  it  more  nearly  approached  the 
temperature  at  which  the  glued  joints  would  be  used.  This 
test  was  later  substituted  in  part  by  an  actual  joint  strength  test. 

The  congealing  point  was  measured  as  the  temperature  at 
which  the  glue,  made  up  in  the  concentration  indicated  by  the 
adhesion  test,  congealed  to  a  jelly.  This  was  found  to  vary  from 
91  to  75°F.,  and  the  interval  of  16°  was  divided  into  ten  " setting 
grades." 

The  test  of  the  glue  was  then  expressed  somewhat  as  follows: 


TABLE  59. — KAHRS'  GLUE  TESTS 


Number 

Price  per 
pound 

Adhesion  (wt. 
per    100   Ib. 
standard 
liquid) 

Economic 
value    (price 
per  100  Ib. 
liquid) 

Cohesion 
(strength  ) 

Congealing 
point 

2942 

18  cents 

29.3 

$5.27 

25 

91.7 

2924 

5^2  cents 

57.8 

3.18 

15 

77.5 

There  are  many  interesting  points  in  such  a  system.  The 
table  will  show  that  although  the  first  glue  costs  more  than 
three  times  the  second,  per  pound,  yet  per  unit  volume  of  liquid 
ready  to  use  it  costs  only  six  tenths  more.  But  it  is  shown  to 
be  worth  more  in  producing,  at  that  dilution,  a  greater  strength, 
and  in  setting  at  a  higher  temperature,  i.e.,  more  rapidly  at  any 
given  temperature.  Such  a  system  enables  one  to  see  at  a 
glance  exactly  wherein  the  differences  in  the  glues  lie,  and  to  form 
an  intelligent  basis  for  judging  between  them.  It  at  least  gets 
down  to  salient  and  understandable  data,  which  is  much  more 
than  can  be  said  for  some  systems  now  in  use. 


EVALUATION  OF  GLUE  AND  GELATIN  491 

Thiele1  bases  the  commercial  value  of  edible  gelatin  on  its 
viscosity  (Engler),  melting  point,  and  color  value. 

2.  PRIMARY  AND  SECONDARY  TESTS 

There  is  a  difference  in  opinion  as  to  what  test  should  be 
regarded  as  the  most  fundamental.  In  Germany  the  viscosity 
test  proposed  by  Fels,2  made  by  the  use  of  the  Engler  viscosi- 
meter  at  35°C.,  seems  to  be  in  greatest  favor.  In  Italy  a  com- 
bination of  viscosity  and  melting  point  is  used.3  In  France  and 
England  the  viscosity  test  and  the  melting  point  test  by  Cam- 
bon's  fusiometer  are  employed.  In  this  country  the  jelly  con- 
sistency or  strength  is  probably  more  used  than  any  other  test, 
although  the  viscosity  test  is  in  favor  in  many  houses,  and  the 
melting  point  test  by  various  methods  is  used.  The  quality  of 
the  material  and  the  price  are  primarily  rated  upon  these  tests — 
sometimes  a  single  one,  and  sometimes  a  combination  of  two  or 
more.  Clayton4  concludes  that  the  "  observations  seem  to  show 
that  whilst  it  would  be  rash  to  form  a  judgment  on  glue  from  a 
single  test,  the  evidences  afforded  by  a  number  may  be  irre- 
sistible. The  experts'  surest  system  appears  to  be,  not  to  rely  on 
single  short  cut  tests  of  general  quality,  but  to  employ  a  number 
of  methods,  including  any  having  special  bearing  on  the  prospec- 
tive or  present  uses  of  the  glue,  and  then  to  base  his  conclusions 
on  a  consideration  of  all  the  results  together. "  And  Alexander5 
who  cites  the  above  adds,  "the  truth  of  the  matter  is  that  the 
figures  have  a  partial  value,  and  then  only  to  a  glue  expert." 
That  is  precisely  the  situation  that  the  chemist  should  set  himself 
to  eradicate.  Figures  that  mean  little  or  nothing  should  be  sub- 
stituted if  possible,  by  data  that  do  mean  something,  and  that 
persons  other  than  glue  experts  may  comprehend. 

The  secondary  basis  for  glue  or  gelatin  evaluation  lies  in  many 
or  few  other  tests  which  are  employed  to  determine  the  applica- 
bility of  the  material  for  any  special  service.  For  example,  where 
the  glue  is  to  be  used  in  mechanical  spreaders,  the  tendency  to 
foam  is  undesirable,  and  the  foam  test  indicates  this  tendency. 

1  L.  A.  THIELE,  Personal  Communication. 

2  J.  FELS,  Chem.  Ztg.,  21  (1897),  56;  25  (1901),  23. 

3  V.  L.  CERRI,  Report  to  Italian  Military  Aviation  Board  (1920). 
*  E.  G.  CLAYTON,  J.  Soc.  Chem.  Ind.,  21  (1902),  670. 

5  J.  ALEXANDER,  loc.  cit. 


492  GELATIN  AND  GLUE 

If  the  glue  is  to  be  applied  by  hand,  the  foam  test  is  of  little  or  no 
significance.  If  the  glue  is  for  use  on  paper  as  a  size  or  wall 
paper  as  a  binder  for  the  clay  filler,  grease  should  not  be  present 
in  large  amount,  as  otherwise  little  droplets  of  this  substance 
form,  making  elliptical  "eyes"  or  spots  on  the  paper.  In  admix- 
ture with  certain  dyes  the  presence  of  acid  or  of  alkali  is  not 
permissible  as  the  dye  would  be  affected  in  one  way  or  another. 
Suitable  tests  must  accordingly  be  made  upon  glues  designed  for 
such  purposes.  Gelatin  for  use  in  photographic  films  or  in 
printing  rollers  must  have  high  jelly  strength;  if  used  for  food  or 
medicinal  purposes  it  must  be  free  from  harmful  impurities; 
if  used  in  making  marshmallows  or  other  emulsions,  a  high 
viscosity  and  foam  are  desirable.  Such  a  list  could  be  greatly 
extended,  and  the  adaptability  of  any  glue  or  gelatin  for  its 
several  uses  is  largely  determined  by  such  secondary  tests. 

The  influence  which  such  properties  should  exert  upon  the 
selling  price  of  the  product  should  be  proportional,  however, 
only  to  the  extra  cost  involved  in  manufacturing  any  specialized 
type  of  material,  and  on  the  laws  of  supply  and  demand.  If  an 
extra  clear  glue  is  required  the  consumer  should  be  properly 
expected  to  pay  for  the  extra  cost  of  clarification  and  filtration, 
and  such  extra  cost  should  properly  be  figured  on  a  sliding  scale, 
dependent  upon  the  final  color  and  clarity,  to  be  added  to  the 
cost  of  the  untreated  glue  of  the  corresponding  grade.  For  the 
textile  trades  where  precipitation  with  alum  must  not  occur;  for 
veneer  glues  where  foam  is  very  objectionable,  and  for  all  other 
trades  requiring  glues  of  specfic  properties,  the  price  should 
similarly  be  based  upon  a  sliding  scale  to  be  applied  to  the  market 
price  of  the  regular  corresponding  grade.  Where  no  extra  cost  is 
involved  in  the  production  of  a  specific  glue,  then  the  sliding 
scale  may  apply  according  to  the  usual  dictum  of  supply  and 
demand,  but  it  would  seem  most  expeditious  to  base  all  such  varia- 
tions upon  a  standard  primary  evaluation. 

3.  THE  DIVERSITY  OF  THE  PROPOSED  TESTS 

The  numerous  methods  that  have  been  proposed  for  the  testing 
of  glue  and  gelatin  for  the  purpose  of  evaluation  have  been 
described  in  detail  in  Chaps.  VIII  and  IX,  and  need  not  be 
repeated  at  this  place.  A  brief  consideration  of  the  merits  of  the 
procedures  is,  however,  necessary  to  the  intelligent  understanding 
of  the  situation. 


EVALUATION  OF  GLUE  AND  GELATIN  493 

Recent  researches  upon  gelatin  have  brought  to  light  many 
relationships  that  should  be  incorporated  into  the  scheme  of 
primary  evaluation.  From  the  point  of  view  of  the  chemist 
gelatin  is  a  chemical  compound,  a  pure  protein,  and  glue  is  a  mix- 
ture of  gelatin  with  the  products  of  gelatin  hydrolysis,  sometimes 
referred  to  as  /3  gelatin,  and  other  impurities  in  varying  amounts. 
Commercial  gelatin  and  glue  should,  therefore,  from  the  stand- 
point of  chemical  constitution  be  primarily  evaluated  in  terms  of 
the  proportion  of  pure  unhydrolyzed  gelatin  which  is  contained 
in  the  material.  From  the  point  of  view  of  the  major  glue  trade, 
i.e.,  the  use  of  glue  as  an  adhesive,  glue  should  be  evaluated  in 
terms  of  the  strength  of  the  joint,  produced  under  the  most 
favorable  conditions,  which  may  be  made  with  the  material. 
Fortunately  these  two  points  of  view, — that  of  the  chemist  and 
that  of  the  joiner. — have  been  shown  to  be  identical.  The  glue 
with  the  largest  amount  of  unhydrolyzed  gelatin  produces  the 
strongest  joint.1  In  the  primary  evaluation  of  the  material, 
therefore,  one  or  the  other  of  these  two  properties  should  be 
measured,  or  else  some  variable  which  has  been  found  by  repeated 
and  exhaustive  tests  to  be  directly  dependent  upon  these  proper- 
ties, and  to  express  them  correctly. 

The  proposed  methods  may  be  divided  into  physical  tests  and 
chemical  tests.  The  latter  aim  in  nearly  all  cases  to  precipitate 
out  the  gelatin  and  rate  the  material  in  accordance  with  the 
amount  of  nitrogenous  matter  so  precipitated.  An  error  by  this 
procedure  was  shown2  to  lie  in  the  fact  that  precipitation  with 
alcohol,  tannin,  and  saturated  solutions  of  zinc-,  magnesium-, 
and  ammonium  sulphate  threw  down  not  only  the  unhydrolyzed 
gelatin  but  also  the  proteoses.  The  actual  value  of  a  gelatin  or 
glue  has  been  found  to  be  defined  by  the  content  of  unhydrolyzed 
gelatin,  and  the  adhesive  strength  of  a  giue  is  proportional  to  the 
gelatin:  proteose  ratio.3  It  is,  therefore  the  gelatin  alone,  rather 
than  the  sum  of  the  gelatin  and  the  proteose  that  is  the  criterion 
for  gelatin  and  glue  grade. 

It  has  been  shown  that  by  the  precipitation  of  the  protein  with 
half  saturated  salt  solutions  (as  above)  the  undegraded  gelatin 
only  is  thrown  down,  and  an  accurate  evaluation,  based  upon 
actual  gelatin^content,  may  be  obtained.  This  operation  appears 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  197. 

2  Vide  Chap.  IX. 

3  Vide  page  28. 


494  GELATIN  AND  GLUE 

to  be  a  fundamental  one  (so  far  as  a  true  division  is  possible 
of  being  drawn  between  protein  and  proteose)  and  when  per- 
formed under  conditions  of  exact  control1  (especially  temperature 
and  acidity)  is  capable  of  ready  duplication.  It  seems  therefore, 
in  the  absence  of  any  more  promising  and  fundamental  pro- 
cedure, that  this  should  be  regarded  as  the  primary  basis  for 
evaluation. 

The  technical  requirements  in  the  laboratory,  however,  make 
it  imperative  that  the  testing  should  be  susceptible  of  rapid  and 
not  too  exacting  manipulation.  For  this  reason  it  becomes  highly 
desirable  that  a  physical  test,  or  series  of  such  tests,  that  may  be 
made  with  rapidity  and  assurance  should  be  found  that  parallel 
to  a  very  close  margin  the  relative  evaluation  that  would  result 
from  the  chemical  examination  described  above. 

Of  the  available  physical  tests  that  have  been  applied,  the 
jelly  consistency,  the  viscosity,  and  the  melting  point  are  the 
ones  that  have  received  most  attention,  and  the  first  two  are 
about  equally  divided  in  favor  among  glue  and  gelatin  testers. 
The  melting  point,  has  not  been  so  much  used  on  account  of  the 
greater  difficulty  of  manipulation. 

The  types  of  apparatus  that  have  been  devised  for  testing 
these  properties  are  numerous,  and  each  new  method  recognizes  a 
possible  source  of  error  in  older  procedures  and  endeavors  to 
correct  it.  The  elimination  of  the  error  due  to  "skin  formation" 
on  the  jelly  in  testing  the  jelly  consistency  has  inspired  a  number 
of  inventions.  The  shape  and  size  of  the  container  has  brought 
forth  others,  until  the  most  recent  device  of  Sheppard2  leaves 
little  to  be  desired  in  the  way  of  scientific  perfection. 

All  kinds  of  devices  have  been  utilized  for  measuring  viscosity, 
from  improvised  pipettes  and  torsion  viscosimeters  to  cans  fitted 
with  a  small  stopcock. 

An  indirect  method  making  use  of  the  property  possessed  by 
gelatin  of  mutarotation  between  the  temperatures  of  15  and 
35°C.,  has  been  employed  by  C.  R.  Smith3  with  highly  interesting 
results. 

The  actual  adhesive  strength  has  at  times  been  utilized  but  it  is 
so  difficult  to  obtain  accurately  duplicatable  results  on  account  of 
the  many  uncertainties  in  the  conditions  of  the  test,  and  also 


Vide  pages  26-27. 
Vide  pages  378-9. 
Vide  pages  413-15. 


EVALUATION  OF  GLUE  AND  GELATIN  495 

because  of  the  expense  of  a  machine  for  making  such  tests,  that 
it  has  not  gained  favor  except  in  a  very  crude  way  among  joiners, 
and  among  scientific  investigators.  The  tensile  strength  of  a 
machined  piece  of  glue  of  specified  dimensions  has  been  suggested, 
but  has  not  yet  been  proven  practicable. 

4.  A  SCIENTIFIC  BASIS  FOR  EVALUATION 

From  among  these  physical  tests,  then,  we  are  confronted 
with  the  task  of  making  a  selection,  but  we  must  keep  in  mind 
the  imperative  necessity,  if  our  selection  is  to  be  altogether 
justifiable  and  sound,  of  utilizing  only  such  a  property  as  will 
conform  to  the  more  fundamental  requirement  of  chemical 
constitution  mentioned  above. 

Extensive  tests  have  been  made  in  the  author's1  laboratory 
upon  the  relations  which  the  jelly  strength,  the  viscosity,  and  the 
melting  point  bear  to  both  gelatin  content  and  to  joint  strength, 
and  the  data  obtained  show  clearly  that  if  the  viscosity  be  held 
constant  the  gelatin  content  and  joint  strength  will  vary  as 
the  jelly  consistency,  while  if  the  latter  be  held  constant  these 
properties  will  vary  as  the  viscosity.  But  the  jelly  consistency 
and  the  viscosity  are  also  shown  to  bear  the  same  relation  to  the 
melting  point,  while  the  latter  appears  to  define  with  precision  the 
gelatin  content  or  joint  strength.  Any  method  therefore,  which 
accurately  estimates  the  melting  point,  or  which  differentiates 
the  glues  in  the  same  order  as  would, result  from  the  melting 
point  test,  should  be  a  satisfactory  basis  for  evaluation.  In 
most  glues  and  gelatins  the  viscosity  and  the  jelly  consistency 
are  perfectly  parallel  functions,  and  a  given  jelly  consistency 
will  imply  a  definite  viscosity,  or  vice  versa,  but  there  are  many 
exceptions  to  this  generality, — so  many  in  fact  that  were  we  to 
use  either  the  jelly  consistency  or  the  viscosity  alone  as  a  basis 
of  evaluation,  a  large  number  of  glues  would  be  incorrectly 
graded.  For  example,  a  given  glue  of  say  1^  grade  (Peter 
Cooper)  and  22  viscosity  (Alexander)  may  show  a  joining 
strength  of  say  2,000  pounds  per  square  inch.  The  same  grade 
with  a  viscosity  of  23  may  show  perhaps  2,200  pounds.  Or  a 
grade  of  1%  and  a  viscosity  of  22  may  show  1,800  pounds.  Obvi- 
ously, if  evaluation  were  correctly  based  upon  jelly  consistency, 
the  first  two  examples  should  show  the  same  strength,  and  if 

1  R.  H.  BOGUE,  lor.,  cit. 


496  GELATIN  AND  GLUE 

based  upon  viscosity,  (at  60~800C.),  the  first  and  third  examples 
should  be  identical. 

The  melting  point  appears  to  be  controlled  by  both  the  jelly 
consistency  and  the  viscosity,  and  would  therefore,  in  the  cases 
cited,  be  highest  in  the  second,  intermediate  in  the  first,  and 
lowest  in  the  third,  which  is  also  the  order  of  the  strength.  The 
melting  point  test  seems  in  fact  to  be  the  most  readily  available 
and  practicable  test  as  the  basis  for  glue  and  gelatin  evaluation. 

There  are  many  methods  by  which  the  melting  point  may  be 
measured,  but  most  of  these  are  inexact  and  on  account  of  the 
time  required  for  the  gel  and  sol  forms  to  come  to  a  true  equili- 
brium, the  procedures  intending  to  measure  the  precise  tempera- 
ture of  melting  of  the  jelly,  or  setting  of  the  sol,  are  apt  to  be 
inaccurate.  In  an  attempt  to  find  a  more  rapid  and  satisfactory 
method  for  measuring  this  property,  the  author  observed  that  by 
plotting  the  curve  of  viscosity  at  regularly  decreasing  tem- 
peratures, and  extrapolating  to  the  temperature  where  the 
viscous  flow  would  be  nil,  the  figures  derived  corresponded 
remarkably  well  with  those  derived  by  several  other  methods 
of  melting  point  determination,  both  direct  and  indirect.  But  it 
was  further  observed  that  the  same  order  of  differentiation  of  the 
glues  was  obtained  by  merely  taking  the  viscosity  readings  at  a 
low  temperature  (32  to  35°C.).  This  order  was,  in  most  cases, 
the  same  as  the  order  of  viscosity  at  60°C.,  and  the  order  of  jelly 
consistency  at  15°C.,  but  in  all  of  those  glues  in  which  the 
viscosity  was  abnormal  to  the  jelly  consistency,  or  vice  versa,  the 
viscosity  at  32  to  35°  was  found  to  give  a  value  intermediate 
between  those  two  properties;  to  correspond  with  the  true  melting 
point;  and  to  be  precisely  indicative  of  the  gelatin  content  and 
the  joint  strength  of  the  product,  which  was  not  true  of  any 
other  test.1 

The  most  satisfactory  means  for  making  this  low  temperature 
viscosity  test  was  found  to  be  by  the  use  of  the  MacMichael 
viscosimeter.  The  advantages  attendant  upon  the  use  of  this 
instrument  were  found  to  be  as  follows:  (1)  The  tests  could  be 
made  very  rapidly.  In  fact,  if  the  glues  are  ready  in  a  water 

1  The  temperature  of  32  to  35°  was  shown  by  C.  R.  Smith  and  by  R.  H. 
Bogue  to  be  especially  significant.  Smith  found  this  to  be  the  temperature 
above  which  the  gel  form  could  not  exist,  and  Bogue  found  this  to  be  the 
temperature  above  which  evidence  of  plastic  flow  could  not  be  observed. 
See  pages  116  and  211. 


EVALUATION  OF  GLUE  AND  GELATIN  497 

bath  at  the  proper  temperature,1  the  viscosity  tests  may  be 
made  at  the  rate  of  about  one  in  a  minute.  (2)  The  instrument 
is  a  standard  make,  obtainable  anywhere,  so  that  its  adoption 
would  eliminate  the  multitudinous  array  of  pipette  and  other 
forms  of  viscosimeter  now  in  use.  This  would  make  for  stand- 
ardization which  is  so  sorely  needed  in  glue  testing  practice.  (3) 
The  instrument  is  especially  well  adapted  to  a  rapid  conversion 
of  the  readings  into  the  absolute  degree  of  viscosity,  the  centi- 
poise.  The  calibration  of  the  instrument  takes  but  a  short 
time,  and  a  conversion  curve,  which  is  a  perfectly  straight  line, 
may  be  plotted,  so  that  the  MacMichael  degrees  may  be  read  off 
in  centipoises  by  a  mere  glance  of  the  curve.2  The  centipoise 
unit  as  a  standard  for  expressing  all  viscosity  measurements 
cannot  be  too  strongly  urged.  The  value  would  be  entirely 
independent  of  the  size  of  torsion  wire  used,  or  the  speed  of 
rotation  of  the  cup,  and  expresses  almost  exactly  the  specific 
viscosity  of  the  material,  water  taken  as  unity,  at  20°C.  (4) 
Under  the  conditions  at  which  the  instrument  would  be  used  in 
glue  testing,  the  errors,  which  are  of  considerable  magnitude 
with  many  types  of  viscosimeter,  e.g.,  the  development  of  turbu- 
lent flow,  the  increasing  loss  in  head  during  the  measurement, 
inconstant  and  faulty  drainage,  the  change  of  temperature  during 
measurement,  the  abnormal  values  obtained  when  insoluble 
material,  as  zinc  oxide,  is  present,  etc.,  are  almost  entirely 
eliminated.  The  straight  line  nature  of  the  conversion  curve 
also  makes  for  greater  accuracy  in  computing  the  absolute 
viscosity  from  the  instrument  reading,  and  by  a  proper  adjust- 
ment of  the  wire  and  the  speed  of  rotation  the  absolute  viscosity 
in  centipoises  may  be  read  directly. 

Different  grades  of  glues  and  gelatins  normally  vary  in  water 
content  from  about  10  to  17  per  cent,  the  higher  grades  retaining 
the  larger  amount  of  water.  Of  even  greater  importance  in 
evaluation  is  the  ability  of  any  given  sample  of  any  grade  to 
take  up  or  lose  water  according  to  the  humidity  and  temperature 

xThe  glues,  after  soaking  in  the  proper  amount  of  water  (see  below) 
should  be  warmed  to  60°C.,  and  then  allowed  to  cool  to  35  degrees  before 
taking  the  viscosity.  If  they  are  not  thus  preliminarily  heated  to  60°  the 
readings  will  be  erratic,  and  unreliable,  no  matter  how  tested. 

2  See  Appendix,  page  608,  for  the  conversion  of  MacMichael  degrees  to 
centipoises. 

32 


498 


GELATIN  AND  GLUE 


of  storage.  Under  ordinary  conditions,  glues  have  been  found 
to  vary  in  water  content  from  9  to  18  per  cent  from  this  cause 
alone.  It  is  obvious,  therefore,  that  a  20  gram  sample  may 
contain  between  16.4  and  18.2  grams  of  dry  glue,  and  since 
this  is  made,  for  the  viscosity  test,  to  100  grams  with  water, 


HS      He     H7     H6     H5 
Grade  in  Order  of  Decreasing  Jelly  Strength 

FIG.  99. — The  relation  of  viscosity  to  jelly  strength.     Hide  glues. 


the  percentage  of  dry  glue  used  in  the  tests  would  vary  between 
16.4  and  18.2  per  cent.  This  is  sufficient  to  modify  very  seriously 
the  viscosity  or  any  other  test  made  which  varies  with  the 
concentration.  In  order  to  eliminate  this  uncertain  variable, 
it  is  necessary  to  make  a  moisture  determination  before  weigh- 
ing up  the  samples  for  further  tests,  and  it  is  suggested  that 
the  amount  of  glue  to  be  used  for  the  viscosity  test  be  stipulated 
as  the  equivalent  of  18  grams  of  dry  glue  made  up  to  100  grams 
with  water. 


EVALUATION  OF  GLUE  AND  GELATIN 


499 


A  comparison  of  the  viscosities  as  determined  by  the  Mac- 
Michael  instrument  and  the  capillary  tube  (of  the  type  described 
80 


B0   Bg     Bg      B7      BG     B5      B4      £3 
Grade  in  Order  of  Decreasing  Jelly  Strength 
FIG.  100. — The  relation  of  viscosity  to  jelly  strength.     Bone  glues. 

by  Fernbach)  for  nine  grades  each  of  hide  and  of  bone  glues  is 
shown  graphically  in  Figs.  99  and  100. 


5.  A  RATIONAL  SYSTEM  OF  EVALUATION 

Our  discussion  has  shown,  therefore,  that  both  the  gelatin 
content  of  a  glue  or  gelatin,  and  also  the  joint  strength  of  a  glue, 
may  be  correctly  indicated  by  a  melting  point  determination, 
while  neither  may  be  correctly  assumed  to  be,  in  all  cases,  pro- 
portional to  either  the  jelly  consistency  or  the  viscosity  at  60°C. 
alone.  Inasmuch  as  the  primary  evaluation  of  the  material 
should  be  based  upon  some  fundamental  and  scientifically 
selected  property  or  properties,  it  seems  that  gelatin  content  and 
joint  strength  should  be  chosen.  It  is  especially  happy  that 
these  two  properties  are  parallel.  Since  the  melting  point  has 
been  shown  to  indicate  the  gelatin  content  and  the  joint  strength, 
it  seems  that  this  determination,  either  directly  or  indirectly 
made,  should  be  selected  as  a  measure  of  the  fundamental 
constitution  and  properties  of  the  material.  The  measurement 


500  GELATIN  AND  GLUE 

of  the  viscosity  of  an  18  per  cent  solution,  dry  basis,  at  35°C.,  by 
means  of  the  MacMichael  viscosimeter  has  been  shown  to  be 
especially  well  adapted  as  an  indirect  estimation  of  the  differ- 
entiation of  glues  and  gelatins  in  the  order  of  their  melting 
points,  and  is  accordingly  recommended  as  the  basis  for  the 
primary  evaluation  of  these  products. 

If  this  test  be  accepted  for  the  above  stated  purpose,  it  follows 
that  the  tests  for  jelly  consistency  and  for  viscosity  at  60°C.  are 
no  longer  of  service  for  primary,  evaluation,  and  may  be  safely 
discarded  as  such.  They  may  however,  be  of  great  value  in 
secondary  evaluation,  i.e.,  in  determining  the  adaptability  of  a 
given  glue  to  a  given  service.  For  example,  the  jelly  consistency 
would  be  of  value  in  selecting  glues  for  printers  rollers,  and  the 
rapidity  of  setting  of  the  jelly  as  well  as  the  viscosity  at  working 
temperatures  would  be  desirable  data  for  the  wood-working 
industry. 

Another  test  which  recent  investigation  has  shown  to  be  of 
considerable  importance  in  determining  the  properties  of  a  glue 
or  gelatin  is  the  hydrogen  ion  concentration.1  If  the  pH  value  is 
4.7  the  viscosity,  swelling,  etc.,  are  low,  and  the  product  nearly 
insoluble.  On  either  side  of  this  point  these  properties  increase 
very  considerably,  attaining  their  maximum,  on  the  acid  side  at 
pH  3.5  and  on  the  alkaline  side  at  pH  9.0.  At  greater  acidity 
than  pH  3.5  or  at  greater  alkalinity  than  pH  9.0  these  properties 
again  decrease.  The  pH  value  indicates  therefore,  not  only  the 
reaction  of  the  material,  and  the  degree  of  acidity  or  alkalinity, 
but  also  the  proximity  of  the  substance  to  the  points  of  maximum 
or  minimum  properties.  The  measurement  may  be  made  by 
either  electrometric  or  colorimetric  means.2  One  per  cent  solu- 
tions are  best  used  in  either  case,  and  the  results  expressed  in 
terms  of  pH  to  the  nearest  tenth. 

The  methods  that  may  be  employed  for  the  estimation  of  the 
secondary  properties  should  likewise  receive  attention  that  the 
results  may  be  expressed  in  uniform  terms.  The  jelly  consistency 
test  is  perhaps  most  conveniently  made  by  the  use  of  the  instru- 
ment described  by  the  Forest  Products  Laboratory3  and  expressed 

1  See  J.  LOEB,  /.  Gen.  Physiol,  1  (1918-19),  39,  237,  363,  483,  559;  3 
(1920-21),  85,  247,  391.     Cf.  Chap.  V. 

2  See  W.  M.  CLARK,  "The  Determination  of  Hydrogen  Ions,"  Baltimore. 
1920. 

3  Forest  Products  Laboratory,  see  pages  374-5. 


EVALUATION  OF  GLUE  AND  GELATIN  501 

in  millimeters  of  depression,  although  for  some  more  exacting 
requirements,  as  in  the  selection  of  gelatin  for  photographic 
purposes,  the  more  elaborate  and  scientific  method  of  Sheppard1 
may  be  used  to  advantage.  The  viscosity  at  working  tempera- 
tures, 60°C.,  may  be  made  with  the  MacMichael  viscosimeter 
upon  a  20  per  cent  solution  and  the  results  expressed  either  in 
centipoises,  or,  following  the  suggestion  of  Kahrs,  in  pounds  of 
dry  glue  which,  when  made  up  to  a  weight  of  100  pounds  with 
water,  will  produce  a  solution  of  a  given  standard  viscosity  at 
60°C.,  as,  for  example,  600  centipoises.  The  foam  test  may  be 
made  upon  200  c.c.  of  a  20  per  cent,  solution  in  a  standard  glass, 
by  means  of  an  egg  beater  turned  at  a  stated  velocity  of  about 
four  revolutions  per  second  for  30  seconds,  and  measured  after  10 
seconds  as  millimeters  of  foam.  The  grease  test  may  usually  be 
made  with  sufficient  satisfaction  by  making  a  streak  of  the  glue, 
to  which  a  dye,  as  turkey  red,  has  been  added,  on  a  sheet  of 
paper.  Better  still,  the  glue  may  be  mixed  with  the  calsomine 
or  color  base  with  which  it  is  intended  to  be  used,  and  streaks 
made  on  paper.  The  appearance  may  be  specified  according  to 
the  information  desired.  The  form  of  the  product,  as  flake, 
sheet,  ribbon,  foil,  or  ground,  should  be  noted.  If  the  glues 
have  been  treated  to  produce  a  clear  light  product,  the  degree 
of  clarity  and  color  should  be  indicated.  This  may  usually  be 
done,  with  sufficient  satisfaction,  by  such  terms  as  light,  clear, 
medium,  amber,  etc.,  or  by  numerical  designations  as  No.  1, 
No.  2,  etc.  Exact  color  data  are  best  obtained  by  the  elaborate 
instrument  of  the  Eastman  Kodak  Co.  If  the  glue  is  colored, 
or  crazed,  or  presents  any  particular  property  it  should  be  men- 
tioned. The  odor  should  be  noted  in  the  warm  solution,  and  a 
strong  or  sour  odor  should  not  develop,  in  good  glues,  within  48 
hours  at  30  to  40°C. 

In  some  cases  special  tests  will  be  required  as  for  moisture,  ash, 
precipitation  with  aluminum  salts,  etc.,  and  in  the  case  of  gelatin 
for  edible  purposes  copper,  zinc,  arsenic,  and  sulphur  dioxide  may 
be  determined,  and  in  some  cases  qualitative  tests  for  preservatives 
are  necessary.  These  are  made  in  accordance  with  the  custom- 
ary scheme  for  the  examination  of  foods,  as  set  forth  in  the  official 
publications  of  the  Association  of  Official  Agricultural  Chemists,2 
and  need  not  be  repeated  here. 

1  S.  E.  SHEPPARD,  see  pages  378-9. 

2  Association  of  Official  Agricultural  Chemists  "Methods  of  Analysis," 
1920,  147.     Cf.  Chap.  IX. 


502  GELATIN  AND  GLUE 

For  the  purpose  of  fixing  an  abstract  valuation,  or  for  the 
estimation  of  tariff  duties  on  imports,  the  primary  evaluation 
only  need  be  made. 

6.    DIFFERENTIATION    BETWEEN    EDIBLE    GELATIN    AND    GLUE 

The  methods  in  common  use  for  distinguishing  between  edible 
gelatin  and  glue  are  based  upon  a  few  tests  that  are  admittedly 
inadequate.  The  material  is  examined  for  copper,  zinc,  and 
arsenic,  the  maximum  permissible  in  edible  gelatin  being  30, 
100  and  1.4  parts  per  million  respectively.  The  total  ash  is 
determined,  the  assumption  being  made  that  a  glue  is  usually 
much  higher  in  ash  than  a  pure  gelatin.  The  jelly  consistency 
is  noted,  it  being  assumed  that  a  glue  will  show  much  lower 
values  for  this  property  than  gelatin.  And  the  general  appear- 
ance, color,  and  odor  are  noted,  only  reasonably  clear  and  per- 
fectly sweet  gelatin  being  passed  favorably.  Sulphur  dioxide 
is  sometimes  determined,  but  its  presence  is  permitted  in  reason- 
able amounts.  A  bacteriological  examination  might  be  of  value 
as  an  additional  test,  but  many  instances  have  been  met  where  a 
gelatin  gave  off  an  offensive  odor,  but  was  found  to  be  practi- 
cally sterile.  In  such  cases  the  decomposition  had  obviously 
taken  place  at  some  stage  in  the  manufacture,  but  had  subse- 
quently been  stopped,  probably  by  the  addition  of  a  germicide. 
A  periodical  inspection  of  the  care  and  sanitation  exercised  in  the 
several  steps  of  manufacture  and  in  the  selection  of  the  stock 
used,  would  probably  be  more  valuable  as  a  basis  for  passing 
upon  gelatin  than  any  chemical  or  other  tests  upon  the  product 
that  could  be  made.  Provided,  however,  that  a  gelatin  passed 
the  requirements  as  an  edible  product,  then  its  evaluation  as  a 
gelatin  should  be  determined  primarily,  as  in  the  case  of  glues, 
upon  its  content  of  the  unhydrolyzed  protein,  or  which  has  been 
shown  to  be  the  same,  upon  the  melting  point,  or  viscosity  at 
35°C. 

7.  THE  DESIGNATION  OF  GRADE 

The  grade  designation  of  the  product,  as  ascertained  by  the 
primary  evaluation,  may  conveniently  be  expressed  by  consecu- 
tive numbers,  1,  being  the  lowest,  following  the  name  or  initial 
letter  of  the  type  or  product.  Thus  hide  glues  may  be  designated 


EVALUATION  OF  GLUE  AND  GELATIN 


503 


as  Hide  glue  No.  1,  Hide  glue  No.  2,  or  HI,  H2,  and  so  on  up  to 
perhaps  Hi5,  and  bone  glues  as  Bone  glue  No.  1,  or  BI  to  Bone 
glue  No.  15  or  Bi5.  If  the  primary  evaluation  is  measured,  as 
suggested,  by  a  determination  of  the  viscosity  in  centipoises  of  an 
18  per  cent  solution,  dry  basis,  at  35°C.,  then  HI  or  BI  would 
correspond  to  a  viscosity  of  less  than  20  cp.,  and  Hi5  or  Bi5  to 
above  150  cp.  The  arrangement  might  well  be  along  the  follow- 
ing lines : 

TABLE  60. — DESIGNATION  OF  GRADE 


Viscosity  of  an  18 

Viscosity  of  an  18 

Designation 

per  cent  solution, 
dry  basis,  at  35°C., 

Designation 

per  cent  solution, 
dry  basis,  at  35°C., 

in  centipoises 

in  centipoises 

Hi  or  Bi 

Below  20 

H9  or  B9 

90  to    99 

H2  or  B2 

20  to  29 

Hioor  Bio 

100  to  109 

H3  or  B3 

30  to  39 

Ha  or  BH 

110  to  119 

H4  or  B4 

40  to  49 

Hi2or  B12 

120  to  129 

H5  or  B5 

50  to  59 

His  or  Bis 

130  to  139 

H6  or  B6 

60  to  69 

HH  or  Bu 

140  to  149 

H7  or  B7 

70  to  79 

His  or  Bis 

150  to  159 

H8  or  B8 

80  to  89 

etc. 

Obviously  the  highest  grades  would  be  attained  only  by  the 
very  pure  gelatins,  while  only  the  material  that  was  exceedingly 
poor  would  reach  the  lowest  designation.  Both  classes  of  glues, 
i.e.,  the  hide  and  bone  types,  would  be  rated  upon  the  same 
standard,  but  the  initial  letter  or  name  would  serve  the  desirable 
purpose  of  differentiating  between  the  type  of  stock  used.  Edible 
gelatins  could  be  referred  to,  if  desired,  as  Gi2,  GM,  etc.,  but  the 
numerical  designations  should  always  refer  to  a  standard  visco- 
sity, or  other  value  fixed  as  the  primary  standard  for  evaluation. 

The  laboratory  test  sheet  would  appear  as  follows: 


504 


GELATIN  AND  GLUE 


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EVALUATION  OF  GLUE  AND  GELATIN  505 

8.  ADVANTAGES  OF  THE  PROPOSED  SYSTEM 

The  advantages  of  such  a  system,  or  of  any  other  that  secures 
a  real  standardization  based  upon  fundamental  and  scientific 
principles,  are  strikingly  apparent.  Where  the  jelly  consistency 
is  taken  as  the  basis  for  determining  grade,  a  set  of  " standard" 
glues  must  be  maintained,  and  in  the  course  of  a  few  years  these 
standard  types,  through  occasional  renewal,  must  inevitably 
alter.  Furthermore,  the  curves  for  the  jelly  consistency  of 
various  glues  at  varying  temperatures  are  not  parallel,  but  a  glue 
that  is  weaker  than  the  standard  at  10°C.  may  be  the  stronger  at 
15°.  There  is  surely  no  good  reason  why  the  tests  would  be 
made  at  one  temperature  in  preference  to  any  other,  but  the 
decision  upon  this  very  arbitrary  point  determines  the  rating 
which  a  glue  may  receive  by  the  current  methods. 

Mention  has  already  been  made  of  the  diversity  of  instruments 
used  in  making  the  viscosity  test,  and  the  impossibility,  without 
profound  instrumental  corrections,  of  expressing  the  readings 
obtained  in  terms  of  absolute  viscosity,  or  any  other  kind  of 
viscosity  that  will  be  intelligible  to  a  person  using  any  other 
instrument. 

And  most  important,  the  figures  obtained  by  the  jelly  strength 
and  the  viscosity  at  high  temperature  methods  do  not  give  data 
which  are  invariably  expressive  of  any  fundamental  property. 

These  objections  the  proposed  changes  seek  to  remedy.  The 
primary  evaluation  involves  the  use  of  no  arbitrarily  selected 
" standard"  glues,  but  gives  results  that  are  in  themselves 
complete  without  reference  to  any  other  hypothetical  product. 
The  use  of  a  standard  instrument  which  readily  permits  of  the 
employment  of  absolute  degrees  enables  the  readings  to  be 
universally  understood,  and  the  data  obtained  are  indicative 
of  fundamental  properties  of  the  material. 

The  grade  designation  is  simple  and  very  easily  understood. 
The  letter  referring  directly  to  the  type  of  stock,  and  the  number 
to  the  absolute  viscosity,  reduces  to  the  vanishing  point  any 
mystery  connected  with  glue  grades,  and  enables  the  layman  to 
define  a  glue  with  nearly  the  same  degree  of  intelligence  as  may 
be  exercised  by  the  expert. 


CHAPTER  XI 
THE  USES  AND  APPLICATIONS  OF  GLUE 

Materials  are  made  one  from  bullish  glue. 
Lucretius   (About  50  B.C.) 

PAGE 

1.  The  Handling  of  Glue 506 

Glue  Room  Economy  and  Technology 506 

Losses  in  Glue  Due  to  Overheating 510 

The  Bacterial  Decomposition  of  Glue 515 

2.  Glue  as  an  Adhesive 517 

The  Conditions  Affecting  the  Adhesive  Strength  of  Glue 517 

Methods  of  Making  the  Adhesive  Strength  Test 526 

The  Selection  of  Glues  for  Joint  and  Panel  Work 534 

The  Application  of  Glue  as  an  Adhesive. 536 

3.  Glue  as  a  Sizing  Agent •. 538 

The  Sizing  of  Paper 538 

The  Sizing  of  Textiles 542 

The  Sizing  of  Wooden  Containers 544 

Glue  as  a  Size  in  Paints  and  Calsomine 544 

4.  Glue  as  a  Binding  Agent 545 

5.  Glue  as  a  Colloidal  Gel 547 

6.  Glue  as  a  Protective  Colloid 550 

To  discuss  adequately  all  of  the  uses  to  which  glue  has  been 
put  in  recent  years  would  necessitate  a  complete  volume  in  itself, 
and  in  the  end  would  not  serve  a  highly  important  purpose. 
The  effort  is  made,  therefore,  in  the  present  chapter  rather  to 
consider  in  some  detail  the  principles  entering  into  the  use  of 
glue  as  an  adhesive,  and  to  make  clear  the  essential  character- 
istics of  the  employment  of  glue,  and  its  selection  for  service,  in 
several  of  the  other  major  uses  to  which  it  is  put.  No  attempt 
is  made  at  an  exhaustive  treatment,  but  some  of  the  most  impor- 
tant qualities  entering  into  the  selection  of  glue  for  its  several 
classes,  of  employment  are  presented. 

1.  THE  HANDLING  OF  GLUE 

Glue  Room  Economy  and  Technology. — There  is  in  com- 
mon practice  in  many  glue  rooms  so  much  waste  of  material,  so 
much  loss  of  energy,  so  much  ignorance  of  the  basic  principles 

506 


APPLICATIONS  OF  GLUE  507 

of  glue  handling,  that  the  executives  of  every  glue-consuming 
plant  would  do  well  to  take  steps  to  assure  themselves  positively 
whether  such  practice  prevails  in  their  own  domain.  While  the 
trained  mechanical  or  electrical  engineer  is  employed  to  develop 
modern  efficiency  and  to  scrap  unsound  practice  in  other  depart- 
ments of  the  plant,  the  glue  room  foreman  has  been  left,  for  the 
most  part,  severely  alone  to  rule  over  his  province  with  whatever 
of  business  or  scientific  efficacy  or  of  indifference  he  may  elect. 
Since  the  average  glue  foreman  has  learned  his  trade  at  the  hands 
of  older  glue  foremen  it  is  not  surprising  that  progress  has  not 
been  as  rapid  as  should  otherwise  have  been  expected  in  this 
department. 

The  only  way  to  overcome  this  hereditary  prejudice  is  by  a 
system  of  deliberate  constructive  education  of  the  glue  work- 
men. Not  by  means  of  books :  Technical  or  scientific  literature 
makes  little  impression,  but  by  the  employment  of  a  few  com- 
parative tests  which  he  may  see  and  appreciate.  A  few  of  the 
special  precautions  that  should  be  introduced  are  set  forth  in  the 
following  paragraphs. 

Preparation  of  the  Batch. — Many  glue  workmen  make  up  their 
batch  of  glue  by  measuring  out  a  number  of  buckets  or  barrels  of 
glue  and  adding  to  that  a  number  of  buckets  or  barrels  of  water. 
This  procedure  may  result  in  a  considerable  variation  in  the 
concentration  of  the  liquid  glue  resulting.  Glue  is  marketed  in  a 
number  of  different  forms :  As  sheet  glue,  flake  glue,  ground  glue, 
etc.,  and  each  of  these  in  turn  may  be  made  in  thicknesses  vary- 
ing from  about  a  fiftieth  to  a  quarter  of  an  inch.  Where  the 
glue  is  made  very  thin,  a  12  quart  pail  full  of  the  flake  will  weigh 
only  about  6  pounds,  while  in  the  very  thick  cut  material  the 
same  volume  will  weigh  up  to  15  pounds.  In  the  case  of  ground 
glues  the  variation  may  be  from  about  12  to  21  pounds.  Obvi- 
ously, if  a  foreman  who  had  been  using  a  medium  thick  cut  glue 
was  supplied  with  a  thin  cut  material,  his  pail,  now  holding  a 
smaller  weight,  would  measure  off  for  him  a  smaller  concen- 
tration for  his  liquid  and,  if  the  glue  was  of  the  same  grade  as 
formerly,  all  of  the  tests  resulting  would  of  course  be  low. 

But  even  in  the  measuring  out  of  glue  from  the  same  barrel  the 
weights  are  apt  to  vary  as  much  as  two  pounds  in  a  16  quart  pail, 
which  shows  that  consistent  work  cannot  be  accomplished  by 
measuring,  even  under  the  most  favorable  conditions.  The 
optimum  concentration  by  weight  should  be  determined 


508  GELATIN  AND  GLUE 

once  for  all  for  a  given  grade  of  glue  and  a  given  service,  and  it 
should  then  be  insisted  upon  that  all  batches  should  be  made  up 
by  weight:  so  many  pounds  of  glue  to  so  many  pounds  of  water. 

There  has  been  no  standard  of  optimum  consistency  or  body  of 
a  glue  that  would  produce  the  best  results  yet  proposed.1  Each 
house  has  its  own  conception  upon  this  point.  In  the  absence  of 
more  definite  information,  however,  it  is  safe  to  say  that  many 
glue  workmen  use  solutions  of  too  high  a  viscosity.  The  glue  is 
too  thick  oftentimes  to  properly  penetrate  the  pores  of  the  wood, 
and  at  the  same  time  it  sets  too  rapidly.  A  higher  dilution, 
keeping  the  temperature  at  60°C.,  would  result  many  times  in 
better  penetration,  slower  set,  and  in  general  more  satisfactory 
results,  in  addition  to  lowering  the  cost  per  square  foot  of  area 
covered.  A  soft  porous  wood  cannot,  however,  stand  as  great  a 
dilution  as  can  a  denser  wood,  on  account  of  the  rapidity  with 
which  the  thinner  solution  would  be  soaked  up  in  the  former 
instance. 

In  making  any  test  upon  the  spreading  capacity  of  a  glue, 
attention  must  be  given,  not  only  to  the  actual  weight  of  glue  in  a 
given  weight  of  water,  rather  than  a  volume  relation,  but  also 
very  carefully  to  the  temperature  of  the  test.  The  viscosity 
increases  very  rapidly  with  every  decrease  in  temperature,  and 
while  a  40  per  cent  solution,  for  example,  will  produce  a  desirable 
viscosity  at  70°C.,  a  35  per  cent  solution  would  be  sufficient  at 
60°,  and  if  the  temperature  were  allowed  to  go  to  40°  a  25  per 
cent  solution  would  produce  the  same  consistency.  These 
tests  are  best  taken  always  at  60°C. 

Soaking  the  Glue. — Unless  special  precaution  is  taken  to  insure 
the  complete  covering  of  the  glue  by  the  water  during  the  soaking 
process,  some  of  the  glue  very  often  will  extend  above  the  water 
and  not  become  softened.  In  the  case  of  thin  cut  bulky  glue, 
this  is  frequently  the  case,  the  water  sometimes  covering  only 
half  or  less  of  the  material.  Upon  warming  a  batch  of  glue 
prepared  in  this  way  the  unsoaked  portion  will  resist  the  action 
of  the  warm  liquid  and  a  greatly  prolonged  heating,  or  an  exces- 
sively high  temperature,  is  necessary  to  bring  it  all  into  solution. 
And  such  heating  has  been  shown  to  result  in  extensive  loss  due 
to  the  hydrolyzing  action  of  the  water. 

To  overcome  this  difficulty  the  glue  may  be  added  in  portions, 
the  first  portion  being  swollen  before  the  second  is  added,  or  the 

1  See  pages  519-20. 


APPLICATIONS  OF  GLUE  509 

whole  mass  may  be  turned  over,  the  top  being  placed  on  the 
bottom.  In  the  case  of  ground  glues  the  entire  batch  of  glue 
should  be  stirred  into  about  three  quarters  of  the  water  required, 
and  after  soaking  for  a  time  the  balance  of  the  water  added. 
This  procedure  prevents  any  of  the  light  flakes  from  floating  on 
the  water  and  so  not  being  properly  soaked. 

The  temperature  of  the  water  should  not  be  high,  for  above 
20°C.  the  bacteria  may  become  active.  A  temperature  near  the 
freezing  point  does  no  harm  but  an  actual  freezing  of  the  swollen 
glue  is  probably  harmful.  If  flake  glue,  or  any  very  thick  cut 
material,  is  used,  the  glue  should  soak  for  10  to  12  hours,  but  if 
ground  glue  is  employed,  an  hour  or  two  is  usually  sufficient. 
In  either  case  the  flakes  of  glue  should  be  completely  swollen, 
and  not  show  any  hard  places,  as  these  will  not  be  easily  dissolved. 

The  water  used  should  be  as  pure  as  is  practicable  to  obtain. 
Acids  or  alkalies,  dissolved  salts,  or  suspended  material  in  water 
each  exert  an  effect  upon  the  gelatin  or  other  constituents  of  the 
glue,  and  undesirable  results  often  may  be  traced  to  this  source. 
The  bacteria  in  contaminated  water  may  be  the  cause  of  unusu- 
ally rapid  spoiling  of  a  batch  of  glue. 

Melting  the  Glue.1 — Heat  may  be  applied  to  the  swollen  glue  in 
a  number  of  ways.  The  most  advantageous  method,  however, 
is  to  apply  steam  to  an  outer  water  jacket  in  which  the  glue  con- 
tainer is  placed.  The  glue  should  be  stirred  slowly  during  solu- 
tion, and  the  temperature  not  allowed  to  exceed  65°C.  The 
water  in  the  outer  jacket  should  not  at  any  time  exceed  70°C., 
and  after  solution  is  complete  the  temperature  in  both  sections 
should  be  adjusted  to  60°.  A  thermometer  is  indispensable,  for 
without  its  use  the  temperature  cannot  be  correctly  gauged  or 
controlled.  The  kettle  should  be  covered  to  prevent  evapora- 
tion, and  the  glue  is  best  delivered  into  the  workmen's  pots  by 
means  of  a  cock  placed  near  the  bottom'  of  the  kettle.  Only 
that  amount  of  glue  which  will  be  used  within  two  or  three 
hours  should  be  kept  in  the  glue  kettle.  Properly  soaked  glue 
may  be  brought  into  a  condition  ready  for  use  in  a  few  minutes, 
and  the  saving  in  the  strength  of  the  glue  warrants  this  procedure. 

Another  method  of  operation  that  is  employed  in  some  plants 
is  to  make  up  the  glue  into  the  form  of  a  concentrated  jelly, 
and  to  distribute  this  jelly  to  the  workmen  as  needed.  The 
workmen  add  a  given  amount  of  water  to  a  given  weight  of 

1  See  pages  510-15. 


510  GELATIN  AND  GLUE 

jelly,  warm  it  over  their  bench  heater,  and  it  is  then  ready  to 
apply.  While  this  procedure  reduces  to  a  minimum  the  loss 
due  to  prolonged  heating,  it  produces  conditions  which,  unless 
very  carefully  controlled,  are  favorable  for  the  rapid  develop- 
ment of  bacteria,  and  large  batches  of  jelly  are  apt  to  spoil  before 
being  used.  If  the  jelly  is  kept  in  an  ice  box  this  danger  is 
averted,  and  very  satisfactory  results  may  be  secured. 

The  Application  of  the  Glue.1 — After  due  consideration  has  been 
given  to  the  concentration  of  the  glue  solution  to  be  used  for  a 
certain  work,  the  joiner  must  see  to  it  that  the  joint  is  true,  that 
the  temperature  of  his  glue  is  correct,  that  the  temperature  of  the 
wood  is  correct,  i.e.,  warm  but  not  hot;  and  then  apply  his  glue 
quickly,  rubbing  it  in  as  if  he  were  painting  it;  apply  the  glue 
to  both  sections  of  wood  in  the  joint,  rub  the  joint  to  squeeze  out 
as  much  of  the  excess  glue  as  he  can;  and  apply  the  pressure 
quickly  and  uniformly  before  the  glue  can  have  time  to  set. 
It  has  been  shown  that  the  most  favorable  pressure  is  about 
200  pounds  to  the  square  inch  of  glued  surface,  but  in  practice 
only  about  30  pounds  are  ordinarily  employed.  The  joint 
should  then  be  placed  in  a  warm  dry  place  and  allowed  to  remain 
undisturbed  for  8  to  12  hours.  It  is  then  safe  to  remove  the 
pressure,  but  the  maximum  strength  is  not  developed  for  several 
days. 

Losses  in  Glue  Due  to  Overheating. — It  was  not  many 
years  ago  that  it  was  a  common  practice  among  joiners  and 
cabinet  makers  to  procure  the  highest  quality  of  glue,  and  boil  it 
down  to  obtain  a  particular  consistency  which  was  regarded  as 
most  desirable.  They  argued  that  in  order  to  do  the  best  work 
a  glue  must  be  of  the  highest  grade,  but  that  such  glues,  on  the 
other  hand,  were  too  thick  to  spread  well  at  the  dilutions  they 
regarded  as  necessary,  and  also  chilled  too  quickly.  It  there- 
fore became  common  practice  to  cook  the  material  until  it  did 
possess  the  desired  qualities. 

That  there  might  be  some  justification  for  such  a  practice 
would  be  inferred  from  a  paper  by  J.  Herold.2  He  made  the 
highly  interesting  statement  that  although  the  protein  gelatin 
is  the  gelatinizing  constituent  of  gelatin,  and  that  the  best 
gelatin  contains  the  largest  amount  of  this  unhydrolyzed  protein, 
yet  that  gelatin  in  glue  is  actually  a  drawback  to  its  adhesive 

1  See  pages  517-26. 
J.  HEROLD,  Chem.  Ztg.,  35  (1911),  93. 


APPLICA TIONS  OF  GLUE  511 

power,  the  best  glue  containing  no  gelatin,  but  in  its  place  a 
non-gelatinizing  proteose  which  may  be  made  from  the  gelatin 
either  by  the  action  of  alkalies,  or  by  prolonged  heating  with 
water.  He  actually  considers  a  low  melting  point,  a  low  jelly 
strength,  and  a  low  viscosity  as  the  desiderata  for  a  high  grade 
glue.  Just  what  adhesive  and  other  properties  would  be  revealed 
by  a  substance  containing  say  90  per  cent  or  more  of  proteose, 
obtained  by  the  partial  hydrolysis  of  gelatin,  cannot  be  stated, 
because  such  a  substance  has  not  yet  been,  with  certainty, 
prepared  and  tested.  In  all  ordinary  hydrolyses  of  gelatin, 
either  by  the  action  of  acids,  alkalies,  enzymes,  or  water  alone, 
the  hydrolytic  cleavage  produces  peptones  in  abundance,  as  well 
as  proteoses,  and  in  many  cases  the  amino-acids,  ammonia,  etc., 
are  likewise  formed.  And  in  all  of  these  cases  the  adhesive 
strength  is  found  to  decrease  proportionally  to  the  extent  of  the 
hydrolysis,  or  in  other  words  to  be  directly  proportional  to  the 
content  of  unhydrolyzed  gelatin. 

A  number  of  experiments  have  been  performed  in  the  author's1 
laboratory  which  illustrate  very  strikingly  the  decrease  in  strength 
which  is  brought  about  by  the  prolonged  heating  of  a  glue. 
Samples  were  made  up  in  the  usual  way  by  the  use  of  one  part 
of  glue  to  two  and  a  half  parts  of  water,  and  were  heated  for  12 
hours  in  a  water  bath  maintained  at  a  temperature  of  80°C.  (176°F.) 
All  loss  in  water  due  to  evaporation  was  made  up  each  hour.  At 
the  beginning  of  the  operation,  and  at  the  end  of  2,  4,  8,  and  12 
hour  periods,  portions  of  the  glue  were  removed  and  applied  to 
maple  blocks  which  had  been  very  carefully  surfaced,  and  the 
glued  joints  subjected  to  a  uniform  pressure  of  200  pounds  to  the 
square  inch  by  placing  between  the  parallel  plates  of  a  universal 
testing  machine.  After  16  hours  they  were  removed,  allowed 
to  set  for  a  week,  and  then  carefully  sawed  to  exactly  4  square 
inches  glued  surface,  and  sheared  apart.  The  data  obtained 
show  very  forcibly  the  serious  loss  due  to  prolonged  heating,  for 
the  value  of  the  glue  dropped  approximately  one  grade  for  each 
2  hours  of  heating,  or  from  a  very  high  to  a  very  low  hide  grade 
in  12  hours.  In  actual  figures  the  loss  in  strength  averaged 
about  85  pounds  per  square  inch  per  hour,  or  about  1,000  pounds 
per  square  inch  in  the  twelve  hours.  Differently  expressed,  the 
loss  in  strength  amounted  to  more  than  5  per  cent  of  the  original 

1  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  201. 


512 


GELATIN  AND  GLUE 


strength  per  hour  of  heating,  or  to  67  per  cent  loss  in  the  12  hours. 
The  curve  expressing  the  decrease  in  strength  is  shown  in  Fig.  101. 
This  is  not  an  exceptional  case,  for  many  other  investigators 
have  found  somewhat  similar  results.  For  example,  Linder  and 
Frost1  working  at  the  somewhat  lower  temperature  of  150°F. 
(65.5°C.)  obtained  a  decrease  in  strength  of  from  30  to  45 


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Hours  Heated 
FIG.  101. — The  effect  of  heating  a  high-grade  glue  in  solution  upon  strength 

per  cent  on  heating  a  glue  for  20  hours.  The  United  States  For- 
est Service2  has  conducted  similar  tests.  Solutions  of  a  high 
grade  joint  glue  and  a  veneer  glue  were  heated  for  48  hours  at 
104,  140  and  176°F.  (40,  60  and  80°C.)  and  tested  every  few 
hours  during  this  period  for  strength  and  viscosity.  "In  the 
first  7  hours  of  heating  at  176°F.  the  veneer  glue  lost  approxi- 
mately one-half  its  joint  strength,  and  the  high  grade  joint  glue 
weakened  almost  as  much.  The  greatest  loss  in  the  strength  of 
the  glue  joints  occurred  at  this  temperature.  In  the  solutions 
kept  at  104°  there  was  a  sudden  drop  in  the  strength  of  the  joints 
made  with  the  high  grade  glue  after  31  hours  of  heating,  due 
possibly  to  a  combination  of  bacterial  and  chemical  action.  The 
veneer  glue  joints  showed  a  more  gradual  decrease  at  this  tem- 
perature. The  most  favorable  of  the  three  temperatures  used 
was  140°,  but  even  at  this  temperature  an  appreciable  weakening 
in  both  glues  was  noted  at  the  end  of  7  hours,  and  longer  heating- 
caused  greater  loss."  ,  The  viscosity  of  the  two  glues  decreased 
rapidly  throughout  the  experiment. 

BINDER  and  FROST,  Eng.  News,  72  (1915),  178. 

2  U.  S.  Forest  Service  Technical  Notes  104  (1920). 


APPLICATIONS  OF  GLUE  513 

Consider  for  a  moment  what  these  losses  mean  in  dollars  and 
cents.  Taking  the  price  of  the  highest  grade  of  glue  at  about  46 
cents  per  pound  (in  barrel  lots),  the  price  of  the  grade  to  which 
it  was  brought  by  twelve  hours  of  the  heating  (in  the  author's 
experiments)  is  32  cents.  That  is,  a  loss  of  14  cents  per  pound 
is  sustained  in  12  hours,  or  a  little  over  a  cent  per  pound  per  hour. 
The  average  loss  per  day  of  8  hours  would  therefore  amount  to 
about  4J^  cents  per  pound.  This  does  not  look  bad,  but  let  us 
carry  the  proposition  to  its  conclusion.  Suppose  the  amount 
of  glue  kept  heated  at  all  times  is  approximately  100  gallons. 
That  will  represent  perhaps  400  pounds  of  dry  glue,  and  during 
the  day  the  loss  will  become  therefore,  4J^j  X  400  or  $18.  In 
the  course  of  a  year  this  rises  to  the  surprising  amount  of  $5,600. 
Surely  the  prevention  of  such  a  loss  is  well  worth  a  little  time  and 
thought. 

Glue  does  not  deteriorate  by  standing  for  12  to  24  hours  in 
cold  water  (unless  an  already  putrid  glue  is  used,  or  the  tem- 
perature of  the  water  is  allowed  to  rise  to  above  70°F.).  And 
glue  that  is  properly  soaked  may  be  brought  into  a  condition  ready 
for  use  very  quickly  by  merely  raising  the  temperature  to  140°F. 
with  stirring.  There  should  be  no  difficulty  experienced  there- 
fore, in  keeping  the  amount  of  glue  actually  in  solution  down  to 
just  above  the  quantity  which  will  be  immediately  used,  and  so 
entirely  eliminate  any  loss  from  prolonged  heating. 

Of  even  more  serious  importance  than  the  foregoing  is  the 
possible  failure  of  the  joint  which  is  made  from  the  weakened 
glue.  As  was  previously  stated,  the  loss  in  strength  amounted 
to  about  67  per  cent  of  the  original  value.  Assuming  that  the 
necessary  strength  was  2,500  pounds  to  the  square  inch,  and 
that  the  glue  was  bought  to  carry  3,000  pounds  to  the  square 
inch,  thus  giving  a  500  pound  margin  of  safety,  any  depreciation 
beyond  this  500  pound  margin  would  be  disastrous.  But,  in  the 
case  cited,  a  6  hour  heating  resulted  in  a  500  pound  drop.  The 
glue  user  has  learned  to  buy  upon  a  high  margin  of  safety, 
and  failures  are  not  as  common  as  might  be  expected,  but  when 
one  does  occur  the  user  invariably  concludes  that  he  was  sold  an 
inferior  glue,  that  the  lot  in  question  was  not  up  to  his  previous 
buys — that  the  glue  manufacturer  had  "put  one  over"  on  him. 
Would  it  not  be  well  for  him  to  look  rather  to  his  glue  room? 

May  we  not  also  properly  ask  why  it  should  be  necessary  for 
the  user  to  allow  such  a  wide  margin  in  strength  for  safety?  The 

33 


514  GELATIN  AND  GLUE 

fact  that  he  does  is  in  itself  an  admission  that  his  use  of  it  is 
uncertain;  that  he  does  not  at  all  times  get  his  full  value  out  of 
the  material.  Just  as  soon  as  he  has  learned  to  control  his 
modus  operandi  to  the  end  that  he  shall  get  the  full  value  from 
his  glue,  just  so  soon  may  he  in  confidence  and  with  safety  cut 
down  his  strength  margin  to  a  narrower  limit,  and  buy  a  less 
expensive  glue,  or  apply  his  original  glue  at  a  greater  dilution — a 
procedure  which  will  again  save  him  many  dollars  on  his  glue 
bill  in  a  year.  The  only  reason,  in  fact,  why  glue  failures  under 
the  conditions  described  are  not  many  instead  of  few  is  that  as 
the  heating  is  in  progress,  so  at  the  same  time  is  evaporation 
taking  place,  and  instead  of  an  original  55  gallons  there  will  be 
left  after  several  hours  of  heating  only,  perhaps,  45  gallons. 

If  the  evaporation  of  water  from  the  glue  which  takes  place 
on  heating  were  made  up  by  occasional  additions  of  water  it 
would  very  soon  become  apparent  that  the  glue  rapidly  became 
thinner  as  heating  progressed  until  it  was  altogether  too  watery 
to  apply.  In  general  this  decrease  in  viscosity  or  spreading 
capacity  escapes  attention  and  remains  quite  unobserved  because 
of  the  fact  that  the  same  heat  treatment  which  is  responsible  for 
it  also  results  in  evaporation  of  the  water  and  consequent  con- 
centration of  the  glue  in  the  solution. 

One  other  point  in  this  connection  is  worth  attention.  The  loss 
due  to  heating  has  been  found  to  be  proportional  to  the  tem- 
perature, increasing  rapidly  as  the  temperature  rises  from  60°C. 
to  the  boiling  point.  The  figures  which  have  been  given  are 
based  upon  a  heating  at  80°.  But  very  often,  where  the  tem- 
perature is  uncontrolled,  much  higher  values  are  reached,  and 
not  infrequently  in  fact  the  glue  is  actually  permitted  to  boil. 
A  very  few  hours  of  boiling  is  sufficient  to  render  the  glue  worse 
than  useless  for  high  class  joint  or  panel  work. 

It  is  obvious,  therefore,  that  prolonged  heating  of  a  glue  results 
both  in  a  loss  in  strength  and  a  decrease  in  the  covering  capacit}^ 
The  latter,  however,  is  concealed  through  evaporation  of  the 
water;  and  the  loss  in  strength  is  usually  unobserved  on  account 
of  the  excess  in  strength  of  the  glue  as  bought  above  that 
necessary  in  the  operation. 

The  higher  the  temperature  the  more  rapidly  will  depreciation 
take  place. 

The  losses  referred  to  may  not  be  made  good  by  the  addition 
of  water  to  make  up  that  lost  by  evaporation,  for  the  product 


APPLICATIONS  OF  GLUE  515 

would  be  too  thin  to  spread  properly,  and  would  result  in  a  weaker 
joint.  All  losses  due  to  overheating  may,  however,  be  minimized 
by  dissolving  only  that  amount  which  will  be  used  immediately, 
and  preventing  the  temperature  from  rising  above  60°C. 

As  soon  as  these  simple  principles  are  efficiently  applied,  a 
lower  margin  of  safety  may  be  confidently  employed,  which 
means  that  a  less  expensive  glue,  or  the  same  glue  at  a  greater 
dilution,  may  be  employed  with  equally  uniform  and  satisfactory 
results. 

The  Bacterial  Decomposition  of  Glue. — The  unsanitary 
appearance  of  the  glue  room  with  its  disagreeable  strenches  is 
proverbial.  It  has  been  taken  as  a  matter  of  course  for  so  long 
that  it  is  too  often  believed  a  necessary  adjunct  to  the  use  of  that 
substance.  Glue  pots  in  some  places  are  never  cleaned.  The 
floor  becomes  covered  with  a  gradually  thickening  layer  of  the 
spillings.  But  glue  is  an  animal  product  and  subject  to  bacterial 
decomposition.  What  wonder,  then  that  the  pots,  the  room, 
the  new  work,  all  imbibe  the  characteristic  putrid  odor!  Prob- 
ably the  greatest  number  of  complaints  which  are  made  by  glue 
consumers  to  the  dealers  and  manufacturers  are  based  upon  the 
affirmation  that  the  glue  in  question  is  "sour. "  Investigations 
show  the  pots  and  kettles  to  be  coated  sometimes  inches  thick 
with  glue  in  an  active  state  of  decomposition  that  has  been 
accumulating  for  months.  It  is  doubtful  if  these  same  men  would 
expect  that  sweet  milk  could  be  poured  into  cans  containing 
decomposed  milk  without  itself  becoming  sour  very  quickly,  but 
they  appear  not  to  have  applied  that  reasoning  to  the  glue  pot. 

Putrefaction  of  glue  takes  place  very  rapidly  at  the  tempera- 
tures which  obtain  in  the  glue  room.  A  sweet  glue  allowed  to 
remain  for  a  few  hours  in  contact  with  decomposing  glue  will 
become  sour,  but  that  indeed  is  not  the  worst  or  only  objection 
to  permitting  of  such  practices.  A  sour  glue  is  not  merely  a  glue 
that  does  not  smell  good,  but  it  is  a  glue  that  has  suffered  a 
chemical  change  in  its  constitution.  That  change  is  an  hydrol- 
ysis, slightly  different,  in  the  nature  of  the  cleavage  products 
produced,  from  hydrolysis  by  water,  acids,  alkalies,  or  enzymes, 
but  nevertheless  an  hydrolysis,  or  a  breaking  down  of  the  gelatin 
molecule.  This  decomposition  results  in  an  alteration  in  the 
properties  of  the  material.  The  viscosity,  the  jelly  consistency, 
the  joint  strength  obtainable,  are  all  greatly  reduced.  Further- 
more, the  action  of  the  bacteria  does  not  stop  upon  the  applica- 


516  GELATIN  AND  GLUE 

tion  of  the  glue  to  the  joint,  but  continues  even  after  drying 
out.  This  means  that  a  failure  of  the  joint  is  very  apt  to  result 
sooner  or  later.  It  makes  no  difference  whether  a  high  grade 
or  a  low  grade  product  is  used  at  the  start,  the  contamination 
with  the  decomposing  material  will  in  a  few  hours  reduce  it  to  a 
low  grade,  and  reduction  in  strength  will  continue  in  the  joint. 

The  loss  in  dollars  resulting  from  such  practice  is  unquestion- 
ably enormous,  not  to  mention  the  harm  done  to  the  house  by 
an  injured  reputation  due  to  failures  in  their  glue  work.  Just 
as  in  the  case  of  prolonged  heating,  a  high  grade  glue  may  be 
bought,  but  by  the  time  it  is  actually  put  into  service  it  may  have 
been  reduced  to  a  very  low  grade.  But  while  the  heating  loss 
ceases  as  soon  as  the  glue  is  applied,  this  is  not  the  case  with  the 
decomposed  glue,  and  the  latter  may,  therefore,  be  productive 
of  even  more  serious  loss  than  the  overheated  material.  This 
fact  is  certainly  indicated,  if  not  proved,  by  the  experiments  of 
the  United  States  Forest  Service  previously  referred  to.1  They 
found  that  the  loss  in  strength  and  viscosity  sustained  by  main- 
taining glues  at  40°C.  for  a  number  of  hours  was  actually  greater 
than  that  sustained  by  maintaining  them  at  60°  for  the  same 
period.  At  the  latter  temperature  the  bacteria  are  inhibited,  if 
not  killed,  so  that  the  loss  in  strength  observed  is  due  entirely 
to  ordinary  water  hydrolysis.  But  at  40°  the  bacteria  are  very 
nearly  at  their  optimum  temperature  for  activity.  The  water 
hydrolysis  is  unquestionably  greater  at  the  higher  temperature, 
so  the  greater  loss  in  strength  and  viscosity  at  40°  is  due  directly 
to  the  decomposing  action  of  the  bacteria,  and  to  nothing  else. 

The  cure  for  this  disastrous  activity  of  bacteria  is  very  simply 
to  keep  the  glue  pots,  kettles,  brushes,  and  all  other  articles  that 
come  in  contact  with  the  liquid  glue,  thoroughly  clean.  There 
is  no  remedy  that  may  be  applied  to  a  decomposed  glue  to  again 
bring  it  into  a  sweet  condition,  and  all  glue  that  has  become  sour 
should  be  discarded  at  once,  and  without  regret.  Attempting 
to  use  it  up  will  bring  even  greater  loss  than  the  price  of  the  glue 
thrown  out.  The  glue  pots  and  kettles  should  be  drained  into  a 
clean  container  each  night,  and  should  be  thoroughly  cleaned 
before  being  used  the  next  day.  The  brushes,  stirrers,  ther- 
mometers, hydrometers,  and  other  equipment  should  likewise 
be  well  cleaned.  The  work  benches  and  floors  should  be  scraped 
free  of  all  glue  that  has  accumulated  on  them  during  the  day. 

1  See  page  512.  » 


APPLICATIONS  OF  GLUE  517 

By  following  these  simple  precautions  bacteria  will  be  unable  to 
gain  sufficient  headway  at  any  time  to  damage  the  glue,  while 
the  general  appearance  and  odor  of  the  glue  room,  and  the  health 
and  morale  of  the  workmen  will  make  such  an  alteration  in  glue 
room  technique  a  decidedly  worth  while  undertaking. 

One  further  point  in  this  connection.  Glue  is  exceedingly 
sensitive  to  the  moisture  conditions  in  which  it  is  stored.  Oc- 
casionally glue  will  be  stored  in  damp  cellars  where  a  consider- 
able absorption  of  water  will  take  place  in  the  glue.  Under  such 
conditions  it  may  happen  that  a  sufficient  amount  of  water  may 
be  taken  up  to  enable  bacteria,  molds,  or  other  microorganisms 
to  attack  the  material.  Molds  especially  grow  under  such 
conditions,  and  good  glue  may  be  completely  spoiled  by  the 
decomposition  effected  by  them.  On  melting  these  glues  the 
viscosity  will  be  low,  the  odor  will  be  strong  and  sour,  and  joints 
made  by  their  use  will  be  weak.  An  excessively  dry  atmosphere 
is  not  to  be  recommended  as  the  glues  become  brittle  but  it  is  to 
be  preferred  to  damp  conditions  of  storage. 

2.  GLUE  AS  AN  ADHESIVE 

Probably  the  oldest  and  most  important  service  for  which  glue 
is  employed  is  as  an  adhesive  for  joining  together  two  pieces  of 
wood.  Cabinet  making  is  an  ancient  craft  and  still  consumes 
large  amounts  of  glue,  and  the  veneer  industry  is  one  of  the 
largest  consumers  of  glue.  Panel  work  is  coming  to  be  regarded 
as  a  highly  specialized  art,  and  while  some  years  ago  it  was  looked 
upon  as  an  " imitation"  process,  with  the  slur  which  usually 
accompanies  the  use  of  that  term,  it  has  now  grown  into  favor 
for  its  own  worth  and  artistic  and  strength  advantages,  and  the 
craft  is  even  contemplating  a  much  greater  substitution  of  built- 
up  and  panel  work  for  the  solid  than  has  yet  been  undertaken. 
The  advantages  of  such  procedure  in  the  producing  of  artistic 
effects,  the  conservation  of  the  more  valuable  timber,  and  the 
production  of  greater  strength  together  with  less  weight  are 
undisputable. 

The  Conditions  Affecting  the  Adhesive  Strength  of  Glue. — 
For  all  glues  that  are  to  be  used  for  joining,  panel,  or  other 
adhesive  purposes,  the  ultimate  criterion  for  value  must,  of  course, 
be  the  actual  adhesive  strength  developed  and  maintained  in 
service.  It  would  seem  at  a  glance  that  the  importance  of  this 


518  GELATIN  AND  GLUE 

property  would  be  so  fundamental  that  it  must  inevitably  con- 
stitute the  principal  test  upon  which  selection  would  depend. 

This  would  undoubtedly  be  the  case  were  it  not  for  the 
technical  difficulties  encountered  in  the  making  of  the  test.  The 
importance  of  the  adhesive  strength  has  indeed  always  been 
recognized,  and  relative  approximations  of  this  property  are 
constantly  being  made.  A  glue  foreman  will  be  asked  to  select 
between  several  glues  submitted  to  him,  and  among  other  tests  he 
will  usually  glue  up  a  few  joints  of  the  several  samples,  clamp 
them  up  in  the  usual  manner,  and  after  standing  a  few  hours,  or  a 
day,  break  them  with  a  hammer  or  a  chisel,  and  note  which 
appears  to  be  the  strongest  joint.  But  attempts  at  measuring 
the  joint  strength  have  not  been  confined  to  the  unscientific 
treatment  of  the  glue  foreman.  Many  times  the  trained  chemist 
has  bent  his  energies  upon  the  problem,  but  his  efforts  have  been 
awarded  with  only  a  questionable  degree  of  success.  He  has, 
however,  succeeded  in  demonstrating  that  the  strength  attained 
in  any  joint  is  dependent  upon  many  factors.  These  factors 
have  been  studied,  one  at  a  time,  and  the  effect  produced  upon 
the  joint  strength  by  varying  any  of  them  is  now  quite  generally 
understood.  But  only  by  taking  the  utmost  precaution  to 
eliminate  every  variable  can  truly  comparative  data  upon 
strength  be  obtained. 

Among  the  factors  which  have  been  found  to  influence  the 
strength  obtainable  in  a  glued  joint,  made  with  any  given  glue, 
are  the  following: 

The  concentration  of  the  glue  solution. 

The  time  occupied  and  temperature  employed  in  heating  the  glue. 

The  temperature  of  the  glue  when  applied. 

The  species,  density,  porosity,  and  moisture  content  of  the  wood  used. 

The  trueness  of  the  joint. 

The  amount  and  method  of  application  of  the  glue  in  the  joint. 

The  temperature  of  the  wood  when  glue  is  applied. 

The  rapidity  of  handling. 

The  pressure  applied  to  the  joint,  and  its  uniformity  of  distribution. 

The  time  under  pressure,  and  time  of  curing. 

The  temperature  and  humidity  during  curing. 

The  method  of  breaking  the  joint. 

The  hydrogen-ion  concentration  of  the  solution. 

From  a  survey  of  the  above  list  of  factors  which  may  influence 
the  joint  strength  of  any  given  glue,  it  becomes  more  easily 
understood  why  the  strength  test  has  not  met  with  greater 


APPLICATIONS  OF  GLUE  519 

success.  A  perfect  control  of  each  of  the  above  mentioned 
variables  is  an  exceedingly  difficult,  if  not  quite  impossible,  task. 
A  brief  account  of  the  particular  influence  of  each  of  these 
factors  is  given  below. 

The  Concentration  of  the  Glue  Solution. — It  is  self-evident 
that  any  wide  variation  in  the  concentration  of  the  glue  solution 
used,  either  above  or  below  a  definite  optimum,  would  reveal 
itself  by  the  production  of  weaker  joints.  This  is,  of  course, 
recognized  in  a  general  way,  but  it  is  noteworthy  that  no  precise 
determinations  have  ever  been  made,  to  the  author's  knowledge, 
of  the  exact  relationships  involved,  or  of  the  exact  specification 
of  the  optimum  value.  In  a  general  way  it  is  recognized  that 
the  optimum  concentration  is  very  largely  dependent  upon  the 
viscosity  of  the  glue  under  consideration;  that  the  higher  the 
viscosity  the  lower  may  be  the  concentration  required  to  produce 
the  best  results.  But  whether  the  viscosity  is  the  only  factor 
which  defines  the  optimum  concentration,  and  exactly  what  this 
concentration  is  for  any  given  glue  viscosity,  has  not  been  defi- 
nitely established.  It  is  common  practice  to  use  the  lower 
grades  of  glues  in  concentrations  of  from  35  to  50  per  cent,  and 
the  higher  grades  at  from  25  to  35  per  cent.  Kahrs1  worked  out 
an  elaborate  scheme  by  which  the  most  advantageous  concentra- 
tion could  be  read  off  on  a  chart,  if  the  viscosity  (by  Kahrs'  glue 
tester)  was  known.  This  was  based  on  the  principle  that  the 
viscosity  was  the  only  factor  which  affected  the  most  desirable 
concentration.  The  chart  merely  indicated  the  amount  of  water 
necessary  to  be  added  to  a  given  amount  of  glue  of  any  viscosity 
in  order  to  produce  a  batch  of  the  specified  optimum  viscosity. 

The  author  is  of  the  opinion  that  such  a  method,  although  it 
possesses  some  distinct  advantages,  would  not  be  equally  applic- 
able to  all  glues  or  to  all  woods.  A  fictitious  viscosity  is  some- 
times produced,  which  might  alter  the  optimum  value.  But  of 
more  importance,  a  viscosity  that  would  give  the  best  results 
upon  maple  or  other  hard  dense  wood  would  certainly  not  be  so 
desirable  upon  basswood,  or  any  other  porous  type.  Glue  joins 
by  penetrating  the  pores  of  the  wood,  resulting  in  a  multiplying 
of  fine  interlacing  threads  of  the  material  extending  from  one  to 
the  other  of  the  two  sections  of  the  joint.  A  degree  of  viscosity 
must  be  selected  such  that  a  fair  amount  of  penetration  may 
result.  If  the  glue  is  too  dilute  it  will  penetrate  without  any 

1  F.  KAHRS,  Glue,  No.  4,  p.  9;  No.  8,  p.  4;  No.  10,  p.  10. 


520  GELATIN  AND  GLUE 

question,  but  the  individual  fibrils  of  glue,  when  dried,  will  be  too 
thin  to  impart  the  greatest  strength.  On  increasing  the  viscosity 
the  penetration  will  diminish,  but  the  individual  fibers  of  dried 
glue  will  be  thicker  and  stronger,  resulting  in  a  stronger  joint. 
If  the  viscosity  is  further  increased,  the  penetration  will  have 
diminished  until  the  strength  obtainable  again  becomes  small. 
It  is  on  this  account  that  glue  is  not  a  good  adhesive  for  metal  or 
nonporous  substances.  The  finer  the  pores  in  a  wood,  the 
thinner  also  must  be  the  glue  solution  in  order  that  a  fair  degree 
of  penetration  may  take  place.  Thus  dense  woods  require  a 
rather  thin  glue.  More  porous  woods  do  not  require  such  thin 
solutions,  and  if  such  are  used  the  liquid  will  sink  deep  into  the 
wood,  resulting  in  very  fine  fibers  of  the  dried  glue.  Conse- 
quently more  viscous  solutions  are  required  for  the  more  porous 
woods. 

The  optimum  viscosity  will  depend  also  upon  the  use  to  which 
the  glue  is  put.  Veneer  work  does  not  require  as  viscous  solu- 
tions as  joint  work.  The  glue  must  be  sufficiently  mobile  to  be 
easily  squeezed  out  during  the  pressing,  or  imperfections  in  the 
smoothness  of  the  veneer  surface  will  result. 

The  Time  Occupied  and  Temperature  Employed  in  Heating 
the  Glue. — One  of  the  very  common  practices  which  results  in  a 
great  lessening  in  the  joint  strength  is  the  tendency  to  melt  up 
large  batches  of  glue,  and  keep  in  this  condition  at  a  relative^ 
high  temperature  for  many  hours  before  being  completely  used. 
Oftentimes  a  large  amount  of  the  glue  which  was  melted  early  in 
the  morning  is  still  unused  at  night,  and  goes  into  the  next  day's 
allotment.  Such  a  practice  results,  as  detailed  on  page  511,  in  a 
decided  weakening  of  the  glue  strength.  If  evaporation  is 
prevented,  either  by  the  use  of  closed  glue  pots,  or  by  the  addition 
of  water  to  compensate  for  that  evaporated,  then  the  joint 
strength  may  drop  as  much  as  50  per  cent  in  a  day's  heating. 

The  temperature  to  which  the  glue  is  heated  and  maintained  is 
of  equal  importance.  A  very  few  hours  of  boiling  is  sufficient 
to  make  a  glue  practically  worthless  as  an  adhesive.  It  is 
unnecessary  ever  to  heat  the  glue  above  60°C.,  and  any  higher 
temperature  will  result  in  an  unnecessary  weakening  in  the 
strength  of  the  glue. 

The  Temperature  of  the  Glue  when  Applied. — If  the  glue  is  at 
too  low  a  temperature  when  applied  to  the  joint  it  will  be  unneces- 
sarily viscous,  and  will  set  too  quickly.  It  should  be  sufficiently 


APPLICATIONS  OF  GLUE  521 

warm  so  that  it  will  readily  penetrate  the  pores  of  the  wood,  and 
so  that  it  will  not  set  before  the  joiner  is  able  to  finish  his  ma- 
nipulation, and  apply  the  pressure  upon  it.  On  the  other  hand, 
if  the  temperature  is  higher  than  necessary,  the  viscosity  may 
have  become  too  low;  and  a  loss  in  strength  will  also  be  sustained 
on  account  of  the  excessive  temperature,  as  set  forth  in  the 
previous  paragraph. 

The  Species,  Density,  Porosity,  and  Moisture  Content  of  the 
Wood  Used. — If  a  given  glue  at  a  given  concentration  and 
temperature  is  applied  to  different  types  of  wood,  the  strength 
of  the  joint  may  be  expected  to  vary  according  to  the  particular 
suitability  of  the  viscosity  used  to  the  porosity  of  the  woods. 
The  wood  of  that  porosity  which  is  most  suitable  for  the  viscosity 
used  will  show  the  greatest  strength.  Density  is  more  or  less  a 
measure  of  porosity.  That  is,  the  greater  the  density,  the  less 
the  porosity.  If  the  moisture  content  is  high  as  a  result  of 
incomplete  curing,  then  the  pores,  since  they  already  contain 
water,  will  be  incapable  of  absorbing  very  much  glue,  and  the 
strength  must  accordingly  suffer. 

The  Trueness  of  the  Joint. — The  strength  of  a  glued  joint 
must  be  proportional  to  the  area  of  the  effective  surface  of 
the  joint.  If,  in  a  joint  of  4  square  inches  only  3  square 
inches  are  in  intimate  contact,  it  must  follow  that  the  strength 
obtained  will  be  much  less  than  if  the  whole  joint  is  in  contact. 
Very  little  strength  is  developed  by  using  glue  as  a  mortar  is 
used.  The  joined  surfaces  must  be  perfectly  true  to  obtain  the 
maximum  strength.  Sand-paper  should  not  be  used  to  remove 
the  last  imperfections  in  the  surface,  for  the  powdered  wood 
which  is  scraped  off  fills  up  the  pores  of  the  wood,  and  retards  the 
penetration  of  the  glue.  A  good  plane  should  be  the  final  tool 
employed.  Under  no  circumstances,  however,  should  tests  be 
made  upon  joints  which  are  not  perfectly  true. 

The  Amount  and  Method  of  Application  of  the  Glue  in  the 
Joint. — One  often  hears  warnings  against  the  " starving"  of 
joints  by  using  too  little  glue  upon  them.  Of  course,  it  is  possible 
to  use  too  little  glue  and  so  obtain  a  weak  joint,  but  if  the  material 
is  applied  with  a  brush,  or  even  a  roller  that  is  properly  supplied, 
the  danger  of  applying  too  little  glue  is  almost  negligible.  The 
tendency  among  the  consumers  of  glue  is  rather  to  use  far  more 
than  is  needed,  and  squeeze  out  the  excess.  The  waste  in  this 
procedure  is  often  very  great. 


522  GELATIN  AND  GLUE 

But  little  is  ever  heard  regarding  the  other  opposite  possibility 
—that  of  " overfeeding'7  the  joint.  If  a  large  amount  of  glue 
is  applied,  and  the  excess  is  not  squeezed  or  rubbed  out,  the 
danger  of  "overfeeding"  may  be  a  very  real  one.  As  the  glue 
dries  out  in  the  joint  it  contracts,  forming  a  honeycomb  appearing 
structure,  and  joints  of  exceeding  weakness  may  result.  When- 
ever a  broken  joint  reveals  this  uneven,  honeycomb-appearance, 
it  is  sufficient  proof  that  either  there  was  too  much  glue  allowed 
to  remain  between  the  two  pieces  of  wood,  or  that  the  joint  was 
not  true,  resulting  in  the  same  effect.  The  common  practice 
of  rubbing  the  joints  together  after  applying  the  glue  is  beneficial 
only  in  that  it  works  out  of  the  joint  the  excess  of  glue  or  air. 
It  may  safely  be  said  that  the  dangers  of  " starving"  a  joint  are 
mostly  illusionary,  but  the  dangers  of  "  overfeeding "  it  are 
always  imminent.  If  the  amount  applied  is  adequate,  it  is  vir- 
tually impossible  to  rub  or  squeeze  out  so  much  that  the  joint  is 
weakened  thereby. 

The  Temperature  of  the  Wood  When  Glue  is  Applied. — When 
a  warm  glue  solution  is  applied  to  a  cold  piece  of  wood,  the  layer 
of  glue  immediately  adjacent  to  the  wood  will  be  chilled  to  the 
setting  point  before  it  can  properly  penetrate  the  pores  of  the 
joint.  This  must  result  in  a  joint  considerably  weaker  than  the 
maximum  obtainable.  The  wood  should  be  warm,  but  not  hot. 
Excessive  heating  will  expel  more  water  than  is  desirable,  and 
tend  to  warp  the  wood,  making  a  true  joint  difficult  to  obtain. 
For  this  same  reason  the  temperature  of  the  glue  room  should 
be  as  high  as  is  consistent  with  the  comfort  of  the  workmen,  and 
a  brief  warming  of  the  wood  just  prior  to  the  joining  will  be 
sufficient. 

The  Rapidity  of  Handling. — A  condition  is  observable  in 
some  plants  where,  on  account  of  ignorance  or  indifference  of  the 
results  incurred,  an  unnecessary  lapse  of  time  is  allowed  between 
the  application  of  the  glue  and  the  placing  of  the  joint  under 
pressure.  In  such  cases  the  glue  has  chilled  and  set  to  a  jelly 
before  the  pressure  is  applied.  In  order  to  understand  the 
detrimental  effect  of  such  a  practice  it  must  be  emphasized  that 
the  influence  of  pressure  is  only  to  squeeze  out  the  excess  of  glue, 
and  to  minimize  the  inequalities  in  the  perfection  of  the  joint. 
By  permitting  the  glue  to  gel  before  applying  the  pressure  is  to 
rob  the  pressure  of  its  usual  effect.  A  jelly  will  not  readily  be 
squeezed  out.  The  result  is  a  joint  which  contains  an  excess  of 


APPLICATIONS  OF  GLUE 


523 


glue  between  the  two  sections,  and,  as  shown  in  a  previous 
paragraph,  this  very  materially  weakens  the  joint.  The  pres- 
sure should  always  be  applied  as  quickly  as  is  consistent  with 
good  workmanship  after  the  spreading  of  the  glue. 

The  Pressure  Applied  to  the  Joint,  and  its  Uniformity  of 
Distribution. — So  far  as  the  author  is  aware,  the  only  carefully 
conducted  tests  that  have  been  reported  upon  the  relation  of  the 
joining  pressure  to  the  strength  of  the  joint  are  those  described 
by  Gill1  in  1915,  and  the  investigations  of  the  author2  in  1920. 


3200 
3000 
2&00 
2600 
2400 
2200 
2000 
1800 
1600 
1400 
1200 
1000 
800 


Average  Strength  of  Wood 


FIG. 


Joining  Pressure 
102. — The  effect  of  joining  pressure  upon  strength. 


Gill  employed  a  joining  pressure  varying  between  10  and  100 
pounds  to  the  square  inch,  and  reported  that  "a  pressure  of  30 
pounds  to  the  square  inch  gave  a  joint  about  15  per  cent  stronger 
than  either  the  10  or  100  pound  pressure."  The  data  obtained 
for  the  tests  at  the  different  pressures  are  not  given,  but  the 
variation  between  the  maximum  and  minimum  values  obtained 
is  very  great — averaging  in  the  neighborhood  of  200  per  cent. 
This  being  the  case  it  seems  difficult  to  regard  the  15  per  cent 
variation  reported  for  different  pressures  as  significant. 

Experiments  which  have  been  conducted  in  the  author's 
laboratory  have  shown  that  high  pressures  are  not  only  harmless, 
but  are  decidedly  beneficial  from  the  standpoint  of  the  ultimate 
strength  developed.  Joining  pressures  varying  from  10  to  1,400 

1  A.  H.  GILL,  J.  Ind.  Eng.  Chem.,  7  (1915),  102. 

2  R.  H.  BOGUE,  Chem.  Met.  Eng.,  23  (1920),  200. 


524  GELATIN  AND  GLUE 

pounds  to  the  square  inch  were  used.  These  pressures  were 
obtained  by  placing  the  joints,  very  carefully  prepared  in  accord- 
ance with  the  outline  on  page  530,  between  the  parallel  plates 
of  an  Olsen  Universal  Testing  Machine.  A  ball-bearing  was  used 
both  above  and  below  the  steel  plates  to  insure  a  perfectly  uni- 
form distribution  of  the  pressure.  Previous  tests  made  by 
using  clamps  had  shown  the  impossibility  of  obtaining  either  a 
constant,  or  a  uniformly  distributed,  pressure  by  that  means. 

The  data  show  the  strength  of  the  joint  to  rise  rapidly  with 
increasing  joining  pressure  between  10  and  200  pounds  to  the 
square  inch.  Above  the  latter  pressure  the  strength  continued 
to  increase  slightly  to  1,000  pounds,  and  thereafter  remained 
constant  to  1,400  pounds.  This  is  illustrated  by  the  curves  in 
Fig.  102. 

In  the  light  of  the  frequent  assertions  made  that  "too  much 
pressure  must  not  be  used  in  gluing  surfaced  wood,  as  the  glue 
may  be  pressed  out  too  completely  from  the  joint,  producing  a  so- 
called  starved  joint,"1  a  consideration  of  the  meaning  of  these 
data  may  not  be  out  of  place.  The  author  has  repeatedly 
emphasized  that  a  glue  is  not  a  cement.  Let  us  exaggerate  the 
conditions  for  a  moment  in  order  to  fix  our  point.  Suppose  that 
it  is  possible  to  make  a  joint  of  two  pieces  of  wood,  and  that  a 
large  enough  excess  of  glue  is  added  to  produce  a  layer  of  jelly 
an  eighth  of  an  inch  thick  between  the  two  pieces.  This  jelly, 
we  all  know,  is  two-thirds  or  more  water.  On  setting,  there- 
fore, the  water  is  evaporated  off,  leaving  the  one-third  or  less  of 
solid  glue  material.  The  evaporation  of  the  water  sets  up  strains 
and  stresses  in  the  material.  The  jelly  contracts,  draws  away 
from  the  surface,  and  becomes  contorted.  Such  stresses  may 
even  produce  cracks  in  the  dried  glue,  and  a  certain  degree  of 
porosity  may  be  observed.  Obviously  a  joint  so  made  would 
show  great  weakness.  This  is  an  absurd  case.  No  glue  man 
would  think  of  making  such  a  joint.  But  assume  that  the  jelly 
layer  is  reduced  to  a  fiftieth;  or  a  hundredth  of  an  inch.  Then 
we  have  conditions  that  are  common.  But  the  effects  outlined 
above  would  still  be  present,  and  in  exact  proportion  to  the 
thickness  of  that  layer.  As  the  layer  of  glue  between  the  wood 
pieces  is  made  vanishingly  small,  the  strength  of  the  joint  will 
approach  its  maximum  value.  The  bond  between  the  two 
pieces  of  wood  should  consist  only  of  the  vertical  and  interlaced 

1  National  Advisory  Committee  for  Aeronautics,  Report  66  (1920),  9. 


APPLICATIONS  OF  GLUE  525 

threads  of  glue  passing  from  the  pores  in  one  piece  to  the  pores 
of  the  other. 

If  this  is  the  case,  and  we  seem  justified  in  making  that  con- 
clusion, then  it  must  follow  that,  provided  the  glue  is  properly 
applied,  and  in  a  condition  such  that  a  fair  degree  of  penetration 
results,  there  can  be  no  such  an  effect  as  a  " starving"  of  a  joint, 
and  no  such  thing  as  too  much  pressure,  as  far  as  the  glue  is 
concerned.  The  advantageous  results  realized  by  high  pressures 
can  be  explained  only  upon  the  assumption  that  such  pressures 
are  necessary  to  squeeze  out  the  maximum  amount  of  excess  glue 
in  the  joint,  and  to  minimize  any  slight  inequalities  in  the 
perfection  of  the  joint. 

It  should  be  stated  that  the  crushing  strength  of  the  wood  used 
sets  a  very  definite  limit  to  the  pressure  which  may  be  applied 
with  safety  in  any  case.  The  porous  types  of  wood  will  be 
injured  by  pressures  which  have  no  effect  upon  the  denser 
varieties.  And  a  thick  piece  of  wood  will  stand  without 
injury  much  higher  pressures  than  could  be  used  in  thin  cut 
veneers. 

The  Time  under  Pressure  and  Time  of  Curing. — If  the 
pressure  is  removed  from  the  joint  before  the  glue  has  been  given 
a  sufficient  time  to  harden,  the  two  pieces  will  separate  slightly, 
resulting  in  a  weakened  joint,  and  if  the  joint  is  broken  before 
the  glue  has  thoroughly  dried  out,  the  strength  will  be  that  of  the 
partially  solidified  jelly,  rather  than  of  the  completely  cured  glue. 
The  author  finds  that  the  pressure  should  be  maintained  for 
about  10  to  12  hours,  and  the  curing  for  about  seven  days,  in 
order  to  obtain  the  maximum  strength. 

The  Temperature  and  Humidity  During  Curing. — The  ra- 
pidity and  thoroughness  of  drying  out  depend  in  large  measure 
upon  the  temperature  and  humidity  of  the  curing  room.  A  low 
temperature  and  a  high  humidity  make  it  almost  impossible  to 
obtain  a  perfectly  cured  joint.  The  glue,  under  these  conditions, 
remains  in  the  form  of  a  partially  dried  jelly,  but  does  not  com- 
pletely dry  out.  The  opposite  conditions  of  a  warm  temperature 
and  low  humidity  are  most  favorable,  both  to  the  rapidity  and 
thoroughness  of  the  curing. 

Some  glue  consumers  are  in  the  habit  of  heating  the  joint, 
after  it  has  been  completed,  to  a  rather  high  temperature  for 
several  hours.  This  practice  has  no  advantages,  as  it  keeps  the 
glue  in  the  liquid  condition  for  a  long  period,  and  the  resulting 


526  GELATIN  AND  GLUE 

hydrolysis  results  in  a  much  weakened  product.  Gill1  has  shown 
that  a  heating  of  the  glued  blocks  to  65°C.  for  several  hours  dim- 
inished the  strength  very  materially.  This  is  fully  corroborated 
by  the  author's  experience. 

The  Method  of  Breaking  the  Joint. — The  results  of  the  so- 
called  strength  test  are  usually  reported  in  pounds  per  square 
inch,  but  it  must  be  urged  that  the  method  by  which  the  break 
is  made  is  of  the  greatest  importance.  As  will  be  shown  in  the 
following  pages,  the  joints  are  made  and  broken  in  several  ways. 
The  joint  may  be  end  to  end  or  side  to  side,  and  it  may  be 
broken  by  pulling  apart,  by  shearing  apart,  or  by  applying  the 
load  at  the  joint  while  the  two  ends  are  supported.  The  results 
obtainable  vary  widely.  Very  few  results  by  different  methods 
are  rightly  comparable,  but  the  author  has  found  the  observed 
"strength"  per  square  inch  to  be  about  500  to  600  per  cent  higher 
when  blocks  of  4  square  inch  glued  area  were  sheared  apart  by 
pressure,  than  when  strips  of  1  square  inch  glued  area  were 
sheared  apart  by  pulling.  A  standard  procedure  should  be 
adopted,  and  the  method  of  the  Forest  Products  Laboratory2  at 
Madison,  Wisconsin,  appears  to  possess  the  greatest  advantages 
of  any  that  have  been  proposed. 

The  Hydrogen-ion  Concentration  of  the  Solution. — In  Chap. 
V  it  was  pointed  out  that  the  viscosity,  jelly  consistency, 
swelling,  and  practically  all  properties  of  a  gelatin  varied  accord- 
ing to  the  hydrogen-ion  concentration,  or  pH,  of  the  solution. 
A  number  of  tests  have  also  shown  that  the  adhesive  strength  of 
a  glue  varies  in  a  similar  way,  the  maximum  being  attained  at  pH 
values  in  the  neighborhood  of  3.5  and  7.5,  while  beyond  these 
limits  the  strength  drops  rapidly.  A  slight  diminution  in 
strength  is  also  found  to  follow  the  bringing  of  the  solution  to 
the  isoelectric  point — pH  4.7.  Sherrick3  has  observed  the  same 
tendency  in  the  adhesive  qualities  of  glues  prepared  for  hecto- 
graphic  plates. 

Methods  of  Making  the  Adhesive  Strength  Test.— On 
account  of  the  many  factors  which  enter  into  any  test  upon 
strength,  on  account  of  the  skillful  technique  necessary  in 
order  to  obtain  reliable  and  comparable  data,  and  on  account  of 
the  rather  elaborate  equipment  required  for  properly  making  the 

1  A.  H.  GILL,  loc.  ciL 

2  See  page  530. 

3  SHERRICK,  personal  communication. 


APPLICATIONS  OF  GLUE  527 

test,  the  strength  test  has  not  received  the  universal  attention 
in  glue  evaluation  that  might  have  been  expected.  In  lieu  of  the 
actual  strength  test  many  other  tests  upon  certain  properties, 
as  jelly  consistency,  viscosity,  and  melting  point,  and  analysis  for 
certain  chemical  groups,  as  a  gelatin,  /3  gelatin,  etc.,  have  been 
proposed  as  being  true  indications  of  the  real  strength  value. 
One  or  another  of  these  propositions  have  been  quite  generally 
accepted  by  the  glue  trade,  and  as  a  result  very  few  consumers 
include  an  actual  strength  determination  in  their  specifications. 
One  of  the  exceptions  to  this  has  been  the  specifications  of  the 
National  Advisory  Committee  for  Aeronautics,  for  glue  to  be 
used  on  airplanes,  but  the  tests  were  made  by  the  Service  at  the 
Forest  Products  Laboratory. 

The  methods  that  have  been  proposed  for  making  the  strength 
test  are  described  briefly  below. 

KahrsJ  Method. — Kahrs1  used  blocks  of  well-seasoned  oak, 
glued  end  to  end,  the  area  of  the  joint  being  either  1  square  inch, 
or  1.44  square  inches  (one  hundredth  part  of  1  square  foot). 
The  length  of  the  former  type  of  block  was  4J^  inches,  and  a 
shoulder  was  left  at  the  end  away  from  the  joint  to  impart 
greater  strength.  The  blocks  were  glued  together  after  a 
careful  weighing  and  measuring  and  varnishing,  placed  between 
clamps,  and  later  broken  apart.  For  this  purpose  holes  were 
bored  near  the  ends  of  the  blocks,  steel  pins  inserted,  and  by 
means  of  an  improvised  machine,  pulled  apart.  The  machine 
used  was  of  a  lever  type,  the  placing  of  weights  at  various  points 
on  the  lever  arm  exerting  a  measured  pull  at  the  joint.  The 
blocks  and  machine  are  illustrated  in  Fig.  103. 

Gill's  Method. — Gill2  first  made  briquettes  of  fuller's  earth, 
diatomaceous  earth,  quartz  sand,  and  sawdust,  using  glue  as  a 
binder.  These  were  dried  at  80°C.  for  6  days,  and  broken  by  the 
well-known  cement  briquette  testing  machine.  Difficulty  was 
experienced  in  drying  completely,  the  briquettes  were  full  of 
blow-holes,  and  no  consistent  results  could  be  obtained. 

Gill  then  used  rectangular  prismatic  maple  blocks,  sawed  into  a 
special  form  as  shown  in  Fig.  104,  and  having  an  end  area  of 
1  square  inch.  The  blocks  were  glued  end  to  end,  and  a  uniform 
pressure  secured  by  placing  in  a  frame  wherein  the  pressure 
employed  was  determined  by  placing  weights  on  a  lever  arm  bear- 

1  F.  KAHRS,  Glue  (1910),  No.  3,  p.  4;  No.  4,  p.  5;  No.  5,  p.  8;  No.  6,  p.  10. 

2  A.  H.  GILL,  loc.  cii. 


528 


GELATIN  AND  GLUE 


FIG.  103. — The  breaking  machine  and  test  pieces  used  by  Kahrs. 


APPLICATIONS  OF  GLUE 


529 


ing  upon  the  joint.  The  pressure  was  maintained  for  3  hours,  and 
the  joints  allowed  to  dry  at  room  temperature  for  2  to  4  days. 
They  were  then  pulled  apart  in  the  cement  testing  machine,  after 
the  ends  had  been  blocked  up  with  bronze  blocks  to  fit  the 
briquette  holder  of  the  machine. 


Machine  for  Drying  the  Glued 
Joints  Under  Pressure 


If- 

r,»-> 

1 

r'-' 

rf] 

c 

k 

& 

W      L 

T             ,~n 

>x 

\ 

J       u 

7          _ 

^Ai 

/          *^r 

'            -^5 

''                  '  —  -  —  ^ 

\o 

feA-  ^r 

/ 

3      r 

\ 

Testing  Glue 

/ 

1 

\     ^ 

f 

\     ^ 

1      ±_ 

A 

FIG.  104. — Blocks  and  apparatus  used  in  Gill's  strength  test. 

Rudeloff's  Method. — Rudeloff1  applied  the  glue  solution  to  the 
planed  end  surfaces  of  two  pieces  of  red  beech  wood,  185  mm. 
long,  125  mm.  broad,  and  50  mm.  thick,  and  placed  the  blocks 
so  that  the  glued  surfaces  crossed  at  right  angles.  A  definite 
joining  pressure  was  used,  and  the  force  required  to  tear  the  pieces 

1  M.  RUDELOFF,  Mitt,  kgl  Materialprufungsamt.,  36  (1918),  2;  37  (1919), 
33.     C/.  Chem.  Abstracts,  13  (1919),  1164;  14  (1920),  2429. 
34 


530 


GELATIN  AND  GLUE 


of  wood  apart  measured  by  means  of  a  suitable  machine.  He 
later  used  both  edge  and  flat  grain  joints  of  beech,  ash,  oak,  and 
fir,  using  50,  40,  33><j,  and  25  per  cent  glue  solutions  at  a  joining 
pressure  of  about  5  kg.  per  square  centimeter  (about  65  pounds 
per  square  inch).  Individual  tests  varied  as  widely  as  from 
63  to  102  kg.  per  square  centimeter. 

Forest  Products  Laboratory  Method. — The  method  developed 
by  the  Forest  Products  Laboratory1  at  Madison,  Wisconsin,  is  as 


>- -Spec/men 


FIG.  105. — The  shear  block  test  specimen  and  shearing  tool  used  by  The  Forest 
Products  Laboratory. 

follows:  Two  blocks  of  hard  maple  about  1  by  2J£  by  12  inches  in 
size  are  glued  together,  subjected  to  a  definite  pressure,  allowed  to 
dry  out  for  a  week,  and  cut  into  blocks  as  shown  in  Fig.  105. 
The  glued  area  of  each  block  is  exactly  4  square  inches.  One  of 
the  two  pieces  is  permitted  to  extend  a  half  inch  beyond  the 
other  to  facilitate  the  technique  and  prevent  a  slip  in  the  shear- 
ing process.  The  blocks  are  broken  by  placing  in  a  shearing 
1  National  Advisory  Committee  of  Aeronautics,  Report  66  (1920),  21 


APPLICATIONS  OF  GLUE 


531 


tool,  as  shown  in  the  figure,  in  which  pressure  is  applied  upon  the 
end  of  the  smaller  block  until  the  joint  separates.  This  pressure 
is  most  conveniently  obtained  by  the  use  of  a  Universal  Testing 
Machine,1  by  which  the  exact  pressure  at  the  breaking  point 
may  be  read  directly. 


'A\ 


m\\ 


'"  ..... 


T 


FIG.  106. — The  plywood  test  specimen  and  plywood  shearing  tool  used  by  The 
Forest  Products  Laboratory. 

'  If  the  break  occurs  entirely  in  the  glue,  a  measure  of  the 
strength  of  the  glued  joint  is  obtained,  but  if,  as  frequently 
happens,  the  break  takes  place  partly  or  wholly  in  the  wood,  the 
breaking  pressure  is  less  than  the  strength  of  the  glue,  and  in 
order  to  obtain  more  exact  data  the  test  must.be  repeated  with  a 
stronger  wood.  The  shearing  strength  of  hard  maple,  using 
similar  blocks,  is  about  3,200  pounds  per  square  inch,  so  the  exact 

1  The  shearing  tool  and  testing  machine  may  be  obtained  from  the  Riehle 
Brothers  Testing  Machine  Co.,  1424  N.  9th  St.,  Philadelphia,  or  The  Olsen 
Testing  Machine  Co.;  500  N.  12th  St.,  Philadelphia,  Pa. 


532  GELATIN  AND  GLUE 

strength  of  glue  which  is  stronger  than  this  cannot  be  measured 
upon  maple.  The  results  obtained  by  this  method  are  reasonably 
satisfactory,  and  by  taking  the  average  of  several  breaks  the  test 
becomes  entirely  practicable. 

An  additional  strength  test  is  made  upon  glues  that  are 
intended  for  plywood  manufacture.  The  form  is  the  one 
adopted  by  the  British  and  American  Governments  for  the 
testing  of  aircraft  plywood.  It  is  shown  in  Fig.  106.  A  piece  of 
the  plywood  3J^  inches  long  by  1  inch  wide  is  sawed  so  that  the 
inner  section  is  cut  through  from  opposite  sides  leaving  a  section 
of  1  square  inch.  This  is  then  placed  in  a  machine,  as  shown, 
designed  for  testing  cement,  but  provided  with  special  grips. 
The  section  is  subjected  to  tension  and  fails  principally  from 
shearing. 

Welmers'  Method. — As  an  illustration  of  the  way  in  which  the 
strength  test  may  be  employed  by  any  glue  shop  foreman,  and  as 
an  example  of  the  more  " advanced"  of  the  modern  glue  shop 
methods,  may  be  cited  the  suggestion  of  Welmers.1  He  glues 
two  pieces  of  wood  together,  measuring  1  by  8  inches,  edge  to 
edge,  so  that  a  right  angle  is  formed.  One  of  the  arms  of  this 
angle  is  fastened  vertically  into  a  rack  on  the  wall,  while  a  beam 
is  secured  to  the  horizontal  arm  as  shown  in  Fig.  107.  At  the 

end  of  the  beam  is  hung  a  pail 
into  which  water  is  allowed  to 
run  evenly  and  slowly.  The 
weight  of  the  pail  with  its  con- 
tents at  the  breaking  point  of 
the  joint  is  the  measure  of  the 
glue  strength.  Such  tests  as  this 
are,  of  course,  crude,  but  if 
made  with  proper  care  some 

FIG.  107. — Welmers'  adhesive  test. 

indication  of  the  relative  strength 

should  be  obtained.  It  is  at  least  a  beginning,  and  if  glue 
consumers  realized  the  advantages  of  such  tests,  it  would  not  be 
long  before  a  more  standard  and  a  more  accurate  procedure 
would  be  generally,  adopted. 

Heinemann's  Method. — A  different  type  of  adhesive  test  has 
been  used  by  Heinemann.2  He  suggests  that  a  determination  of 
the  minimum  strength  of  a  glue  solution  that  exhibits  a  definite 

1  J.  WELMERS,  Veneers,  14  (1920),  No.  11,  35. 

2  A.  HEINEMANN,  Chem.  Ztg.,  24  (1900),  871. 


APPLICATIONS  OF  GLUE 


533 


adhesive  power  on  paper  be  taken  as  the  measure  of  glue  strength. 
Strips  of  strong  smooth  paper  were  used;  one  brushed  over  with, 
say,  a  1  per  cent  solution,  another  strip  of  paper  laid  over  the 
first,  and  pressed  under  a  5  pound  weight  for  1  or  2  minutes.  The 
paper  is  then  taken  up,  and  it  is  observed  if  the  joint  will  support 
a  weight.  Heinemann  found  that  at  or  about  the  critical  degree 
of  dilution  the  joint  will  either  support  a  fairly  heavy  weight — 
even  as  much  as  30  pounds — or  scarcely  no  weight  at  all.  All 
measurements  are  referred  to  a  standard  for  evaluation. 


FIG.  108. — Weidenbusch's  strength  test. 

Other  Methods. — Setterberg1  soaked  strips  of  unsized  paper  in  a 
solution  of  glue  of  definite  concentration,  pressed  the  strips 
between  blotting  papers  to  remove  the  excess  of  glue,  and  when 
dry  broke  them  in  a  paper  testing  machine  by  tearing  tests. 

Weidenbusch2  prepared  exactly  equal  prismatic  rods  of  gyp- 
sum 9.2  cm.  long  with  the  sides  of  the  transverse  section  4  mm., 

1  SETTERBERG,  Cf.  POST,  Chem.  tech.  Rep.  (1911),  ii.,  857. 

2  WEIDENBUSCH,  Cf.  LUNGE  and  KEANE,  ibid.  (1911),  ii,  456. 


534  GELATIN  AND  GLUE 

and  having  a  weight  of  1.7  grams.  These  are  dipped  in  a  glue 
solution  for  5  minutes,  then  removed  and  allowed  to  dry.  The 
rod  is  then  placed  across  an  iron  ring  as  shown  in  Fig.  108,  and  a 
dish  hung  to  the  center  of  the  rod  into  which  weights  are  placed 
until  the  rod  breaks.  Weidenbusch  claims  to  have  obtained 
fairly  consistent  results  by  this  means,  but  Gill1  has  found  such 
methods  unsatisfactory. 

Hauseman2  first  employed  blocks  of  " biscuit- ware"  stone 
which  he  glued  together  and  subsequently  pulled  apart.  Not 
obtaining  satisfactory  results  by  this  means  he  next  used  pieces 
of  straight  grained  walnut  9  X  2  X  %  inches  in  size.  These 
strips  were  glued  so  that  4  square  inches  of  surface  were  in  contact, 
and  after  leaving  in  the  clamps  for  48  hours,  and  curing  for  a 
further  24  hour  period,  they  were  sheared  apart. 

The  Selection  of  Glues]  for  Joint  and  Panel  Work.— 
Although  no  set  rules  can  be  laid  down  for  the  selection  of  a  glue 
for  a  given  purpose,  yet  a  few  general  considerations  should  be 
borne  in  mind.  Too  often  a  consumer  of  glue  fails  to  recognize 
the  wide  differences  that  exist  in  the  several  grades  on  the 
market.  What  he  really  wants  is  the  lowest  priced  glue  that 
will  do  his  work  satisfactorily,  but  his  lack  of  knowledge  upon 
the  subject  leads  him  to  believe  oftentimes  that  price  is  the  sole 
factor  worth  attention,  and  his  work  suffers  accordingly. 

On  recognizing  his  own  ignorance  upon  glue  he  may  put  his 
problems  up  to  the  glue  salesman  and  rely  upon  him  to  provide 
the  proper  material.  Although  most  large  glue  houses  employ 
competent  and  honest  salesmen  to  handle  and  distribute  their 
product,  yet  it  must  be  understood  that  not  all  glue  sales- 
men are  experts  in  glue  engineering,  nor  are  all  salesmen  scru- 
pulously conscientious  in  giving  the  buyer  the  best  for  his  money. 
In  short,  a  consumer  who  relies  entirely  upon  the  salesman  is 
placing  himself  in  a  position  where  he  may  be  sold  more  expen- 
sive glue  than  he  needs;  an  improper  type  of  glue  for  the  service 
desired;  a  poor  glue  for  the  price  of  a  good  material;  or  other 
glues  fraudulently  described.  No  injustice  is  meant  to  be  done 
the  many  reputable  and  competent  dealers  and  salesmen,  but 
not  all  of  these  are  honest,  and  not  all  that  are  honest  are  com- 
petent. The  only  safe  recourse  for  the  consumer  to  follow  is  to 

1  A.  H.  GILL,  /.  Ind.  Eng.  Chem.,  7  (1915),  102. 

2  P.  A.  HAUSEMAN,  ibid.,  9  (1917),  359. 


APPLICATIONS  OF  GLUE  535 

make  or  have  made  proper  tests  upon  all  material  bought,  and 
ascertain  that  the  shipment  is  as  per  specifications. 

The  belief  that  the  price  should  be  the  determining  factor  in 
glue  purchases  should  be  dispelled.  A  higher  priced  glue  should, 
as  has  already  been  shown,  have  a  much  greater  covering  capacity 
than  a  cheaper  product,  and  at  the  same  time  produce  much 
stronger  joints.  These  factors  must  be  properly  balanced  if 
glues  are  to  be  selected  efficiently.  Under  no  circumstances 
should  a  partially  decomposed  glue,  as  revealed  by  the  disagree- 
able putrid  odor,  be  accepted. 

For  high  class  joint  work  only  a  good  grade  of  hide  glue  is 
used.  Although  some  of  the  best  bone  glues  may  give  as  good 
results  as  an  intermediate  or  low  grade  of  hide  glue,  it  is  rarely 
that  large  amounts  are  diverted  to  such  usage.  Glues  that  have 
been  given  a  fictitious  viscosity,  as  by  the  addition  of  alum,  should 
be  regarded  with  suspicion  until  they  have  been  proven  satis- 
factory. An  extraordinarily  high  viscosity  should  be  compared 
with  the  jelly  test,  and  if  the  former  is  found  to  be  incommen- 
surate with  the  latter  a  test  should  be  made  for  ash  and  alum. 
An  unusually  clear  glue  with  high  viscosity  will  often  be  found  to 
have  been  clarified  with  alum.  The  use  of  glues  with  such 
abnormal  viscosities  is  not  ordinarily  attended  with  the  best 
results,  for  either  the  glue  will  be  diluted  to  the  consistency 
at  which  it  is  easily  applied,  and  in  that  case  contain  actually  too 
small  an  amount  of  real  glue  to  make  a  strong  joint,  or  else  it  will 
be  applied  at  the  usual  percentage  of  dilution,  and  in  that  case 
be  too  thick  to  easily  penetrate  the  pores  of  the  wood,  and  fail 
from  that  reason.  If,  however,  the  high  viscosity  is  accompanied 
by  a  similarly  high  jelly  strength,  then  such  a  dilution  that  will 
bring  the  material  to  a  good  working  condition  will  not  result 
in  a  weakened  joint. 

An  abnormally  high  jelly  strength  for  the  viscosity  is  also 
undesirable  as  a  rule  on  account  of  the  rapidity  with  which  such 
a  glue  will  set.  If  the  glue  sets  before  the  joining  operation  is 
completed  and  the  pressure  applied,  the  joint  will  be  weak. 
In  using  such  glues  it  is  necessary  that  the  wood  be  heated  to  a 
higher  temperature,  or  that  more  rapid  operation  be  employed. 

The  hydrogen-ion  concentration,  or  the  degree  of  acidity  or 
alkalinity,  should  be  taken  into  consideration,  as  an  excessively 
alkaline  glue,  pH  9.0  or  higher,  is  usually  indicative  of  overlimed 
stock  which  will  continue  to  weaken  even  after  applying  to  the 


536  GELATIN  AND  GLUE 

joint,  and  an  excessively  acid  glue,  pH  3.0  or  less,  may  affect  and 
weaken  the  fiber  of  the  wood.  Beween  these  values,  however, 
there  are  not  likely  to  be  any  ill  effects  noted.  The  region  of 
pH  4.7  shoud  be  avoided,  as  the  viscosity,  swelling,  strength, 
etc.,  of  the  glue  are  least  at  this  point,  but  this  may  easily  be 
corrected  by  the  consumer  by  the  addition  of  small  amounts  of 
acid  or  alkali.  The  regions  of  pH  3.5  or  8.0  are  ordinarily  most 
favorable. 

If  the  glue  is  to  be  applied  by  hand  the  foam  need  not  be  con- 
sidered. If  spread  by  machine,  excessive  foaming  is  undesirable. 
This  is  ordinarily  depressed  by  the  addition  of  a  little  fatty 
matter,  but  if  too  large  amounts  of  grease  are  present  a  weakening 
of  the  joint  will  result. 

For  veneer  work  a  lower  grade  of  glue  may  be  employed,  but  a 
higher  viscosity  is  desirable.  This  latter  is  necessary  on  account 
of  the  tendency  of  a  thin  liquid  to  penetrate  the  pores  of  the  thin 
sheet  of  wood  and  show  itself  on  the  opposite  surface.  The 
higher  viscosity  of  the  liquid  used  tends  to  offset  this  tendency, 
and  may  be  secured  either  by  using  a  higher  concentration  of 
the  glue,  or  by  employing  a  glue  of  a  higher  viscosity.  Since  an 
exceptionally  high  grade  is  not  necessary,  the  desired  results  are 
very  well  obtained  by  using  a  rather  high  grade  of  bone  glue 
and  at  a  somewhat  greater  concentration.  This  gives  the  higher 
viscosity  necessary  without  increasing  the  cost.  Where  pene- 
tration does  not  occur,  or  when  it  makes  no  difference  if  such 
does  take  place,  a  thinner  liquid  may  be  used. 

The  Application  of  Glue  as  an  Adhesive. — In  addition  to 
the  use  of  glue  in  the  wood-joining  trades,  the  adhesive 
properties  of  the  material  are  made  use  of  in  a  number  of  other 
industries. 

The  weakest  grades  of  glues,  either  hide  or  bone,  are  usually 
suitable  for  the  making  of  paper  boxes  and  cartons.  Only  those 
glues  that  are  in  a  state  of  decomposition  may  be  regarded  as 
unfit.  Of  course,  a  certain  amount  of  adhesive  strength  is 
required,  but  since  the  strength  necessary  is  only  that  of  the 
paper  or  cardboard  used  in  the  box,  it  is  rarely  that  an  animal 
glue  of  the  lowest  grade  will  not  be  sufficiently  strong.  Because 
no  great  strength  is  required  for  this  service,  other  substitutes 
for  glue  have  to  a  great  extent  replaced  the  animal  product  for 
this  class  of  work.  Sodium  silicate,  or  water  glass,  is  greatly 
used,  especially  in  corrugated  fiber  boards,  and  cheap  starch 


APPLICATIONS  OF  GLUE  537 

and  dextrin  glues,  as  well  as  the  lower  grades  of  liquid  fish  glues, 
are  abundantly  employed. 

For  bookbinding  Zaensdorf1  recommends  that  while  pastes 
should  be  employed  for  morocco,  calf,  russia  and  vellum,  all 
leather  with  an  artificial  grain  should  be  glued,  as  a  greater- 
body  is  imparted  to  the  leather  by  the  glue  and  the  grain  is 
preserved.  Cloth  bindings  are  dipped  in  glue  and  turned  in  at 
once.  For  each  special  type  of  cover,  as  velvet,  silk,  etc.,  the 
procedure  used  is  varied  to  suit  the  conditions.  A  rather  high  or 
medium  grade  of  hide  glue  is  usually  employed  for  this  work,  and 
it  should  be  clear  in  most  cases.  For  fastening  the  sections  of 
the  book  together  at  the  back  a  less  expensive  glue  may  be 
employed,  bone  glue  being  generally  used.  It  is  applied  with  a 
brush,  and  in  Germany  is  worked  in  with  a  special  hammer  and 
the  excess  taken  off  with  a  brush. 

A  very  satisfactory  paste  for  fastening  paper,  leather,  etc., 
is  made  by  dissolving  4  parts  of  good  glue  in  80  parts  of 
water,  and  pouring  this  mixture  into  a  solution  of  starch  made 
by  dissolving  30  parts  of  starch  in  200  parts  of  cold  water  and 
warming. 

A  leather-metal  adhesive  is  prepared  by  digesting  a  small  amount 
of  powdered  nutgalls  in  8  parts  of  water  for  6  hours ;  then  dissolv- 
ing 1  part  of  glue  in  8  parts  of  water.  The  leather  is  moistened 
with  the  nutgall  preparation,  and  the  glue  applied  to  the  metal 
which  has  just  been  roughened  and  heated.  The  leather  is 
placed  on  the  metal  and  dried  under  pressure.  It  is  said  the 
leather  will  split  before  the  joint  will  give  way. 

Waterproof  glue  can  be  made  by  the  addition  of  a  solution  of 
potassium  bichromate  or  calcium  chromate  to  a  strong  glue 
solution.  The  glue  is  then  applied  to  whatever  service  is  desired, 
and  after  geling  is  exposed  to  the  direct  light  of  the  sun.  This 
results  in  a  change  in  the  composition  of  the  mixture2  by  which 
the  mass  becomes  insoluble  and  waterproof.  Such  a  glue  may 
be  used  for  waterproofing  fabrics,  canvas,  awnings,  roofing 
papers,  etc.  It  has  been  used  as  a  cement  for  joining  crockery 
and  sealing  cover  slides.  Its  use  in  photography  is  described 
elsewhere.3 

Tannin  is  also  used  in  connection  with  glue  in  waterproofing 

1  ZAENSDORF,  "Art  of  Bookbinding,"  93. 

2  See  page  573. 

3  See  page  571. 


538  GELATIN  AND  GLUE 

fabrics.  The  goods  usually  are  dipped  alternately  into  a  very 
dilute  solution  of  glue  and  then  into  a  moderately  strong  solution 
of  tannin.  After  each  dipping  the  fabric  is  pressed  between 
rollers,  and  allowed  to  dry  for  some  time,  then  the  process 
repeated  until  the  mesh  of  the  goods  is  quite  lost  in  the  insoluble 
material  surrounding  them. 

Glue  rendered  insoluble  by  the  addition  of  formaldehyde  may 
also  be  used  for  waterproofing,  but  the  technical  difficulties  of 
handling  the  material  have  greatly  limited  such  use. 

Rubber  cements  are  of  various  types,  but  very  good  ones  are  in 
use  that  are  made  of  mixtures  containing  glue,  admixed  with 
glycerin,  chloroform,  and  water.  The  chloroform  renders  the 
product  permanent  against  decomposition.  A  mixture  of  glue 
with  sulphur,  barium  sulphate,  alum,  collodion,  and  sulphuric, 
acetic,  nitric,  and  formic  acids  has  been  patented  as  a  "gelatinous 
resilient  composition."  The  use  of  acids  is  objectionable,  how- 
ever, as  the  product  will  weaken  due  to  hydrolysis,  and  if  the 
cement  is  brought  in  contact  with  metal  a  corrosive  action  will 
take  place. 

Frosted  glass  is  a  form  of  glass  that  is  much  used  in  the  doors 
and  partitioning  windows  of  offices  where  privacy  is  desired. 
It  is  made  by  allowing  a  glue  or  gelatin  to  rapidly  dry  out  upon  a 
plate  of  ordinary  rather  thick  glass.  As  the  glue  loses  its  mois- 
ture it  contracts,  and  the  power  of  the  gelatin  is  so  great  it  tears 
away  the  surface  of  the  glass  itself,  chipping  it  into  characteristic 
fern-like  patterns.  The  general  appearance  of  the  design  can 
be  modified  by  varying  the  properties  of  the  glue  used;  i.e.,  a 
brittle  glue  will  give  a  different  pattern  than  a  tough  glue,  and 
the  addition  of  salts  also  modifies  the  patterns.  A  strong  gelatin 
solution  containing  6  per  cent  of  alum  gives  exceptionally  fine 
designs. 

3.  GLUE  AS  A  SIZING  AGENT 

The  Sizing  of  Paper. — Glue  is  caused  to  serve  for  two  dis- 
tinct purposes  in  the  manufacture  of  watt  paper.  It  is  employed 
as  a  binder  for  the  clay,  paris-white,  or  other  material  with  which 
the  papers  are  grounded,  and  also  as  a  sizing  agent  for  the  ground 
colors,  and  in  some  cases  for  the  top  colors.  The  latter  are, 
however,  more  commonly  applied  with  a  dextrin  or  starch 
mixture.  Casein  has  at  times  been  employed  for  the  ground 


APPLICATIONS  OF  GLUE  539 

size,  but  its  normally  high  price  and  more  difficult  working 
properties  have  prevented  any  great  substitution  of  this  material 
for  glue.  Other  attempts  to  rind  a  suitable  size  for  the  paper 
industry  have  been  made,  but  without  notable  success.  Parti- 
ally saponified  rosin  mixed  with  some  forms  of  starch  has  been 
tried  but  the  failure  of  these  mixtures  to  give  uniform  results 
with  different  colors  has  retarded  their  adoption. 

In  the  selection  of  glues  that  will  be  satisfactory  for  use  upon 
wall  papers  the  greatest  diversity  of  possibilities  which  may  make 
a  given  glue  excellent  or  worthless  for  the  purpose  present  them- 
selves. In  fact,  for  no  other  service  for  which  glue  is  employed 
is  the  exact  nature  of  the  materials,  both  of  the  color  bases  and 
the  glue  itself,  so  necessary  to  take  into  careful  consideration. 
The  importance  of  doing  so,  and  the  type  of  result  that  would 
obtain  from  a  failure  to  do  this,  is  best  shown  by  a  consideration 
of  the  nature  of  the  colors  that  are  used  on  wall  papers. 

Soluble  dyes  cannot  be  employed  directly  as  they  would  pene- 
trate the  pores  of  the  paper  and  spread.  Insoluble  lakes  would 
require  incorporation  with  water  before  mixing  with  the  glue 
solution,  and  mixing  machinery  would  then  be  required  to  be 
installed  at  the  plant.  The  most  general  practice,  therefore,  is 
to  precipitate  .the  color  directly  upon  an  insoluble  base  such  as 
finely  divided  barium  sulphate,  draw  off  the  precipitated  mass  after 
setting,  wash  to  free  it  of  excess  of  precipitant  or  reagent,  and 
then  separate  from  the  excess  of  water  by  running  through  a 
filter  press  or  a  centrifugal  hydro-extractor.  This  treatment 
leaves  the  color  material  as  a  heavy  insoluble  paste,  sometimes 
spoken  of  as  pulp  color,  and  in  this  form  is  easily  incorporated 
with  the  glue  solution  in  the  preparation  of  the  sized  material. 

The  precipitation  of  the  color  upon  the  insoluble  base  may  be 
performed  either  by  causing  the  precipitation  of  the  base  and 
the  color  simultaneously,  or  by  precipitating  the  color  upon  the 
base  suspended  in  the  solution.  The  former  are  productive  of 
the  best  results  but  are  somewhat  more  expensive  in  preparation. 
The  latter  type  may  be  made  by  suspending  the  base,  as  barium 
sulphate,  in  a  water  solution  of  one  of  the  color  reagents,  as 
for  example,  potassium  bichromate  for  the  preparation  of  Lemon 
Yellow,  and  adding  the  precipitant,  in  this  case  lead  acetate. 
A  pulp  color  so  produced  is  weak  in  coloring  power  and  not  so 
firmly  fixed  as  the  other  type. 

As  an  example  of  the  simultaneously  precipitated  variety, 


540  GELATIN  AND  GLUE 

Bordeaux  eosine  is  prepared  by  adding  to  an  aqueous  solution  of 
eosine,  first  a  solution  of  sodium  carbonate,  followed  by  one  of 
barium  chloride.  The  latter  solution  precipitates  the  color  and 
at  the  same  time  reacts  with  the  sodium  carbonate  to  form  in- 
soluble barium  carbonate.  A  solution  of  magnesium  chloride 
is  then  added  and  this  is  followed  by  one  of  sodium  hydroxide. 
These  react  to  form  insoluble  magnesium  hydroxide.  The  color 
is  in  this  way  much  more  intimately  admixed  with  the  precipi- 
tates than  could  be  possible  if  the  latter  were  suspended  in  the 
solution  prior  to  the  precipitation  of  the  color. 

In  the  preparation  of  some  pulp  colors  a  number  of  chemicals 
are  employed  in  order  that  the  exact  shade  of  color  desired  may 
be  produced.  The  viscosity  of  reagents  employed,  and  the 
frequent  failure  to  wash  out  completely  the  excess  of  precipitant 
or  reagent,  makes  the  proper  selection  of  glue  for  service  with 
them  a  matter  of  concern.  And  even  the  pulp  colors  produced 
as  above  described  are  frequently  loaded  down  to  an  astonishing 
extent  with  further  additions  of  heavy  chemicals.  Aluminum 
sulphate,  lead  acetate,  barium  chloride  and  other  chemicals 
are  regularly  added.  As  an  example  may  be  cited  a  formula  for 
Bremen  Blue: 

POUNDS  POUNDS 

Sodium  carbonate 163     Barium  sulphate 50 

Aluminum  sulphate 200     Malachite  green 5 

Barium  chloride 350     Tannic  acid 8 

It  is  intended,  of  course,  that  there  shall  be  no  excess  of  any 
reagent  left,  after  the  washing,  in  a  soluble  condition  in  the 
mixture,  but  this  desideratum  is  rarely  attained  in  practice, 
especially  as  cheapness  is  one  of  the  major  requirements  for  these 
pulp  colors  when  used  upon  the  general  run  of  wall  paper.  And 
any  excess  of  any  one  of  the  above  chemicals  may  be  expected  to 
affect  the  glue  in  one  way  or  another.  The  aluminum  sulphate 
and  the  tannic  acid  especially  would  seriously  influence  the 
properties  of  a  glue,  causing  a  streakiness  or  "livering"  of  the 
sized  mass. 

The  presence  of  the  large  excess  of  sodium  carbonate  which  is 
introduced  into  most  pulp  colors  is  usually  more  than  sufficient  to 
counteract  any  acidity  that  may  be  introduced  through  the  glue. 
Except  in  unusual  instances  the  acidity  or  alkalinity  of  a  glue 
will  not  be  excessive,  but  after  mixing  with  the  color  base  the 


APPLICATIONS  OF  GLUE  541 

mixture  will  usually  be  slightly  alkaline.  It  should  be  pre- 
viously determined,  in  such  cases,  if  this  change  in  reaction  of  an 
acid  glue  produces  any  concomitant  change  in  the  properties 
that  will  be  deleterious.  If  no  such  buffer  is  present  the  influence 
of  slight  changes  in  hydrogen  ion  concentration  upon  the  colors 
should  be  observed,  and  the  selection  of  glue  made  accordingly, 
or  its  reaction  properly  adjusted  prior  to  use. 

The  presence  of  grease  in  small  amounts  is  probably  not 
deleterious  to  the  satisfactory  working  of  a  glue  in  wall  paper 
sizing.  There  is,  however,  a  difference  of  opinion  upon  this 
point.  Some  houses  employ  greasy  glues  constantly,  and  even 
add  more  oil  if  the  glue  foams  badly,  while  other  houses  regard 
the  presence  of  even  traces  of  grease  as  prohibitive.  Its  presence 
is  considered  by  its  advocates  to  brighten  the  colors  and  to 
produce  a  more  uniform  flow  of  the  color  material  from  the  rollers 
of  the  printing  machine,  in  addition  to  lessening  the  foam.  It  is 
argued,  on  the  other  hand,  that  grease  will  penetrate  the  paper 
producing  grease  spots,  and  the  customary  method  of  making  the 
test,  by  painting  strips  of  paper  with  the  glue  to  which  has  been 
added  a  little  dye,  produces  this  effect.  If,  however,  a  pulp 
color  such  as  is  actually  used  in  practice  were  mixed  with  the 
glue,  rather  than  the  straight  dye,  most  of  those  glues  which,  by 
the  former  method,  showed  the  presence  of  grease  would,  by  the 
latter  procedure,  show  no  trace  of  it.  The  reason  for  this  lies  in 
the  power  of  the  mineral  matter  of  the  pulp  color  to  absorb  the 
fatty  matter,  or  dissipate  it  until  its  effect  is  negatived.  If 
present  in  large  amounts,  however,  the  grease  may  actually 
react  with  some  constituent  of  the  pulp  color  resulting  in  the 
production  of  insoluble  metallic  soaps  which  would  interfere  with 
the  uniformity  of  the  color  on  the  paper. 

A  consideration  of  viscosity  in  the  selection  of  glues  for  wall 
paper  use  is  of  importance  on  account  of  the  mechanical  spreading 
of  the  mixture.  The  sized  colors  are  applied  from  a  roller  and  if 
the  flow  is  not  rapid  and  uniform  there  will  be  difficulty.  In 
general,  therefore,  the  lower  the  viscosity  the  better  will  be  the 
glue  for  this  service.  This  property  must  not,  however,  be 
carried  to  the  point  where  other  necessary  properties  are  lost. 
The  actual  binding  strength  need  not  be  great,  but  it  is  important 
that  the  material  have  some  strength.  For  the  clay  ground  work 
a  low  grade  of  bone  glue  is  usually  satisfactory,  but  for  the  color 
size  the  lower  grades  of  hide  glue  are  preferred.  It  is  very 


542  GELATIN  AND  GLUE 

desirable  that  the  glues  used  with  the  colors  be  clear,  as  any 
cloudiness  will  result  in  a  lessening  in  the  brilliance  of  the  dye. 

In  ordinary  sized  paper  the  glue  is  applied  in  one  of  two  ways. 
The  glue  is  either  put  into  the  beater  with  the  paper  pulp  previous 
to  making,  or  more  frequently,  as  that  process  is  very  wasteful, 
the  paper  is  run  through  a  dilute  bath  of  glue  just  before  drying. 
In  sized  paper  there  is  seldom  anything  used  with  the  glue  except 
at  times  a  little  alum  to  increase  the  body  and  give  the  paper 
a  somewhat  harder  finish.  Coated  paper  is  made  by  applying  a 
mixture  of  glue  and  various  pigments  or  fillers  about  the  con- 
sistency of  cream  to  the  paper  after  it  has  been  finished.  It  is 
usually  a  separate  operation  and  is  performed  on  a  separate 
machine.  This  coating,  unless  calendared  afterwards,  will  give  a 
matte  finish.  High  gloss  papers  are  usually  coated  papers. 

The  adaptability  of  a  glue  for  sizing  and  coating  is  dependent 
upon  exactly  the  same  factors  as  obtain  in  the  case  of  glues  for 
wall  papers.  Low  and  medium  grade  hide  glues  are  mostly 
employed,  although  high  grade  bone  glues  are  sometimes  used. 
Clearness  is  one  of  the  most  important  properties  which  glues 
for  this  purpose  should  possess.  If  the  glue  is  used  as  a  size  on 
white  paper,  the  absence  of  any  brownish  color  is  imperative. 
Casein  has  largely  replaced  glue  for  this  service  on  account  of  its 
whiteness,  but  glue  is  still  used  in  considerable  amounts.  Water- 
proof coatings  are  sometimes  applied  by  employing  a  glue  solu- 
tion to  which  has  been  added  potassium  bichromate  solution. 
The  coated  paper  is  then  exposed  to  the  influence  of  sunlight 
which  renders  the  paper  practically  water-proof. 

The  Sizing  of  Textiles. — Glue  and  gelatin  find  extensive  use 
in  the  manufacture  of  wool,  silk,  and  other  fabrics,  both  as  a  size 
to  be  mixed  with  the  dye,  and  as  a  finishing  size  for  the  whole. 
For  service  in  textile  manufacture  the  purity  of  the  glue  is  of  the 
utmost  importance.  The  most  objectionable  impurity  which 
must  be  avoided  is  sulphur  dioxide  or  sulphites  left  in  the  glue 
from  the  bleaching  process.  And  yet  very  clear  glues  only  may 
be  used  as  dull  glues  would  dim  the  brilliance  of  the  color. 

The  colors  employed  for  dyeing  fabrics  are  much  more  delicate 
than  those  used  in  paper,  and  are  usually  soluble.  For  this 
reason  the  presence  of  traces  of  mineral  acids  or  alkalies  must  be 
ascertained  and  their  exact  effect  on  the  colors  to  be  employed 
noted.  Wools  dyed  black  or  any  other  dark  color  are  dyed 
with  mixtures  containing  logwood  (haematoxylin  and  haematin) 


APPLICATIONS  OF  GLUE  543 

extracts  containing  glucose.  These  may  be  badly  spotted  if  the 
glue  used  contains  any  notable  proportion  of  mineral  acids  or 
sulphites. 

This  applies  to  an  even  greater  degree  with  silks  as  even  more 
delicate  colors  and  mixtures  of  colors  are  used.  For  example  a 
pale  blue  color  is  produced  on  silk  by  dyeing  with  a  fast  red  in 
combination  with  a  blue  and  a  violet.  Sulphur  dioxide  will 
react  with  the  blue  and  the  violet,  discharging  those  colors, 
and  leaving  the  red  unaffected.  The  cloth  appears  then  to  be 
covered  with  red  spots.  Since  a  large  number  of  the  shades 
upon  silk  are  produced  by  combining  a  number  of  colors  it  follows 
that  if  any  constituent  of  the  glue  affects  any  one  of  these  colors, 
a  change  either  in  the  shade  or  the  nature  of  the  color  is  affected, 
spoiling  the  goods.  On  the  other  hand  if  unbleached  glues  are 
used,  the  brilliance  so  much  desired  upon  silks  will  be  lost. 

Black  silks  are  often  treated  with  a  mixture  of  oil,  acid,  soda, 
and  glue  as  a  finishing  process  to  dispel  the  fuzziness  resulting 
from  the  drying  process.  The  color  of  the  glue  is  not  of  great 
importance  here,  but  it  must  be  clear.  Gelatin  or  clear  dark 
brown  glues  are  frequently  used  for  this  purpose. 

Glue  is  used  on  cotton  goods  and  on  cotton  batting  to  stiffen 
the  material.  The  sizing  solution  is  applied  at  one  end  of  a 
machine,  and  the  goods  are  expected  to  emerge,  after  passing 
through  a  drying  chamber  heated  to  200°F.,  in  a  dry  condition. 
There  is  difficulty  in  obtaining  exactly  the  correct  consistency  of 
glue  for  this  purpose.  If  the  latter  is  too  thin  it  will  penetrate 
the  pores  of  the  cotton  fiber  to  such  a  degree  that  the  latter  will  be 
altogether  too  stiff  to  use,  while  if  it  is  too  viscous  it  will  not  be 
absorbed  at  all,  and  will  fail  to  dry  out  during  its  passage  through 
the  drying  chamber.  To  obtain  the  desired  results  it  is  common 
practice  to  treat  a  very  dilute  solution  of  a  medium  grade  clear 
glue  with  a  solution  of  alum.  The  alum  thickens  the  solution 
and  is  satisfactory,  provided  no  precipitation  results.  Many 
glues  are,  however,  precipitated  with  alum,  and  unless  a  given 
glue  is  found  by  preliminary  test  to  remain  clear  for  several  hours 
at  least  after  the  addition  of  alum  it  should  not  be  subjected  to 
such  treatment  in  the  plant. 

Carpets,  tapestries,  burlap  wall  coverings,  curtains,  etc.,  are  all 
heavily  sized  with  glues.  Where  stiffness  is  desired  as  in  window 
curtains  a  strong  high  grade  glue  is  used.  Otherwise  dilute 
solutions  of  low  grade  hide  or  bone  glues  are  employed. 


544  GELATIN  AND  GLUE 

All  straws  used  in  the  manufacture  of  hats  are  heavily  sized. 
In  this  case  a  product  that  is  more  or  less  resistant  to  the  action 
of  water  is  desired,  and  certain  hardening  agents  are  commonly 
added  to  the  glue  solution,  prior  to  its  application,  for  this 
purpose.  Formaldehyde,  potassium  bichromate,  tannin,  and 
alum  are  employed.  The  glue  used  must  be  very  clear  and  light 
colored  or  the  straw  appears  yellow  and  "shop  worn."  A  final 
bleaching  is  often  given  the  material,  after  applying  to  the  straw, 
by  the  use  of  sulphur  dioxide,  hydrogen  peroxide,  or  oxalic 
acid. 

The  Sizing  of  Wooden  Containers. — Barrels  and  casks  that 
are  to  be  used  as  containers  for  liquids  are  sized  with  some 
material  before  use,  as  otherwise  the  liquid  would  penetrate  the 
wood  and  be  lost,  besides  resulting  in  a  decay  of  the  wood. 
Where  the  liquid  to  be  stored  is  of  an  aqueous  type,  glue  is  not 
well  adapted  on  account  of  the  softening  and  swelling  it  under- 
goes in  water,  but  for  the  sizing  of  oil-containing  barrels  it 
serves  very  well.  A  first  treatment  is  given  to  fill  all  of  the 
cracks  and  imperfections,  and  a  second  to  size  the  whole  inner 
surface.  A  few  quarts  of  the  glue  in  solution  are  introduced  into 
each  barrel  and  steam  applied  under  a  low  pressure  to  force  the 
solution  well  into  the  pores  of  the  wood.  The  barrels  are  rotated 
and  finally  drained  while  still  hot. 

Glue  as  a  Size  in  Paints  and  Calsomine. — A  large  amount 
of  glue  is  diverted  to  the  painters'  trade.  It  is  employed  both 
as  a  size  for  the  treatment  of  walls  prior  to  the  application  of 
paint,  merely  to  fill  up  the  pores  of  the  wall,  for  which  work  a 
cheap  low  grade  of  bone  glue  is  satisfactory,  or  it  may  be  mixed 
with  a  little  paint,  an  insoluble  base,  and  water,  in  the  prepara- 
tion of  a  calsomine.  For  the  latter  service  glues  that  have  been 
rendered  opaque  by  the  addition  of  some  white  filler  as  calcium 
carbonate,  zinc  oxide,  or  lead  carbonate  have  been  most  used. 
In  the  higher  grades  of  these  calsomines  which  must  be  used  with 
hot  water  high  grade  glues  are  used,  but  for  the  cheaper  cold 
water  products  low  grades  are  employed. 

Prepared  sizes  are  obtainable  which  contain  mixtures  of  glue 
with  chalk,  clay,  whiting,  or  some  other  material.  These  are 
made  ready  for  use  by  the  addition  of  boiling  water.  In  the 
preparation  of  such  mixtures  it .  must  previously  have  been 
ascertained  that  no  residual  salts  are  contained  in  the  glue  that 
might  react  with  any  constituent  of  the  mixture.  Glues  of  low 


APPLICATIONS  OF  GLUE  545 

viscosity    are    preferable    provided    that   sufficient   strength    is 
developed  to  bind  the  basic  material  of  the  size. 

4.  GLUE  AS  A  BINDING  AGENT 

A  considerable  quantity  of  glue  is  used  in  the  manufacture 
of  matches.  The  pieces  of  wood  are  dipped  into  a  mixture 
containing  the  several  ingredients  for  the  ignition  of  the  head 
upon  ''striking,"  together  with  a  porous  material  such  as  plas- 
ter of  paris,  ground  glass  or  sand,  and  glue  to  serve  as  a  bin- 
der and  to  secure  portions  of  the  mass  to  the  head  of  the 
match.  The  igniting  constituents  may  include  a  sesqui-sul- 
phide  of  phosphorus,  elementary  phosphorus,  potassium 
chlorate,  lead  oxides,  manganese  dioxide,  and  other  chemicals. 
Since  phosphorus  is  easily  oxidized  by  the  oxygen  of  the  air  the 
glue  must  also  be  sufficiently  strong  to  form  a  firm  film  about 
the  mass  to  exclude  the  air,  and  no  chances  may  be  taken  of  the 
rubbing  off  of  this  film  in  the  handling  of  the  product.  This 
latter  obligation  requires  the  use  of  a  moderately  strong  glue, 
which  also  should  be  rather  viscous.  The  admixture  with  the 
chemicals  mentioned  necessitates  the  use  of  a  product  free  from 
any  material  which  might  react  with  any  of  the  constituents  of 
the  mixture,  or  which  could  possibly  oxidize  the  phosphorus. 
Low  test  bone  glues  have,  however,  been  occasionally  used. 

Sand  papers  and  Emory  papers  are  also  large  consumers  of  a 
high  grade  of  hide  glue.  The  demand  is  primarily  for  glues  of 
the  highest  attainable  viscosity,  but  a  strong  jelly  strength  is  also 
deemed  important.  The  paper  is  passed  between  rollers  which 
supply  a  rather  concentrated  solution  of  the  glue  to  the  upper 
surface,  and  the  sand  or  emery  (garnet,  carborundum,  alundum, 
etc.)  of  the  selected  size  of  grain  is  sprinkled  profusely  upon  it. 
The  excess  of  sand  is  shaken  off  and  the  paper  then  passed 
slowly  along  to  a  second  set  of  rollers  which  it  reaches  in  about  a 
half  hour.  In  passing  through  this  second  set  it  is  again  treated 
with  a  layer  of  the  same  glue,  but  of  a  weaker  concentration, 
which  binds  the  sand  firmly  that  it  may  not  become  easily 
loosened.  The  paper  is  then  passed  slowly  over  heated  pipes 
for  about  an  hour  and  then  wound  into  rolls,  or  cut  to  appropriate 
sizes  and  baled. 

Glue  is  also  used  to  a  limited  extent  as  a  binder  for  abrasive 
wheels,  only  the  highest  grades  being  considered  sufficiently 
reliable  for  this  purpose. 

35 


546  GELATIN  AND  GLUE 

Artificial  leather  contains  glue  as  an  important  constituent. 
In  one  process  Italian  hemp  and  coarse  cleaned  wool  are  finely 
cut  and  carded  together,  being  formed  into  wadding.  This  is 
felted  by  packing  in  linen  and  treatment  with  hot  acid  vapors. 
The  product  is  washed,  dried,  and  impregnated  with  a  mixture 
the  composition  of  which  varies  according  to  the  type  of  material 
to  be  produced.  For  imitating  a  sole  leather  the  following 
mixture  is  made :  50  parts  of  boiled  linseed  oil  are  mixed  with  20 
parts  of  calophony,  25  parts  of  French  turpentine,  10  parts  of 
glycerin,  and  10  parts  of  vegetable  wax,  and  the  whole  heated  on 
a  water  bath  with  a  little  ammonium  hydroxide.  When  the 
mass  has  become  homogeneous,  25  parts  of  a  very  high  grade 
glue  soaked  in  an  equal  weight  of  water,  and  a  casein  solution 
made  by  dissolving  50  parts  of  moist  freshly  precipitated  casein 
in  a  saturated  solution  of  16  parts  borax,  and  adding  10  parts  of 
potassium  bichromate,  are  added,  and  the  mixture  boiled  until 
it  becomes  sticky.  After  impregnating  the  felt  in  this  mixture 
it  is  allowed  to  dry  for  24  hours,  then  laid  in  a  solution  of 
aluminum  acetate,  dried,  and  finally  pressed  between  hot  rollers. 

Moulding  material  is  made  from  a  mixture  of  basic  lead  car- 
bonate (white  lead),  calcium  sulphate,  sawdust,  and  glue,  and  is 
used  extensively  in  making  moldings  for  picture  frames,  mirror 
frames,  rosettes,  etc.  Other  plastic  substances  such  as  dolls, 
toys,  ornaments,  and  the  like  are  made  of  materials  in  which 
glue  is  an  important  constituent.  A  very  hard  horny  substance 
is  made  of  50  parts  glue,  35  parts  rosin  or  wax,  15  parts  of  glycerin, 
and  30  parts  of  zinc  oxide  or  other  earthy  material.  By  increas- 
ing the  glycerin  and  lowering  the  rosin  content,  a  softer  material 
results. 

Billiard  balls  are  made  by  dissolving  90  parts  of  glue  in  a 
minimum  of  water,  and  adding  5  parts  of  barium  sulphate,  4 
parts  of  calcium  carbonate,  and  1  part  of  linseed  oil.  A  small 
ball  is  formed  and  allowed  to  dry.  This  is  then  made  larger  by 
adding  another  layer  and  again  drying.  This  process  is  repeated 
until  the  ball  is  as  large  as  desired.  This  is  allowed  to  dry  for  3 
months  and  then  turned.  The  finished  ball  is  immersed  in  a 
solution  of  aluminum  sulphate  for  an  hour,  dried,  and  polished 
for  use. 

Composition  cork  employes  glue  or  gelatin  as  the  binder.  The 
cork  is  first  softened,  then  dried  and  ground  to  the  size  desired, 
this  being  usually  about  %G  of  an  inch  in  diameter.  The  ground 


APPLICATIONS  OF  GLUE  547 

cork  is  then  warmed  and  mixed  in  a  machine  with  a  glue-glycerin 
mixture  for  about  30  minutes,  or  until  the  latter  is  thoroughly 
incorporated  into  the  cork.  At  the  end  of  the  mixing  a  little 
paraformaldehyde  is  added  and  well  stirred  in.  The  mass  so 
prepared  is  placed  in  a  cold  room  to  be  drawn  upon  as  needed. 

The  glue  or  gelatin  used  is  usually  of  a  high  grade.  The  glue 
mixture  is  prepared  by  dissolving  from  40  to  60  parts  of  the 
glue  or  gelatin  in  from  60  to  40  parts  respectively  of  a  mixture 
consisting  of  about  55  per  cent  glycerin  and  45  per  cent  water. 
The  amount  of  paraformaldehyde  added  at  the  end  of  the  mixing 
is  slightly  less  than  1  per  cent  of  the  weight  of  dry  glue  or  gelatin 
used.  This  serves  both  as  a  hardening  agent  to  render  the 
product  water  resistant,  and  as  an  antiseptic.  From  10  to  30 
per  cent  by  weight  of  the  glue-glycerin  binder  is  incorporated 
into  the  cork. 

One  of  the  principal  uses  of  this  cork  mixture  is  in  the  manufac- 
ture of  disks  for  sealing  foods  preserved  in  glass  jars.  For  this 
purpose  the  mixture  is  stuffed  into  iron  cylinders,  baked  at 
100°C.,  for  3  or  4  hours,  then  forced  out  by  rods,  and  cut  with 
circular  knives  into  thin  disks.  These  are  immersed  in  paraffin, 
centrifuged,  and  cooled. 

Thick  cork  sheets  are  also  made  of  the  mixture  for  use  as 
gaskets,  washers,  and  insulation  between  steel  plates. 

An  imitation  of  hard  rubber  is  prepared  by  mixing  a  thick  solu- 
tion of  glue  with  a  solution  of  sodium  tungstate  in  hydrochloric 
acid.  A  heavy  precipitate  forms  of  gelatin  tungstate  which  is 
said  to  be  sufficiently  pliable  and  elastic  at  a  temperature  of 
86  to  104°F.,  to  permit  of  being  molded,  or  drawn  into  sheets. 
On  cooling,  it  becomes  hard  and  brittle,  but  again  becomes  soft 
on  heating. 

5.  GLUE  AS  A  COLLOIDAL  GEL 

Printing  rollers  are  .made  up  with  a  combination  of  a  high  grade 
of  hide  glue  and  glycerin,  or  some  other  substance  added  to  give 
the  product  elasticity  and  springiness.  A  composition  used  by 
one  large  plant  is  made  by  soaking  16  parts  of  gelatin  in  the  same 
weight  of  water,  and  after  melting  adding  24  parts  of  linseed  oil 
at  65°C.  After  these  are  well  mixed  20  parts  of  molasses  and 
1  part  of  calcium  chloride  are  stirred  in,  and  the  whole  digested 
at  90°C.,  for  three  hours.  For  especially  tough  compositions, 


548 


GELATIN  AND  GLUE 


a  mixture  of  2  parts  of  rosin  dissolved  by  warming  in  linseed  oil 
is  added  to  the  above.  The  substitution  of  bismuth  carbonate 
for  calcium  chloride  is  said  to  make  the  product  non-absorbent 
of  water.  Another  composition  is  made  by  dissolving  32  pounds 
of  gelatin  and  4  pounds  of  glue  in  water,  and  adding  4  pounds  of 
glucose,  72  pounds  of  glycerin,  and  1  ounce  of  methylated 
spirit.  After  digesting  for  4  to  6  hours  it  is  cast  into  rollers. 
This  composition  is  said  to  be  unaffected  by  temperature,  to 
retain  its  elasticity,  and  not  to  shrink.  Formaldehyde  is  fre- 
quently added  to  assist  in  keeping  the  roller  firm  and  insoluble 
in  water. 

Dawidowsky1  gives  the  following  formulas  as  expressive  of  the 
types  of  material  entering  into  the  composition  of  printing 
rollers. 

TABLE  61. — THE  COMPOSITION  OF  PRINTING  ROLLERS 


Glue 

8 

10 

4 

2 

32 

2 

1 

3 

]Molasses 

1? 

8 

1 

^?, 

6 

2 

8 

Paris  white 

1 

1 

Sugar                                          .      .  . 

10 

Glycerin 

12 

56 

Isinglass     

1U 

India  rubber  in  naphtha        .     ... 

10 

Hectograph  plates  are  manufactured  by  causing  a  glue  or 
gelatin-water-glycerin  mixture  to  adhere  to  oiled  paper  or  cloth. 
The  latter  in  widths  varying  between  8%  and  20  inches,  is 
allowed  to  unwind  from  the  roller  which  dips  into  a  mixture  of 
the  glue  solution.  After  passing  between  another  set  of  rollers 
adjusted  so  as  to  leave  a  uniform  rather  thick  (^{Q  inch)  layer 
of  the  glue  upon  the  paper  or  cloth,  the  latter  is  cut  into  lengths 
of  about  14J^  feet  and  transferred  to  shelves  for  drying.  From 
2  to  6  hours  are  allowed  for  the  sheets  to  become  sufficiently  dry. 
They  are  then  spread  on  the  under  side  with  oil  or  upon  the 
jelly  side  with  talcum  powder  to  prevent  sticking  upon  rolling, 
and  spindles  fastened  to  each  end  of  the  sheets  by  means  of 
cloth  strips.  The  sheets  are  then  rolled  upon  the  spindle  and 
are  ready  for  service. 

Steinitzer2  gives  the   following  formulas  as  representative  of 

1  DAWIDOWSKY,  "Glue,  Gelatin,  etc."  (1905),  157. 

2  F.  STEINITZER,  Kunststoffe,  4  (1914),  161. 


APPLICATIONS  OF  GLUE 


549 


the  composition  of  the  glue  mixtures  that  are  commonly  success- 
fully employed : 


TABLE  62. — COMPOSITION  OF  HECTOGRAPH  PLATES 


Glue  or  gelatin 

15 

10  0 

25 

10 

Water  

20 

40  0 

50 

20 

Glycerin,  30°Be      .  . 

60 

50  0 

90 

50 

Kaolin,  kieselguhr  etc 

2  5 

10 

10 

Sugar  

10 

In  addition  to  the  above  ingredients  a  small  amount  of  some 
hardening  agent  as  formaldehyde,  alum,  potassium  bichromate, 
or  tannin,  is  usually  added,  and  a  little  salicylic  acid  or  other 
preservative  may  be  introduced,  althougn^Eemitzer  claims~lhe 
latter  is  not  necessary  as  the  ^due-glycerin  mixture  is  in  itself 
resistant  to  bacterial  or  other  decomposition. 

In  general,  only  the  high  grades  of  hide  glue  or  gelatin  are  used^ 
for  hectograph  plates. 

Artificial  silk  has  been  prepared  which  consists  entirely  of  a 
very  high  grade  of  glue  or  gelatin,  treated  with  potassium  bichro- 
mate to  render  the  product  insoluble,  pliable,  and  firm.  The 
solution  is  forced  through  fine  holes  by  a  strong  pressure,  and 
the  resulting  threads,  after  being  rendered  insoluble  by  exposure 
to  the  sun,  are  dyed  if  desired,  dried,  and  woven  into  cloth. 
Water  makes  the  product  limp  but  it  again  becomes  firm  upon 
drying.  ^ 

Artificial  ivory  has  in  the  last  few  years  come  into"very  popular 
use.  It  is  made  essentially  of  the  constituents  which  have  been 
found  to  exist  in  the  natural  material.  The  following  formula  is 
illustrative  of  the  method  of  manufacture:  300  parts  of  calcium 
oxide  are  treated  with  sufficient  water  to  convert  it  into  calcium 
hydroxide,  but  before  the  reaction  is  complete  75  parts  of  aqueous 
phosphoric  acid  are  added,  and  this  followed  by  16  parts  of 
ground  calcium  carbonate,  2  parts  of  magnesium  oxide,  5  parts 
of  aluminum  oxide,  and  15  parts  of  a  high  grade  glue  or  gelatin 
dissolved  in  20  parts  of  water.  After  a  very  intimate  mixing 
the  mass  is  allowed  to  stand  for  a  day,  and  then  pressed  into  the 
desired  form  in  molds  and  dried  in  a  current  of  air  at  65°C. 
About  3  or  4  weeks  of  curing  are  then  necessary  before  the  product 
is  perfectly  hard. 


550  GELATIN  AND  GLUE 

Gelatin  veneers  is  a  name  given  to  preparations  of  glue  with 
earthy  material  by  which  imitations  of  marble,  mother  of  pearl, 
lapis  lazuli,  malachite,  ivory,  tortoise  shell,  travertine,  etc., 
may  be  made  by  a  proper  coating  of  a  board  or  stone  or  piece  of 
glass  or  crockery  with  the  "veneer"  desired.  The  marble  or 
glass  is  made  of  the  shape  required,  and  glass  surfaces  should  be 
ground.  All  surfaces  are  chalked  and  rubbed  dry. 

For  imitating  marbles  and  enamels,  a  glue  solution  made  from  a 
high  grade  of  glue  and  containing  finely  ground  zinc  oxide  is 
poured  upon  the  prepared  surface,  placed  in  a  horizontal  position. 
The  design  required  is  traced  with  a  glass  rod,  and  glue  solutions 
that  have  been  colored  by  the  appropriate  dyes  are  poured 
on  to  conform  with  the  design.  If  the  color  design  is  to  be 
blended,  the  solutions  should  not  be  too  thick,  and  should  be 
poured  on  in  quick  succession,  after  which  they  are  caused  to 
intermingle  as  desired  by  the  use  of  a  glass  rod.  Imitation 
malachite  is  prepared  in  the  same  manner,  except  that  the  glues 
used  are  colored  various  shades  of  green. 

For  mother  of  pearl  a  small  amount  of  silver  bronze  is  mixed 
with  the  clear  glue  solution,  and  traces  of  anilin  colors,  especially 
those  producing  fluorescent  colorations,  are  added  to  different 
portions  of  the  glue.  Portions  are  poured  successively  into  a 
prepared  glass  plate  and  the  pattern  produced  by  deft  manipula- 
tion of  a  comb  or  other  instrument. 

The  layers  of  imitation  marble  or  other  material  prepared  as 
above  are  allowed  to  dry  for  a  few  hours  and  then  placed  down- 
ward upon  another  set  of  plates  prepared  in  the  same  manner 
except  that  only  clear  colorless  gelatin  is  used  in  the  second  set. 
These  are  allowed  to  remain  for  a  few  hours  after  which  time  the 
first  plate  is  removed  by  lifting  off  from  the  lower  layer,  loosened 
by  a  knife.  The  veneer  is  then  thoroughly  dried  by  placing  the 
other  plate  in  a  drying  room  and  when  hard  is  removed  by  care- 
fully detaching  the  gelatin  with  a  knife.  The  veneers,  now 
free  of  glass,  are  trimmed  and  may  be  applied  to  any  article  by 
cementing  with  clear  glue  in  the  usual  way.  Exceedingly  fine 
imitations  have  been  made  by  this  procedure. 

6.  GLUE  AS  A  PROTECTIVE  COLLOID 

A  high  grade  of  glue  or  gelatin  has  often  been  found  to  be  of 
great  value  in  electro-metallurgic  processes  for  the  production  of 


APPLICATIONS  OF  GLUE  551 

smooth  hard  deposits  in  electrolysis.  Betts1  found  that 
in  the  refining  of  silver  from  solutions  of  silver  dithionate 
(Ag2S206.H2S2O6);  the  deposit  was  greatly  improved  by  the 
addition  to  the  bath  of  one  part  of  gelatin  or  of  gum  arabic  to  / 

120,000  parts  of  the  silver  solution.  By  employing  a  solution  -  iit  (_ 
of  silver  methyl  sulphate  the  addition  of  one  part  of  gelatin  to 
12,000  parts  of  solution  influenced  the  deposit  favorably.  Kern2 
reports  favorable  results  in  the  deposition  of  tin  from  all  electro- 
lytes when  gelatin  is  present  to  the  extent  of  one  gram  in  500  c.c. 
of  solution.  Miiller  and  Bahntje3  found  that  gelatin  and  egg 
albumin  improved  the  electrolytic  deposition  of  copper  from 
solutions  of  acid  copper  sulphate.  By  using  a  current  density  of 
0.0033  ampere  per  square  centimeter  gelatin  produced  numerous 
vertical  bands  or  stripes  "just  as  though  molten  metal  had 
flowed  down  the  plate,  solidifying  on  its  way."  By  increasing 
the  current  density  the  bands  became  wider  until  at  0.035  ampere 
per  square  centimeter  brilliant  homogeneous  deposits  of  high 
reflecting  power  were  obtained.  The  action  of  gelatin  as  a  pro- 
tective colloid  preventing  crystallization  is  offered  as  the  explana- 
tion of  its  favorable  action. 

The  knowledge  that  the  addition  of  certain  colloidal  substances, 
or  even  some  non-colloids,  to  metal  electro-depositing  solu- 
tions will  prevent  the  spongy,  branching  forms  and  make  for 
smooth  hard  deposition  is  not  a  recent  discovery,  but  has  been 
applied  for  a  long  time.  The  explanation,  however,  is  even 
yet  incomplete.  Gelatin,  for  example,  when  added  to  a  solution 
of  lead  fluosilicate,  which  ordinarily  deposits  a  loose  mass  of 
lead  crystals,  will  cause  a  solid  deposition,  while  gum  arabic  and 
many  other  colloids  have  no  effect  whatsoever.  Gum  arabic, 
on  the  other  hand,  is  somewhat  more  effective  than  gelatin  in 
improving  the  deposition  of  silver  from  solutions  of  silver  dithion- 
ate. Some  non-colloids  are  as  effective  as  colloids,  while  most , 
of  such  are  ineffective.  Pyrogallol  and  resorcin,  for  example,  are 
nearly  as  effective  as  gelatin  in  the  lead  deposition  mentioned, 
while  carbon  disulphide  is  most  favorable  in  the  case  of  silver 
methyl  sulphate.  In  the  presence  of  nitric  acid,  gelatin  no  longex- 
exerts  an  appreciable  influence,  but  in  other  acids  it  functions 
properly.  This,  taken  with  the  effectiveness  of  pyrogallol  and 

1  A.  BETTS,  Trans.  Am.  Electrochem.  Soc.,  8  (1905),  121. 

2  E.  KERN,  ibid.,  33  (1918),  155. 

3E.  MULLER  and  P.  BAHNTJE,  Z.  Electrochem.,  12  (1906),  317. 


552  GELATIN  AND  GLUE 

resorcin,  seem  to  indicate  that  the  reducing  action  of  these  sub- 
stances determines  their  value  in  this  connection,  and,  as  soon  as 
this  reducing  action  is  destroyed,  the  effectiveness  of  the  added 
substance  is  lost.  Betts1  is  convinced  that  a  difference  in  the 
electromotive  force  set  up  in  the  hollows  and  on  the  ridges  of  an 
electro-deposit,  due  to  the  former  enclosing  a  weaker  solution 
than  is  adjacent  to  the  projecting  points,  accounts  for  a  rough 
deposit,  while  the  addition  of  gelatin  would  lessen  this  difference 
and  so  make  for  a  smooth  deposition. 

A  somewhat  simpler  explanation  is  offered  by  Bancroft.2  He 
states  that  "other  conditions  being  the  same  we  shall  get  the 
smallest  crystals,  the  greater  the  potential  difference  between 
the  metal  and  the  solution.  The  addition  of  glue  or  similar 
substances  to  a  solution  tends  to  make  precipitates  come  down  in 
a  colloidal  form.  Following  out  this  analogy  we  should  expect  to 
find  that  addition  of  glue  or  similar  substances  to  an  electrolytic 
bath  would  decrease  the  size  of  the  metal  crystals,  the  limiting 
concentration  being  that  at  which  the  added  substance  causes  a 
bad  deposit  owing  to  its  chemical  properties."  Bancroft  found 
that  the  addition  of  10  grams  of  glue  per  liter  of  acidified  copper 
sulphate  solution  improved  the  quality  of  the  deposit. 

In  rubber  glue  is  finding  extensive  application.  It  seems  that 
glue  is  capable  of  replacing  the  rubber  to  the  extent  of  10  to  20 
per  cent  without  any  very  appreciable  loss  in  the  desirable  quali- 
ties of  the  product.  The  glue  may  be  brought  into  solution  in  a 
minimum  of  water  to  which  a  small  amount  of  formaldehyde  has 
been  added  to  prevent  decay.  The  water  is  then  evaporated  off 
until  a  very  thick  solution  results.  A  quantity  of  rubber  equal  to 
that  of  the  dry  glue  is  then  mixed  into  the  mass  and  the  mixture 
again  evaporated  until  practically  free  from  .water.  This  mate- 
rial is  then  added  in  whatever  quantity  is  desired  to  a  batch  of 
rubber,  but  if  more  than  20  per  cent  of  the  rubber  is  replaced  by 
glue  the  product  begins  to  show  loss  in  strength  and  elasticity. 

This  use  of  glue  is  of  especial  significance  since  the  very  lowest 
grades  appear  to  be  nearly  equal  in  value  for  this  purpose  to  the 
higher  grades.  The  nearly  anhydrous  finely  powdered  glue 
made  by  drying  slowly  on  steam-heated  rollers  has  recently  come 
into  extensive  use  for  this  purpose.  When  this  material  is  used 
it  is  mixed  directly  into  the  rubber  without  first  being  brought 

1  A.  BETTS,  Trans.  Am.  Electrochem.  Soc.,  8  (1905),  53. 

2  W.  D.  BANCROFT,  ibid.,  6  (1904),  27. 


APPLICATIONS  OF  GLUE  553 

into  an  aqueous  solution,  which  offers  a  decided  advantage  in 
plant  practice. 

Technical  comparisons  of  the  several  effects  of  different  ingre- 
dients going  into  the  manufacture  of  rubber  have  been  presented 
by  Wiegand.1  These  include  carbon  black,  lamp  black,  china 
clay,  red  oxide,  zinc  oxide,  glue,  lithophone,  whiting,  fossil  flour, 
and  barytes.  These  were  added  to  a  basic  mixture  consisting  of 
100  parts  of  fine  Para  rubber,  30  parts  litharge,  and  5  parts 
sulphur  by  weight.  The  effect  of  these  substances  on  the 
stress-strain  curve  (elongation  plotted  against  breaking  load  in 
grams  per  square  millimeter)  showed  barytes  to  have  no  effect 
whatever,  except  as  a  dilutent,  while  carbon  black,  at  the  opposite 
extreme,  showed  a  downward  progression  of  the  curves  towards 
the  load  axis  indicating  greater  and  greater  toughness,  without 
sensibly  altering  the  breaking  tensile  strength.  Glue  is  about 
intermediate  between  these  two.  Up  to  20  volumes  there 
was  a  definite  displacement  of  the  curve  indicating  that  glue  is 
not  a  mere  dilutent,  like  barytes,  but  exerts  a  definite  stiffening 
or  toughening  action.  The  tensile  strength  at  break  is,  however, 
lowered. 

Wiegand  furthermore  makes  the  assumption  that  the  energy 
absorption  in  all  cases  may  be  represented  by  the  area  contained 
between  the  stress-strain  curve  and  the  elongation  axis.  By 
measuring  this  area  with  a  planimeter  and  calculating  the  results 
to  foot-pounds  per  cubic  inch  of  original  stock,  he  finds  that  the 
basic  mixture  "stored  up"  450  foot-pounds.  The  addition  of 
barytes,  fossil  flour,  glue,  whiting,  and  red  oxide  all  constantly 
diminished  the  energy  content  with  increasing  amount  added. 
China  clay  produced  a  slight  increase,  zinc  oxide  and  lamp  black 
showed  marked  increases,  and  carbon  black  up  to  25  volumes 
increased  the  energy  content  up  to  nearly  150  per  cent  of  its 
original  value. 

The  results  obtained  in  both  cases  point  to  a  concise  parallelism 
between  the  specific  surfaces,  or  size  of  the  individual  particles, 
and  the  effects  produced.  The  surface,  in  square  inches  per 
cubic  inch,  developed,  for  example,  by  barytes  was  found  by 
direct  measurement  to  be  30,480;  for  glue  152,400;  and  for  carbon 
black,  1,905,000. 

The  above  data  may  be  summed  up  in  the  following  table  of 
Wiegand : 

1  W.  B.  WIEGAND,  Can.  Chem.  /.,  4  (1920),  160. 


554  GELATIN  AND  GLUE 

TABLE  63. — EFFECTS  OF  VARIOUS  SUBSTANCES  ON  RUBBER 


Substance  added 

Apparent 
surface 

Displace- 
ment of 
stress-strain 
curve 

Total  energy 
of  resilience 

Volume  in- 
crease at 
200  per  cent 
elongation, 
per  cent 

Carbon  black  
Lamp  black  
China  clay 

1,905,000 
1,524,000 
304,800 

42 
41 

38 

640 
480 
405 

1.46 
1.76 

Red  oxide  
Zinc  oxide 

152,400 
152,400 

29 
25 

355 
530 

1.9 

0.8 

Glue....  
Lithophone  
Whiting 

152,400 
101,600 
60,950 

23 
17 

344 
410 

4.6 

Fossil  flour 

50,800 

14 

365 

3  5 

Barytes  

30,480 

8 

360 

13.3 

Basic  mixture  

450 

In  acid  pickle  baths  which  are  employed  for  cleaning  and  remov- 
ing scale  from  iron  and  steel,  especially  just  prior  to  galvanizing, 
glue  has  been  found  of  value.  It  apparently  greatly  lessens  the 
consumption  of  acid,  while  giving  results  quite  as  satisfactory 
from  the  point  of  view  of  the  cleansing  action  obtained  as  the 
untreated  solution. 

Foamite  is  a  substance  employed  in  connection  with  the  carbon 
dioxide  generating  fire  extinguishers  for  use  especially  on  oil 
fires.  A  small  amount  of  a  foamy  glue  is  added  to  the  solution 
in  the  tank,  and  the  carbon  dioxide,  formed  by  the  customary 
action  of  the  acid  on  the  carbonate  results  in  the  production  of 
a  heavy  foam.  This  foam  settles  upon  the  oil  and  is  not  dissi- 
pated with  anything  like  the  rapidity  •  that  would  be  the  case 
were  the  carbon  dioxide  not  entangled  in  films  of  glue.  The 
material  is  in  consequence  especially  efficacious  for  such  service. 

India  ink  consists  of  very  finely  divided  carbon  suspended  in 
water  containing  a  little  glue  or  gelatin,  or  some  other  colloidal 
material.  The  pure  finely  divided  lamp  black  is  made  into  a 
thick  paste  with  a  dilute  solution  of  the  glue  containing  a  little 
musk  or  ambergris,  and  then  moulded  and  dried.  In  order  to 
make  the  ink  stand  up  better  in  service,  i.e.,  to  prevent  a  blotting 
upon  passing  a  damp  brush  over  the  lines,  about  1  per  cent  of 
potassium  bichromate  may  be  added  in  the  dry  finely  powdered 
state  to  the  paste. 


CHAPTER  XII 
THE  USES  AND  APPLICATIONS  OF  GELATIN 

A    wound    is    glued  together 

by  myrrh,  incense,  and  gum. 

Celsus  (about  200  A.D.) 

PAGE 

I.  Edible  Gelatin 556 

1.  Gelatin  as  a  Food 556 

2.  Gelatin  as  a  Protective  Colloid  in  Dietetics 558 

The  Infant  and  Invalid  Dietary 558 

Gelatin  in  Ice  Cream  and  Other  Food  Products 561 

3.  Gelatin  as  an  Emulsifying  Agent 567 

4.  Isinglass  and  Gelatin  in  Fining 569 

5.  Gelatin  in  Pharmaceutical  Preparations 569 

II.  Commercial  Gelatin 571 

1.  Gelatin  in  Photography  and  Photolithography 571 

2.  Gelatin  as  a  Bacterial  Culture  Medium 574 

3.  Gelatin  Cells  for  Ultrafiltration 575 

4.  Gelatin  in  Analytical  Procedures 576 

5.  Gelatin  as  a  Medium  for  Demonstrating  Colloidal  Phenomena..  576 

An  attempt  is  made  in  the  preceding  and  present  chapters  to 
distinguish  between  glue  and  gelatin  on  the  basis  of  refinement 
rather  than  upon  jelly  consistency,  viscosity,  or  any  other 
physical  property.  The  highest  grades  of  glue  are  by  physical 
tests  quite  the  equal  of  the  pure  gelatins,  and  many  uses  that  have 
commonly  been  attributed  to  gelatin  solely  on  account  of  the 
usually  higher  tests  of  the  latter,  are  in  this  text  credited  under 
glue.  By  gelatin  we  prefer  to  imply  a  material  that  has  either 
been  made  under  conditions  of  such  rigid  sanitary  specifications 
that  it  may  properly  be  classed  as  edible  or  medicinal,  or  else  a 
high  grade  of  glue  that  has  been  further  treated  so  as  to  produce 
a  product  that  is  very  close  to  a  chemically  pure  protein  gelatin. 
The  latter  may  not,  however,  be  " edible,"  but  is  utilized  for  the 
arts  requiring  a  highly  refined  product. 

The  most  important  uses  of  pure  gelatin  are  as  a  food  and 
protective  colloid  in  dietetics,  in  photographic  preparations,  and 
in  pharmaceutical  products. 

555 


556  GELATIN  AND  GLUE 

I.  EDIBLE  GELATIN 

1.  Gelatin  as  a  Food. — The  role  of  gelatin  in  animal  economy 
has  been  studied  by  a  number  of  able  physiologists  and  in  the 
light  of  their  findings  there  is  no  question  of  the  value  of  gelatin 
in  the  dietary.  We  are  not  however  permitted  to  regard  this 
delicacy  as  the  equivalent,  in  the  sparing  of  protein  tissue  in  the 
body,  of  the  combined  proteins,  such  as  are  found  in  milk, 
meat,  eggs,  etc. 

That  it  functions  as  a  true  food  seems  satisfactorily  proven, 
but  it  appears  to  be  incapable  of  supplying  more  than  about  a 
third  to  a  half  of  the  required  nitrogenous  matter  necessary  to 
maintain  a  nitrogen  equilibrium  in  the  body.  Thus  Kirchmann1 
reported  that  if  gelatin  is  included  in  the  diet  to  the  extent  of 
12  per  cent  of  the  required  energy,  the  decomposition  of  body 
protein,  or  the  requirement  of  other  proteins  necessary  for 
equilibrium,  is  lessened  by  27  per  cent,  but  further  increases  in 
the  administered  gelatin  failed  to  diminish  the  protein  katabolism 
proportionately.  For  example,  gelatin  to  the  extent  of  62 
per  cent  diminished  the  protein  decomposition  by  only  35  per 
cent.  This  was  the  maximum  obtained.  Up  to  12  per  cent 
practically  all  of  the  gelatin  was  absorbed,  traces  only  being 
found  in  the  faeces. 

Krummacher2  expresses  his  results  in  a  slightly  different  form. 
He  found  that  the  protein  decomposition  in  dogs  during  gelatin 
feeding  was  62.6  per  cent  of  that  which  is  broken  down  during 
inanition.  In  the  average  man  the  amount  of  protein  which 
undergoes  katabolism  in  a  day  is  70  grams.  If  gelatin  is  given 
to  the  extent  of  its  maximum  effect,  assuming  the  same  rela- 
tionship for  man  as  for  dogs,  33  grams  of  gelatin  will  reduce  the 
katabolized  protein  to  56  grams,  or  33  grams  of  gelatin  will 
spare  14  grams  of  protein.  Krummacher  reports  the  heat  value 
of  gelatin  as  5.3676  Cal.,  and  upon  deducting  the  value  of  the 
unburned  products  in  the  urine  and  faeces  it  leaves  3.8835  Cal.  or 
72  per  cent  of  the  total  energy  available.  The  available  energ}- 
in  meat  protein  has  been  placed  at  74.9  per  cent  by  Rubner, 
which  is  but  little  greater  than  that  found  for  gelatin. 

Murlin3  observed  that  protein  nitrogen  might  be  replaced  by 

1  J.  KIRCHMANN,  Z.  BioL,  40  (1900),  54. 
2O.  KRUMMACHER,  Z.  BioL,  42  (1901),  242. 
3  J.  MURLIN,  Proc.  Am.  Phiol.  Soc.,  29  (1904). 


APPLICATIONS  OF  GELATIN  557 

gelatin  up  to  a  half  of  the  starvation  requirement,  while  as  much 
as  two  thirds  may  be  replaced  provided  carbohydrates  are 
present  in  such  amounts  as  to  provide  a  half  to  two  thirds  of  the 
total  calorific  requirement. 

Some  effort  has  been  directed  at  an  explanation  as  to  why 
gelatin  could  not  be  substituted  completely  for  other  types  of 
protein  in  animal  economy,  and  the  conclusions  of  these  studies 
indicate  that  its  failure  in  this  respect  lies  in  the. absence  of  certain 
specific  and  necessary  amino-acid  residues.  Thus  tyrosine, 
cystine,  and  tryptophane  are  practically  absent  in  gelatin. 
Kauffman1  adopted  this  explanation  with  such  confidence  that 
he  carried  on  a  series  of  experiments  to  test  the  point,  not  only 
upon  the  proverbially  unfortunate  dogs,  but  also  upon  himself. 
He  reports  that  gelatin  may  be  substituted  for  protein  normally 
to  the  extent  of  20  per  cent  without  harm,  but  that  "this  can  be 
exceeded  and  the  protein  completely  replaced  by  gelatin  if  the 
latter  is  mixed  with  tyrosine,  cystine,  and  tryptophane.  .  .  . 
Both  dogs,  however,  died."  An  eminently  successful  experi- 
ment. Sherman2  also  came  to  the  belief  that  although  the 
absence  of  glycine  from  the  products  of  hydrolysis  of  a  protein 
was  of  no  significance  as  regards  its  nutritive  value,  yet  the 
absence  of  more  complex  radicals  such  as  tryptophane,  tyrosine, 
etc.,  seriously  affected  its  ability  to  completely  replace  kata- 
bolized  body  protein. 

Murlin3  came  to  the  conclusion  that  any  carbohydrate  which 
was  not  needed  for  satisfying  the  energy  requirement  was  much 
more  efficacious  in  reducing  the  nitrogen  output  than  that  which 
was  necessary  for  combustion.  He  affirmed  that  the  sparing 
action  of  gelatin  is  not  due  to  any  dextrose  that  it  may  give  rise 
to,  but  to  its  content  of  nitrogenous  residues.  Glycine,  which 
is  the  chief  amino-acid  constituent  of  gelatin,  can  be  retained 
temporarily  in  the  body,  and  so  may  serve  to. account  for  the 
high  replacement  of  other  proteins  by  gelatin.  Glycine  is  not 
retained  permanently,  however,  even  in  the  presence  of  an 
abundance  of  carbohydrate. 

Many  other  contributions  have  appeared  upon  this  subject 
and  in  every  case  the  conclusions  point  to  the  insufficiency  of 
gelatin  as  a  complete  protein  food.  But  many  other  pure  pro- 

1  M.  KAUFFMAN,  Pfliiger's  Arch.  Physiol,  109  (1905),  440. 

2  H.  C.  SHERMAN,  "Chemistry  of  Food  and  Nutrition"  (1913),  302. 

3  J.  MURLIN,  Am.  J.  Physiol.,  20  (1907),  234. 


558  GELATIN  AND  GLUE 

terns,  as  albumin,  fibrin,  etc.,  are  also  incomplete,  and  it  should 
be  emphasized  that  in  a  normal  diet  where  a  great  variety  of 
ingredients  go  to  make  up  the  dietary  the  need  for  a  complete 
food  being  embodied  in  any  one  material  is  quite  negative  and  a 
matter  of  indifference.  As  one  of  a  variety,  however,  there  can 
be  no  reasonable  objection  raised  to  the  inclusion  of  a  pure 
gelatin,  for  it  is  a  true  food,  a  preserver  of  nitrogen,  is  easily 
digested,  and  is  -readily  burned  in  the  production  of  energy. 
The  additional  value  of  gelatin  in  the  diet  as  a  protective  colloid 
is  described  in  the  following  section. 

2.  Gelatin  as  a  Protective  Colloid  in  Dietetics. — Many  col- 
loidal substances  and  most  suspensions  are  readily  precipitated 
from  solution  by  the  addition  of  electrolytes.  This  is  especially 
true  of  the  suspensoid  type  of  colloid  such  as  the  colloidal  metals, 
sulphides,  oxides,  etc.,  but  some  proteins  are  similarly  affected. 
For  example,  the  casein  of  cows'  milk  is  coagulated  as  soon  as  the 
bacterium  lactis  acidi  has  produced  a  certain  small  amount  of 
lactic  acid,  or  by  the  direct  addition  of  a  very  little  mineral  or 
organic  acid.  There  are  some  colloids,  however,  that  are  not 
only  practically  uninfluenced  by  the  addition  of  electrolytes, 
but  that  possess  the  striking  and  important  property  of  being 
able  to  stabilize  colloids  that  are  normally  easily  precipitated, 
so  that  a  very  much  greater  concentration  of  electrolyte  is 
required  to  bring  about  a  coagulation.  Exceedingly  small 
amounts  of  these  colloids  are  able  to  protect  very  large  volumes  of 
otherwise  unstable  material. 

The  Infant  and,  Invalid  Dietary. — When  the  casein  of  milk 
is  separated  from  the  other  constituents  it  is  found  very  difficult 
to  bring  it  into  a  state  of  colloidal  solution,  for  even  traces  of 
electrolytes  suffice  to  precipitate  it.  An  examination  of  the 
whole  milk  shows  that  it  contains : 

In  true  solution  In  colloid  solution  In  suspension 

Lactose  Casein  Milk  fat 

Mineral  salts  Lactalbumin 

and  the  proportion  of  these  constituents  is  found  to  vary  in 
different  animals.  The  stability  of  the  milk  and  its  resistance 
to  the  action  of  acids  is  found  to  be  proportional  to  the  content 
of  lactalbumin,  as  shown  in  the  following  table  by  Alexander 
and  Bullowa.1 

1  J.  ALEXANDER  and  J.  BULLOWA,  J.  Am.  Med.  Assn.,  66  (1910),  1196. 


APPLICATIONS  OF  GELATIN  559 

TABLE  64. — COMPOSITION  OF  MILK  FROM  DIFFERENT  SOURCES 


Kind  of  milk 

Casein 

Lactal- 
bumin 

Fat 

Sugar 

Behavior  with  acids  and 
rennin 

Cow 

3  02 

0  53 

3  64 

4  88 

Woman  
Goat  

1.03 
3.20 

1.26 
1.09 

3.78 
4.78 

6.21 
4.46 

Not  readily  coagulated. 

Ewe  

4.97 

1.55 

6.86 

4.91 

Mare  

1.24 

0.75 

1  21 

5  67 

Ass  

0  67 

1  55 

1  64 

5  99 

Not  readily  coagulated 

The  order  of  digestibility  also  corresponds  with  the  content  of 
lactalbumin,  according  to  Jacobi,1  who  finds  that  asses'  milk 
may  often  be  fed  with  success  to  infants  who  are  unable  to  digest 
either  cows7  or  woman's  milk. 

The  specific  action  of  the  lactalbumin  is  obviously  as  a  stabiliz- 
ing agent2  which  keeps  the  casein  moiety  in  a  finely  divided 
state,  and  prevents  a  coagulation  of  the  latter  even  upon  reaching 
the  stomach  with  its  acid  secretions.  The  ultramicroscope  has 
revealed  that  the  size  of  the  casein  particles  is  much  smaller  in 
woman's  than  in  cow's  milk.  It  has  also  shown  that  flocculation 
is  almost  entirely  inhibited  in  cow's  milk  upon  the  addition  of 
small  amounts  of  acid  if  a  little  gelatin  is  previously  added. 
This  experiment,  also  confirmed  by  the  extraordinary  small 
"gold  number"  of  the  gelatin,3  makes  it  appear  certain  that 
gelatin  is  capable  of  functioning  as  a  protective  colloid,  in  con- 
junction with  lactalbumin,  in  preventing  coagulation  of  milk 
during  digestion.  Jacobi4  advocated  the  addition  of  gelatin  or 
gum  arabic  to  cow's  milk  for  infant  feeding  as  early  as  1889  and, 
although  the  exact  nature  of  the  action  was  not  then  understood, 
the  beneficial  results  obtained  by  such  practice  were  well  known. 
It  is  very  probable  also  that  gelatin  functions  in  keeping  the  fat 
in  a  finely  divided  condition.  When  casein  is  precipitated  it 
carries  down  with  it  a  considerable  portion  of  the  fat,  and  troubles 
that  have  been  experienced  by  an  appearance  of  undigested  fat, 
may  be  due  in  fact  to  fat  precipitation  by  the  casein. 

1  A.  JACOBI,  ibid.,  21  (1908),  1216. 

2Cf.  J.  ALEXANDER,  Z.  Chem.  Ind.  Kolloide,  4  (1909),  86;  6  (1909),  101; 
6  (1910),  197. 

3  See  page  123. 

4  A.  JACOBI,  "The  Intestinal  Diseases  of  Infancy  and  Early  Childhood," 
1889. 


560  GELATIN  AND  GLUE 

It  must  be  urged  that  gelatin  will  not  in  all  cases  entirely 
prevent  the  formation  of  casein  curds  in  the  stomach.  The 
acidity  may  become  sufficiently  high  to  produce  coagulation  in 
spite  of  the  protective  colloids  present,  but  these  undoubtedly 
are  of  value  in  retarding  and  diminishing  this  undesirable 
phenomenon.  In  fact  the  size  of  the  flock  produced,  rather 
than  the  entire  absence  of  any  curd,  is  probably  the  more  im- 
portant aspect  clinically,  for  if  the  curd  is  finely  divided  and 
soft,  the  enzymes  of  the  digestive  tract  will  be  easily  able  to 
dissolve  them,  whereas  if  large  hard  lumps  are  formed  the 
enzymes  may  have  but  little  effect  upon  them.  Koplik1  states 
that  the  equilibrium  between  the  acid  content  of  the  infant's 
stomach  and  the  protective  colloid  content  of  woman's  milk  is 
such  that  coagulation  takes  place  late,  and  in  small  soft  masses, 
while  upon  the  ingestion  of  cow's  milk  coagulation  occurs  early, 
and  in  large  masses. 

Herter2  also  finds  the  addition  of  gelatin  to  the  milk  in  cases 
of  serious  malnutrition  to  be  highly  beneficial,  and  to  result  in  a 
much  greater  absorption  of  the  milk  fed.  The  milk  fat  tends 
in  such  cases  to  coalesce  into  relatively  large  masses  which  are 
quite  impossible  of  digestion  in  the  ijifant  organism,3  and  the 
amount  of  fat  fed  is  often  reduced  to  less  than  two  per  cent  with- 
out greatly  improving  the  case,  while  /any  successful  attempt  at 
preventing  the  coagulation  of  the  ^casein  is  simultaneously 
reflected  by  a  perfect  digestion  of  the  fat.  Even  in  adults 
Moore  and  Krombholz4  regard  the  ingestion  of  protective 
colloids  in  the  form  of  albumins  and  gelatin  as  of  the  highest 
importance  in  maintaining  an  emulsion  of  the  fats  which  are 
ingested,  and  in  that  way  preventing  digestive  disorders  that 
would  result  from  the  non-emulsification  of  fat  masses. 

Although  investigations  upon  this  subject  have  been  largely 
confined  to  the  single  food  milk  and  to  its  adaptibility  for  infant 
feeding,  the  principle  of  colloidal  protection  must  be  of  none  the 
less  great  importance  in  many  other  foods,  and  in  the  normal 
dietary,  as  well  as  in  that  of  the  sickroom  and  the  preparation  of 
food  for  invalids.  This  important  phase  of  dietetics  has  not  been 

1  H.  KOPLIK,  "Diseases  of  Infancy  and  Childhood,"  1902. 

2  C.  HERTER,  "On  Infantilism  from  Chronic  Intestinal  Infection,"  1908. 

3  J.  SCHERESCHEWSKY,  Hyg.  lab.  U.  S.  Public  Health  and  Mar.  Hosp. 
Service  Bull.  41. 

4  MOORE  and  KROMBHOLZ,  J.  Physiol,  22  (1908),  54. 


APPLICATIONS  OF  GELATIN  561 

adequately  investigated  but  there  can  be  no  doubt  in  the  light 
of  what  has  already  been  accomplished  that  the  chemical  con- 
stitution of  a  food  is  only  one  of  a  number  of  the  factors  which 
must  properly  be  considered  in  the  selection  of  a  dietary.  If  the 
nitrogen  supply  is  given  wholly  through  the  single  protein  albu- 
min, or  fibrin,  or  gelatin,  the  unfortunate  victim  will  starve  to 
death.  If  a  perfectly  balanced  ration  of  heat  sterilized  foods 
is  given,  the  same  misfortune  will  result.  We  have  discovered 
that  a  single  pure  protein  is  usually  insufficient.  We  have  dis- 
covered the  hypothetical  vitamins.  We  have  observed  that  a 
certain  association  of  foods  may  react  in  the  body  quite  differ- 
ently than  a  certain  other  association.  And  in  this  field  lies  the 
influence  of  the  protective  colloid.  In  vitro  there  is  no  colloid 
that  exhibits  this  property  of  protection  to  the  degree  shown  by 
gelatin,  and  the  value  of  this  substance  as  a  part  of  the  normal 
diet,  especially  to  'those  who  suffer  from  poor  digestion,  is  prob- 
ably far  more  as  a  protective  colloid  and  emulsifying  agent  than 
as  a  food,  but  it  functions  unquestionably  as  both. 

Gelatin  in  Ice  Cream  and  Other  Food  Products. — Gelatin  has 
long  been  used  in  the  manufacture  of  ice  cream,  especially  when 
made  in  large  quantities  by  the  larger  manufacturers.  There 
are  a  few  states  in  which  such  use  of  gelatin  is  prohibited  by  law, 
but  the  old  prejudice  against  gelatin  due  to  its  unrefined  cor- 
relation with  glue  has  fallen  away  with  the  advent  of  more 
sanitary  and  cleanly  conditions  of  manufacture.  When  it  is 
realized  that  upwards  of  250,000,000  gallons  of  the  frozen  delicacy 
are  consumed  annually  in  the  United  States  alone,  and  that  a 
pound  of  gelatin  is  used  for  approximately  each  50  gallons  of 
ice  cream,  it  is  seen  that  the  amount  of  gelatin  which  annually 
finds  its  way  into  the  frozen  cream  is  in  the  neighborhood  of 
5,000,000  pounds. 

The  advantages  obtained  by  the  use  of  gelatin  in  ice  cream 
find  explanation  in  three  colloidal  properties  of  the  substance; 

(1)  the  ability  of  gelatin  to  produce  a  jelly  at  low  temperatures, 

(2)  the  " protective"  nature  of  gelatin,  which  prevents,  or  greatly 
diminishes,  the  tendency  of  other  substances  to  crystallize  or 
separate  from  the  mixture,  and   (3)   the  ability  of  gelatin  to 
function  as  an  emulsifying  agent,  and  so  render  more  permanent 
the  emulsion  of  the  milk  fat  in  its  aqueous  medium. 

It  seems  to  have  been  known  for  a  long  time  that  gelatin 
improved  the  texture  and  keeping  qualities  of  ice  cream,^but 

36 


562  GELATIN  AND  GLUE 

exact  experimentation  on  a  large  scale  was  not  made  until  of 
comparatively  recent  date.  R.  M.  Washburn1  of  the  Vermont 
Agricultural  Experiment  Station,  and  O.  E.  Williams2  of  the 
Dairy  Division,  Bureau  of  Animal  Industry,  have  made  extended 
investigations  upon  the  use  of  gelatin  in  ice  cream.3  , 

The  Improvement  of  "Body"  and  Texture. — The  ways  in  which 
gelatin  acts  beneficially  in  ice  cream  are  three  in  number.  Gela- 
tin improves  the  "body"  of  the  product,  it  improves  the  texture, 
and  it  improves  the  stability  or  keeping  qualities.  By  "body" 
is  meant  the  consistency,  the  firmness,  the  structure  of  the 
material.  The  ice  cream  should  be  firm  and  mellow,  but  not 
hard  or  rubbery.  Where  no  gelatin  is  used  the  product  will  be 
weak,  will  yield  easily  to  any  slight  pressure,  will  tend  to  be 
"crumbly,"  and  not  easily  retain  its  shape.  The  jet  from  the 
soda  fountain  directed  upon  a  portion  of  the  frozen  cream  in  a 
glass  would  scatter  the  cream  into  fine  flakes,  which,  as  Washburn 
says,  "could  be  easily  drunk  as  a  gruel,  but  could  not  well  be 
eaten."  A  small  amount  of  gelatin,  however,  acts  as  a  binder, 
making  for  the  retention  of  shape,  for  consistency,  and  firmness. 
It  is  not  desired  to  produce  a  real  jelly,  such  as  is  sought  in  the 
familiar  culinery  desert.  The  presence  of  the  gelatin  would  then 
be  too  manifest,  and  the  taste  of  the  public  has  not  been  trained 
to  be  satisfied  with  an  ice  cream  that  is  of  the  consistency  of  a  real 
jelly.  For  this  reason  the  amount  of  gelatin  which  may  be  added 
is  very  small.  A  good  grade  of  gelatin  will  produce  a  jelly,  at  or 
near  the  freezing  point  of  water,  in  a  1  per  cent  concentration. 
A  concentration  of  about  a  quarter  of  1  per  cent  is  adequate  for 
the  development  of  a  marked  beneficial  effect  upon  the  con- 
sistency and  firmness  of  the  ice  cream,  but  is  not  great  enough  to 
produce  the  undesirable  effect  of  transforming  the  ice  cream  into  a 
jelly. 

The  second  way  in  which  gelatin  has  been  found  of  advantage 
in  ice  cream  manufacture  is  in  its  effect  upon  the  texture  of  the 
product.  This  term  refers  to  the  "smoothness"  of  the  cream. 
It  is  often  incorrectly  assumed  that  the  fat  content  determines 
the  texture,  nearly  to  the  exclusion  of  any  other  factor.  If 

1  R.  M.  WASHBURN,  Ver.  Agr.  Exp.  Sta.  Bull  155  (1910). 

2  O.  E.  WILLIAMS,  Paper  read  before  Wisconsin  Ice  Cream  Dealers  Assoc., 
(1916). 

3  Vide  also  J.  H.  FRANDSEN,  and  E.  A.  MARKHAM,  "The  Manufacture  of 
Ice  Creams  and  Ices"  (1919). 


APPLICATIONS  OF  GELATIN  563 

the  ice  cream  is  coarse,  granular,  crystalline,  it  is  said  to  be  thin, 
and  poor  in  fat,  while  if  it  is  of  a  velvety  texture,  it  is  said  to  be 
rich.  Although  the  milk  fat  is  a  very  important  agent  in  the 
production  of  a  smooth  cream,  it  must  be  urged  that  it  may  be 
of  only  secondary  importance.  When  the  creams  are  freshly 
frozen,  other  factors  being  equal,  the  smoothness  of  texture  will 
be  proportionate  to  the  milk  fat  present,  but  after  the  frozen 
cream  has  been  allowed  to  stand  for  a  day  or  two  it  will  be  found 
that  the  water,  which  in  either  case  is  present  to  the  extent  of 
60  to  70  per  cent,  will  begin  to  crystallize  out  in  the  form  of  sharp 
spiney  crystals.  Lactic  acid  may  also  crystallize  out  as  granular 
sandy  crystals.1  These  are  very  objectionable,  both  to  the 
consumer,  and,  by  virtue  of  a  consequent  loss  in  trade,  to  the 
dispenser  of  the  cream.  But  gelatin  has  been  found,  when 
added  in  very  small  Amounts,  to  greatly  retard,  or  even  entirely 
prevent,  this  crystallization  of  the  water  and  the  lactic  acid. 

The  explanation  of  this  action  of  the  gelatin  lies  in  the  colloid 
nature  of  the  material.  It  has  been  suggested  that  the  gelatin 
forms  a  thin  film  around  the  other  molecules  in  the  soultion, 
thus  preventing  them  from  coming  into  a  sufficiently  intimate 
contact  with  each  other  to  carry  out  the  reactions  which  otherwise 
would  ensue.  It  is  by  the  same  property  that  gelatin  prevents 
the  ionic  precipitation  of  salts,  and  the  coagulation  of  milk  by  the 
acid  of  the  stomach.  It  is  another  of  the  many  examples  which 
have  been  mentioned  of  the  " protective"  action  of  gelatin. 

The  Improvement  in  Stability  and  Keeping  Qualities. — Of 
equally  great,  and  perhaps  greater,  importance  than  the  fore- 
going, is  the  influence  of  gelatin  upon  the  stability  and  keeping 
qualities  of  the  ice  cream.  When  no  gelatin  is  added,  the 
material  must  be  kept  at  a  temperature  of  18  to  20°F.  in  order 
that  softening  and  a  separation  of  the  constituents  may  not  take 
place.  The  fats  in  the  cream  being  lighter,  and  the  sugars 
being  heavier,  than  the  main  mass  of  the  ice  cream,  there  is  a 
tendency  for  the  former  to  rise,  and  the  latter  to  sink  as  soon  as  a 
softening  takes  place.  All  of  these  things  are,  of  course,  unde- 
sirable. When  small  amounts  of  gelatin  are  present  the  cream 
will  remain  in  a  firm  condition  at  temperatures  as  high  as  24°F. 
This  is  only  a  few  degrees  difference  but  it  may  mean  much  on  a 
hot  day.  The  steps  through  which  a  cream  is  passed  between  the 
manufacturer  and  the -consumer  may  be  many.  In  our  American 

i  H.  F.  ZOLLER  and  O.  E.  WILLIAMS,  /.  Ayr.  Research,  21  (1921),  791. 


564  GELATIN  AND  GLUE 

method  of  doing  business  the  ice  cream  industry  has  grown  until 
it  is  no  longer  localized,  but  one  large  plant  may,  and  often  does, 
supply  its  frozen  product  daily  to  communities  a  hundred  miles 
and  more  distant.  In  the  course  of  transporting  such  a  perish- 
able product  as  ice  cream  to  such  distances  it  is  obvious  that  a 
difference  of  a  few  degrees  in  temperature  may  be  a  mar- 
gin of  sufficiently  important  magnitude  to  make  possible  an 
undertaking  which  otherwise  would  have  to  be  regarded  as 
impracticable. 

The  presence  of  a  small  amount  of  gelatin  makes  it  possible 
therefore,  to  keep  the  cream  at  a  somewhat  higher  temperature 
without  suffering  the  possibility  of  a  melting,  a  disintegration,  or 
a  spoiling  of  the  substance.  This  point  finds  an  important 
bearing  in  the  economic  side  of  the  business.  As  pointed  out  by 
Washburn,1  melted  cream  is  in  some  places  taken  back  by  the 
manufacturer,  in  order  to  hold  his  trade,  and  it  has  been  a  com- 
mon practice  of  manufacturers  to  turn  all  melted  cream  back  into 
the  new  batches,  refreeze,  and  send  out  again.  Thus  old  creams 
may  repeatedly  be  permitted  to  contaminate  the  fresh  lots, 
and  putrefactive  decomposition,  with  the  development  of  pto- 
maines, has  been  known  to  result.  The  use  of  gelatin  makes 
melting  while  in  the  hands  of  the  retailer  less  likely  to  occur,  and 
so  discourages  and  renders  unnecessary  the  return  of  melted  lots 
to  the  manufacturer. 

The  Effect  upon  Flavor  and  Swelling. — The  effect  of  gelatin 
upon  the  flavor  of  the  frozen  cream  was  especially  studied  by 
Williams.2  He  found  that  where  a  small  amount  of  a  high  grade 
gelatin  was  used  there  was  very  little  difference  detectable  be- 
tween the  creams  with  and  without  the  gelatin.  If  rather  large 
amounts  of  gelatin  were  employed,  the  fruit  flavors  are  largely 
masked  and  rendered  scarcely  apparent.  This,  he  says,  "can  be 
explained  by  the  law  of  diffusion,  and  the  physiological  function 
of  the  organs  of  taste."  It  seems  simpler  to  the  physical  chemist 
to  say  that  the  gelatin  has  adsorbed  the  flavoring  principle. 
Whenever  low  grades  of  gelatin  were  used,  there  remained  a 
characteristic  and  disagreeable  after-taste,  suggestive  of  glue. 
This  sometimes  occured  also  with  the  higher  grades  of  gelatin, 
but  was  undoubtedly  due  to  a  selection  of  gelatin  produced 
from  poor  or  partially  decomposed  stock.  If  the  gelatin  was 

1  R.  M.  WASHBURN,  loc.  cit. 

2  O.  E.  WILLIAMS,  loc.  cit. 


APPLICATIONS  OF  GELATIN  565 

prepared  on  one  day,  and  not  used  in  the  cream  until  a  later  day, 
there  was  also  produced  an  offensive  taste.  This  results,  of 
course,  from  a  decomposition  by  bacteria  which  takes  place  very 
rapidly  in  a  solution  of  gelatin  at  room  temperature. 

Upon  using  a  gelatin  that  had  been  repeatedly  heated,  an 
ice  cream  resulted  which  showed  a  minimum  of  influence  by  the 
gelatin.  That  is,  the  hydrolyzed  products,  proteose  and  pep- 
tone, do  not  possess  the  beneficial  properties  which  make  gelatin 
desirable  in  ice  cream. 

It  has  sometimes  been  said1  that  gelatin  imparts  an  increase  in 
the  swelling  effect  which  results  in  the  freezing  of  ice  cream,  but 
Washburn  has  reported  that  this  is  not  the  case.  In  fact  a  slight 
decrease  in  the  swelling  was  observed.  In  184  freezings  without 
a  binder  he  obtained  an  average  swell  of  63  per  cent,  while  in  37 
freezings  with  a  binder  the  average  swell  was  only  55  per  cent. 

Objections  to  the  Ute  of  Gelatin  in  Ice  Cream. — There  have  been 
many  objections  raised  against  the  use  of  gelatin  in  ice  cream, 
but  the  most  persistent  of  these  is  based  upon  the  association 
which  gelatin  bears  to  glue,  and  the  difficulty  in  disengaging 
the  popular  mind  from  the  belief  that  gelatin  and  glue  are  one  and 
the  same  material.  The  analogy  is  the  same  as  would  be  met 
between  the  use,  for  the  making  of  a  soup,  of  a  good  clean  fresh 
soup-bone,  and  of  an  indeterminate  mass  of  skin  pieces,  heads, 
feet,  ears,  sinews,  etc.,  of  the  same  animal.  The  latter  might  be 
clean  and  undecomposed,  but  there  would  be,  nevertheless, 
an  aesthetic  objection  to  partaking  of  a  " dainty"  soup  or  ice 
cream  made  from  them.  There  is,  of  course,  always  a  possibility 
that  the  gelatin  used  may  be  low  grade  material,  and  really 
unfit  for  culinary  purposes,  but  it  is  just  as  pertinent  to  apply 
that  same  objection  to  meat,  poultry,  milk,  and  other  readily 
decomposed  substances.  The  use  of  any  gelatin  of  questionable 
origin  or  purity  should  be  most  strenuously  opposed,  but  the 
addition  of  a  pure  high  grade  gelatin  need  not  be  condemned 
because  of  a  possible  misuse  of  the  privilege. 

An  indifferenetyp^the  selection  of  the  gelatin  is  altogether 
indefensible.  TTne  following  table  taken  from  Bulletin  134  of 
the  Iowa  Stairon  shows  the  enormous  differences  that  may  exist 
in  the  bacteria  content  of  different  samples  of  gelatin,  and  of  the 
ice  cream  made  from  the  several  samples. 

1  Cf.  U.  S.  Marine  Hospital  Service,  Bull  41  (1908),  292. 


566 


GELATIN  AND  GLUE 


At  this  place  it  should  be  emphasized  that  there  is  no  economic 
advantage  accruing  to  the  manufacturer  by  the  use  of  a  low 
rather  than  a  high  grade  gelatin.  The  low  grade  material  costs 
less  per  pound,  to  be  sure,  but  in  order  to  obtain  the  same 
advantages  of  improved  body,  texture,  and  keeping  qualities 
of  the  frozen  cream,  it  is  necessary  to  use  much  more  of  the  low 

TABLE  65. — BACTERIA  IN  GELATIN  AND  ICE  CREAM 


Sample  number 

Bacteria  per  gram 

Bacteria  in  1  c.c.  of  ice 
cream  due  to  gelatin 

1    - 

.        113,000,000 

656,000.0 

2 

14,000,000 

70,000.0 

3 

35 

0.2 

4 

4,200 

21.0 

5 

85,000 

425.0 

grade  gelatin  than  would  be  necessary  of  the  high  grade  material. 
This  larger  amount  and  poorer  quality  would  likewise  make  its 
presence  in  the  cream  much  more  easily  detectable,  even  to 
the  unobservant,  by  leaving  an  unpleasant  taste  in  the  mouth. 
This  would  certainly  not  react  to  the  advantage  of  the  manufac- 
turer, especially  if  he  were  in  competition  with  others.  So  the 
use  of  only  the  highest  grade,  and  purest  gelatins  is  equally 
important  from  the  several  points  of  view  of  the  consumer,  the 
manufacturer,  and  the  Board  of  Health. 

Other  arguments  that  have  been  raised  against  the  use  of 
gelatin  in  ice  cream  are:  That  it  conceals  the  age  of  the  cream, 
that  its  use  permits  of  a  warmer  and  therefore  less  safe  holding 
temperature,  and  that  it  is  unwise  to  permit  the  use  of  any 
material  in  ice  cream  other  than  the  cream,  sugar,  and  flavoring 
substance.  The  age  of  the  cream  is  concealed  by  gelatin  because 
the  latter  substance  prevents  the  crystallization  of  the  water. 
Therefore  a  retailer  may  sell  a  cream  several  days  old.  That  is 
true,  but  if  no  gelatin  were  added  the  cream  would  have  become 
unsalable,  and  perhaps  returned  to  the  manufacturer,  refrozen 
with  fresh  stock,  and  resold.  It  seems  that  a  constant  repetition 
of  the  latter  procedure  would  surely  be  more  harmful  than  the 
former. 

A  warmer  temperature  may  be  employed  in  handling  an  ice 
cream  containing  gelatin,  but  the  maximum  temperature  of  24°F. 


APPLICATIONS  OF  GELATIN  567 

is  still  sufficiently  low  to  prevent  any  untoward  decomposition 
by  bacteria. 

That  gelatin  should  be  prohibited  on  the  ground  that  it  is  an 
adulterant,  regardless  of  its  beneficial  effect,  is  unjustified.  If  it 
were  added  for  the  purpose  of  concealing  an  inferior  product,  it 
should  not  be  permitted,  but  as  pointed  out  above,  this  is  not  the 
end  sought  in  adding  gelatin  to  ice  cream. 

Other  Applications  of  Gelatin  in  Food  Products. — Small  amounts 
of  gelatin  are  sometimes  added  to  a  number  of  food  products  for 
much  the  same  purposes  as  have  just  been  described  for  ice 
cream.  In  many  such  cases,  however,  gelatin  may  correctly 
be  regarded  as  an  adulterant.  Fruit  preserves,  jams,  and 
jellies  occasionally  contain  gelatin  added  to  give  an  appearance 
of  a  better  quality  of  product.  Meat  extracts  and  similar 
preparations  are  often  given  a  ficticious  "body"  with  gelatin. 
Cream  is  occasionally  made  to  appear  thicker  and  richer  by 
added  gelatin.  Chocolate  and  cocoa  have  been  treated,  at 
times  with  gelatin  for* the  same  purpose.  Coffee  beans  have  been 
dipped  into  a  gelatin  solution  to  impart  to  them  a  glaze  which  has 
been  found  to  be  pleasing  to  the  eye. 

3.  Gelatin  as  an  Emulsifying  Agent. — The  theory  of  emulsions 
has  been  described  in  Chap.  IV,  and  it  was  mentioned  in  that 
place  that  gelatin  is  one  of  the  most  effective  of  emulsifying 
agents.  Without  the  presence  of  some  third  substance  which 
forms  a  film  about  the  finely  divided  droplets  of  the  internal 
phase  (Bancroft),  or  which  forms  a  solvated  colloid  in  which  the 
internal  phase  may  become  dispersed  (Fischer),  it  is  doubtful 
if  emulsions  of  more  than  transient  duration  may  be  produced. 
But  whatever  the  theories  that  are  advanced  to  account  for  it 
may  be,  the  third  substance  known  as  the  emulsifying  agent,  is  in 
practice  a  real  and  important  constituent  of  all  emulsions.  The 
use  of  gelatin  in  this  capacity  is  of  great  importance. 

In  the  manufacture  of  emulsion  flavors  for  the  baking  and 
confection  trades  gelatin  may  be  employed.  The  emulsion  will 
be  increased  in  viscosity  by  cooling  and  is  very  stable.  Some 
preservative  is  added  to  prevent  decomposition.  The  addition 
of  a  little  gelatin  to  a  thin  cream  will  make  it  easily  possible  to 
whip,  and  solutions  sold  for  that  purpose  often  contain  gelatin 
and  some  vegetable  gum  which  serves  the  same  purpose.  In  the 
manufacture  of  marshmallows  and  other  confectionary  foams, 
gelatin  is  employed  in  much  the  same  way,  and  gelatin  and  egg 


568  GELATIN  AND  GLUE 

albumin  have  both  found  service  in  baking  powders  and  prepared 
flours  to  give  increased  openness  of  texture. 

In  the  making  of  mayonaise  dressings  the  whites  of  eggs  or 
commercial  egg  albumin  has  been  most  used  as  the  emulsifying 
agent  causing  the  olive  or  cottonseed  or  corn  oil  to  emulsify  with 
water  or  vinegar  or  lemon  juice.  Gelatin  may  be  substituted 
in  some  instances,  however,  without  apparent  detriment  to  the 
product.  Fischer  and  Hooker1  observe  that  "lasting  emulsions 
of  oil  in  gelatin  are  obtainable  only  by  dispersing  the  oil  in  a 
gelatin  mixture  of  a  concentration  which  is  just  fluid  at  the 
temperature  at  which  the  experiment  is  carried  out.  If  with 
such  a  gelatin  colloid  the  temperature  is  raised  (and  its  degree  of 
hydration  thereby  decreased)  a  less  permanent  emulsion  results. 
On  the  other  hand,  an  emulsion  of  oil  in  gelatin  remains  fixed 
if  the  mixture  is  chilled  to  below  the  gelation  point  of  the  gela- 
tin." The  very  favorable  results  obtained  with  gelatin  in  ice 
cream  is  probably  due  in  part  to  the  greatly  increased  efficiency 
of  this  colloid  at  low  temperatures. 

Fischer  and  Hooker  have  also  used  gelatin  in  the  preparation 
of  synthetic  milk,  but  as  the  object  in  making  this  substance  is 
primarily  to  produce  an  emulsion  identical  with  the  natural 
product,  the  milk  proteins  are  more  commonly  added. 

When  ingested  as  a  food,  gelatin  also  undoubtedly  functions 
as  an  emulsifying  agent,  as  suggested  in  the  previous  section. 
Especially  in  the  duodenum,  upon  mixing  with  the  bile,  the 
gelatin  must  greatly  favor  a  complete  emulsification  of  the  fat, 
and  a  consequent  active  hydrolysis  and  assimilation. 

In  the  preparation  of  non-edible  emulsions  such  as  are  used 
for  agricultural  and  industrial  purposes,  gelatin  or  a  high  grade 
of  glue  are  also  much  used.  Emulsions  of  the  oil-in-water  type 
are  most  commonly  stabilized  by  the  addition  either  of  soap  or 
gelatin  as  the  emulsifying  agent.  Jones2  has  patented  an  insecti- 
cide and  fungicide  formed  of  1.5  to  3  gallons  of  an  emulsified 
mineral  oil  containing  a  small  proportion  of  cresol  soap,  5  to  15 
gallons  of  lime-sulphur  solution,  8  to  24  ounces  of  ground  glue, 
and  100  to  200  gallons  of  water.  Bordeaux  mixture  may  be 
used  with  the  same  preparation  in  place  of  the  lime-sulphur  solu- 
tion. Hedenberg3  has  advised  the  author  that  glue  is  used, 

1  M.  FISCHER  and  M.  HOOKER,  "Fats  and  Fatty  Degeneration"  (1917),  33. 

2  P.  R.  JONES,  U.  S.  Patent,  1,291,013,  Jan.  14,  1919. 

3  O.  HEDENBURG,  personal  communication. 


APPLICATIONS  OF  GELATIN  569 

together  with  casein  and  borax,  and  a  preservative  such  as 
sodium  fluoride,  to  emulsify  finely  divided  sulphur  in  water  when 
the  sulphur  may  be  present  to  the  extent  of  about  50  per  cent. 
Bourcart1  states  that  Lodeman  has  applied  glue  to  grape  vines 
as  a  cure  for  peronospora  viticola,  mildew  of  the  vine,  and  that 
Del  Quercio  has  observed  that  an  excellent  method  of  destroying 
the  wooly  aphis,  schizoneura  lanigera,  consists  in  coating  colonies 
of  the  louse  with  a  mixture  of  1 J£  pounds  of  glue  and  3  gallons  of 
tar.  Lodeman2  states  that  glue  is  frequently  recommended  as  a 
valuable  addition  to  insecticides  and  fungicides  to  increase  their 
adhesive  properties,  and  gives  a  formula  for  a  paris  green  mixture : 

Glue 1  pound 

Paris  green 1  ounce 

Water 2  gallons 

David3  used  glue  to  the  extent  of  6  kilograms  of  strong  glue  to  800 
liters  of  Bordeaux  mixture  with  beneficial  results. 

4.  Isinglass  and  Gelatin  in  Fining. — In  the  fining  or  clarifica- 
tion of  beer,  wine' vinegar,  etc.,  a  high  grade  of  isinglass  is  usually 
used.     This  consists  essentially  of  the  swimming  bladders  of  the 
sturgeon  and  other  fishes,  and  comes  into  service  in  the  form  of 
the  original  membranes,   washed  and  laid  one  upon  another 
until  a  thick  leaf  is  produced.4     This  material  is  powdered  or 
shredded  and  added  to  the  liquid  to  be  clarified.5     The  numerous 
fine  flakes  of  the  isinglass  gradually  settle  to  the  bottom  carrying 
with  them  the  colloidally  dispersed  particles  which  had  given 
the  liquid  a  turbidity.     The  temperature  of  the  mixture  must 
be  kept  low  during  this  process  as  a  solution  of  the  isinglass  would 
be  disastrous  to  the  object  in  view.     A  high  grade  of  gelatin, 
ground  into  a  powder,  is  also  sometimes  used  but  lacking  the 
fine  membraneous  structure  of  the  isinglass  is  not  nearly  as 
efficient  as  the  latter.     Its  lower  cost,  however,  is  in  its  favor  and 
where  the  highest  quality  of  product  is  not  required  it  serves 
very  well. 

5.  Gelatin  in  Pharmaceutical  Preparations. — When  formalde- 

1  E.  BOURCART — GRANT,  "  Insecticides,  Fungicides,  and  Weed  Killers" 
(1913),  382. 

2 LODEMAN,  "Spraying  of  Plants"  (1916),  147. 

3  DAVID,  J.  agr.  pract.  (1885),  661. 

4  Vide  page  350. 

5  Vide  page  355. 


570  GELATIN  AND  GLUE 

hyde  is  added  to  a  solution  of  gelatin  a  change  is  observed  to  take 
place  in  the  latter  which  to  a  certain  degree  is  dependent  upon 
the  amount  of  formaldehyde  added.  In  amounts  smaller  than 
one  part  in  10,000  there  is  very  little  change  observable.  Added 
to  the  extent  of  0.1  per  cent  a  more  viscous  solution  results  but 
insolubility  is  not  obtained  in  such  a  solution  until  the  gelatin 
has  been  permitted  to  dry  out.  Added  in  greater  concentrations 
formaldehyde  produces  a  jelly  that  may  not  again  be  melted  or 
brought  into  solution  by  heat  or  the  addition  of  more  water. 
This  jelly  differs  from  an  ordinary  gelatin  gel  in  being  rubbery 
and  possessing  less  strength  when  cold.  The  dried  and  powdered 
product,  known  as  for  mo-gelatin  is  employed  as  a  surgical  dress- 
ing. Due  to  the  antiseptic  action  of  the  formaldehyde  it  remains 
sterile  and  is  a  germicide. 

A  small  amount  of  formaldehyde  is  often  added  to  glue  or 
gelatin  employed  for  printers  rollers  and  hectograph  plates,  as  it 
serves  the  double  purpose  of  preventing  any  decomposition,  and 
also  of  hardening  the  product,  that  the  desired  consistency  may 
be  maintained  even  in  warm  weather. 

Capsules  for  use  as  containers  of  doses  of  medicine  are  used  in 
very  large  quantities.  They  are  made  from  a  pure  food  gelatin 
by  dipping  into  a  strong  solution  of  the  latter  containing  a  little 
glycerin  or  sugar,  an  iron  rod,  the  end  of  which  is  shaped  exactly 
as  the  capsules  required.  This  end  is  highly  polished  so  that 
the  gelatin  when  cool  may  be  easily  detached.  The  dipping  and 
cooling  may  be  repeated  until  the  desired  thickness  of  capsule  is 
obtained.  After  removing  from  the  rod  they  are  thoroughly 
dried  and  are  then  ready  for  use.  In  using  them,  the  two  sections 
are  made  so  that  one  fits  down  over  the  other  like  a  cover.  By 
moistening  the  edges  after  filling  the  capsules,  or  by  painting  the 
edges  with  a  weak  gelatin  solution  containing  a  little  gum,  a 
perfecty  tight  joint  is  secured. 

For  coating  pills  gelatin  is  also  much  in  favor.  The  object  in 
this  case  is  not  only  to  eliminate  the  taste  of  the  pill  in  swallowing 
but,  depending  upon  the  specific  case,  to  prevent  evaporation  of 
enclosed  moisture,  to  prevent  crumbling,  to  prevent  sticking 
together,  or  to  prevent  other  undesirable  changes  taking  place 
in  the  pill.  A  solution  of  about  1  part  of  gelatin  to  2  parts  of 
water,  together  with  a  little  glycerin  or  sugar  may  be  used.  The 
pills  are  coated  by  dipping. 

In  Europe  large  quantities  of  gelatin  are  made  into  very  thin 


APPLICATIONS  OF  GELATIN  571 

sheets  of  about  the  thickness  of  ordinary  writing  paper,  called 
gelatin  foils.  A  pure  gelatin  is  brought  into  a  rather  dilute  solu- 
tion, a  little  glycerin  added  and  if  desired  a  little  analin  dye,  to 
give  the  product  any  required  color.  The  solution  is  poured 
out  upon  a  polished  plate  of  glass,  and  rolled  flat,  or  another 
plate  of  glass  placed  over  the  solution.  After  drying  a  few  hours 
the  upper  plate  is  removed,  and  when  thoroughly  dry  the  gelatin 
may  be  pealed  from  the  lower  plate  without  difficulty.  The 
foils  are  used  for  printing  sacred  images,  labels,  visiting  cards, 
in  the  manufacture  of  artificial  flowers,  fancy  articles,  and  for 
covering  small  wounds,  as  the  material  adheres  to  the  skin  and 
can  be  rendered  antiseptic. 

Court- plaster  is  made  from  a  mixture  of  gelatin,  alcohol,  and 
glycerin.  The  gelatin  is  dissolved  in  a  small  amount  of  water. 
A  portion  of  this  is  spread  upon  taffeta  and  allowed  to  dry. 
This  is  repeated  a  few  times,  when  alcohol  and  glycerin  are  added 
to  another  portion  of  gelatin,  and  a  few  more  applications  made 
in  a  similar  manner.  The  reverse  side  is  treated  with  tincture  of 
benzoin. 

Chromate  gelatin  has  been  used  abroad,  especially  in  Germany, 
as  aluting  for  glass  and  cork  stoppers  in  pharmaceutical  con- 
tainers. Alutes  of  this  character  have  been  replaced  largely 
by  cellulose-ester  preparations. 

II.  COMMERCIAL  GELATIN 

1.  Gelatin  in  Photography  and  Photolithography. — A  large 
amount  of  the  highest  quality  of  gelatin  goes  into  service  as  a 
coating  upon  the  films,  plates  and  developing  papers  used  in 
photography.  This  coating  contains  the  silver  salts  and  other 
constituents  of  the  light-sensitive  portion  of  the  film.  If  the 
gelatin  is  not  of  the  purest  quality  there  will  result  reactions 
between  it  and  the  sensitive  silver  salts,  or  in  the  later  stages 
of  the  development,  fixing,  etc.,  of  the  picture  undesirable  reac- 
tions may  take  place.  The  gelatin  solution  must  be  made  such 
that  the  dried  film  will  have  just  the  right  degree  of  porosity  to 
electrolytes,  for  in  all  stages  of  the  development  where  chemicals 
are  used  it  is  necessary  that  they  penetrate  and  impregnate 
the  gelatin  layer  with  considerable  ease,  but  no  trace  of  the 
precipitated  silver  must  be  permitted  to  escape.  It  is  indeed 
due  to  the  protective  action  of  the  gelatin  that  the  precipi- 


GELATIN  AND  GLUE 


tated  silver  remains  finely  divided  as  an  even  thin  deposit. 
Other  colloids,  on  account  of  their  inferior  protective  action, 
are  less  adapted  to  this  service  and  cannot  advantageously  be 
substituted  for  gelatin. 

When  a  gelatin  solution  is  treated  with  a  solution  of  a  soluble 
bichromate  and,  after  setting,  exposed  to  the  action  of  strong 
light,  the  gel  becomes  insoluble.  This  reaction  has  been  applied 
with  success  to  the  process  of  photolithography.  Gelatin  which 
has  been  treated  with  the  bichromate  in  the  dark  and  allowed 
to  form  a  jelly  on  a  glass  plate  is  exposed,  through  a  photographic 
negative  to  a  strong  light.  That  portion  of  the  gelatin  plate 
which  receives  light  through  the  negative  will  be  rendered  insolu- 
ble, and  later,  when  placed  in  water  will  not  swell  to  the  same 
extent  as  the  parts  which  were  protected  from  the  light  by  the 
deposit  of  the  negative.  Furthermore,  the  amount  of  swelling 
which  any  part  of  the  plate  will  undergo  is  in  exact  inverse 
proportion  to  the  amount  of  light  which  penetrates  the  negative 
to  it.  The  picture  is  thus  reproduced  upon  the  gelatin  plate. 
This  plate  may  then  be  covered  with  graphite  and  a  plate  of 
copper  deposited  upon  it  in  an  electrolytic  solution,  in  which  case 
the  copper  plate,  backed  with  easily  fusible  metal  poured  upon 
it,  may  be  inserted  in  the  printing  press  and  prints  made.  In  one 
form  or  another  the  process  has  been  in  use  for  a  great  many 
years. 

The  first  study  of  importance  upon  the  reactions  involved  upon 
the  addition  of  a  soluble  bichromate  to  gelatin,  and  the  subse- 
quent exposure  to  light  was  made  by  Eder1  in  1878.  He  suc- 
c&eded  in  demonstrating  that  light  exerted  a  reducing  action 
upon  the  bichromate  in  the  presence  of  gelatin  with  the  formation 
of  chromium  sesquioxide.  This  in  turn  reacted  with  any  excess 
of  bichromate  forming  chromium  chromate  which  likewise 
decomposed  under  the  prolonged  action  of  light  until  it  was 
eventually  completely  transformed  into  sesquioxide.  Lumiere 
and  Seyewetz2  have  in  a  number  of  later  communications  con- 
firmed the  findings  of  Eder  and  added  to  them  in  making  exact 
determinations  of  the  several  variable  influences  in  the  operation, 
and  in  showing  that  the  reaction  produces  a  product  of  a  per- 
fectly definite  composition. 

1  EDER,  Compt.  rend.  (1878). 

2  A.  LUMIERE  and  A.  SEYEWETZ,  Bull.  Soc.  chirn.,  29  (1903),  1077;  1085; 
33  (1905),  1032,  1040. 


APPLICATIONS  OF  GELATIN  573 

The  reactions  involved  are  written  by  them  as  follows: 
The  bichromate  is  first  reduced  by  the  action  of  the  gelatin  and 
light,  forming  the  sesquioxide  of  chromium, 

K2Cr2O7  -*•  Cr203  +  K2O  +  30. 

The  oxygen  is  absorbed  by  the  gelatin,  participating  in  its 
insolubilization.  The  potassium  oxide  is,  of  course,  changed 
to  the  hydroxide  and  reacts  with  more  of  the  bichromate  with  the 
formation  of  chromate, 

K2Cr2O7  +  2KOH  ->  2K2Cr04  +  H2O. 

The  neutral  chromate  acts  on  the  gelatin  in  the  presence  of  light 
in  a  manner  quite  similar  to  that  of  the  bichromate  but  with 
extreme  slowness, 

2K2CrO4  ->  Cr2O3  +  2K2O  +  3O. 

Finally  the  sesquioxide  reacts  with  the  excess  of  bichromate  to 
form  chromium  chromate, 

K2Cr2O7  +  Cr2O3  -»  Cr03,  Cr203  +  K2Cr04. 
It  is  probable  that  the  latter  equation  might  better  be  written, 

K2Cr207  -4  K2Cr04  +  Cr03, 
3Cr03  +  Cr203  -»  Cr2(CrO4)3, 

and  if  Eder's  observation  that  this  also  goes  slowly  to  the  sesqui- 
oxide is  correct,  the  final  equation  may  be  added, 

2Cr2(Cr04)3  -»  5Cr2O3  +  90. 

Lumiere  and  Seyewetz  found  also  that  the  amount  of  Cr203 
fixed  per  unit  weight  of  gelatin  increased  with  the  amount  of 
bichromate  added,  and  that  the  proportion  of  added  bichromate 
that  actually  entered  into  the  reaction  increased  with  the  duration 
of  the  exposure  to  light.  Upon  studying  the  action  of  the 
bichromates  of  eleven  different  metals,  including  chromic  acid, 
they  found  that  the  susceptibility  to  reduction,  and  the  conse- 
quent insolubilization  of  the  gelatin,  varied  greatly,  the  bichro- 
mate of  iron  having  the  least  effect,  and  that  of  ammonium  the 
greatest.  In  fact  the  quantity  of  chromium  sesquioxide  obtained 
by  the  use  of  ammonium  bichromate  was  nearly  twice  that  result- 
ing from  the  use  of  the  potassium  salt  during  equal  exposures. 

In  the  light  of  more  modern  theory  it  seems  probable  that 
chromium  gelatinate  is  formed.  The  chromium  is  trivalent,  and 
Loeb1  has  shown,  using  aluminum  and  cerium  salts,  that  the 

1  J.  LOEB,  J.  Gen.  PhysioL,  I  (1918-19),  483. 


574  GELATIN  AND  GLUE 

several  properties  of  gelatin,  including  solubility,  swelling,  vis- 
cosity, etc.,  are  depressed  in  proportion  to  the  increasing  valence 
of  the  cation  added.  Thus  the  divalent  calcium  ion,  forming 
calcium  gelatinate,  results  in  a  product  having  lower  solubility, 
swelling,  etc.,  than  the  monovalent  sodium  ion  of  sodium  gela- 
tinate, while  the  trivalent  aluminum  ion,  forming  aluminum 
gelatinate,  produces  a  product  even  more  insoluble.  If  his 
generalization  is  correct,  then  chromium  gelatinate  should  also  be 
insoluble,  as  seems  to  be  the  case.  The  greater  efficiency  ob- 
tained by  adding  the  bichromate  and  allowing  this  to  be  reduced 
to  the  chromate  obviously  results  in  an  increase  in  the  amount 
of  available  Cr203  for  the  reaction,  and  it  is  doubtful  if  the 
oxygen  liberated  as  such  assumes  an  important  role  in  the 
insolubilization  of  the  material.  When  chromic  acid  alone  is 
used  it  is  probable  that  the  product  formed  is  largely  gelatin 
chromate,  but  this  being  a  dibasic  acid,  and  the  pH  of  the 
mixture  probably  being  brought  very  close  to  the  isoelectric 
point,  a  certain  degree  of  insolubility  would  undoubtedly  arise. 

A  thorough  study  of  the  action  of  the  chromates  and  bichro- 
mates upon  gelatin  from  the  standpoint  of  modern  theory  and 
under  rigid  control  of  hydrogen  ion  concentration  should  be 
highly  fruitful  of  a  better  understanding  of  these  reactions. 

2.  Gelatin  as  a  Bacterial  Culture  Medium. — Koch  first 
employed  gelatin  as  a  solid  medium  for  studying  the  behavior  of 
bacteria  and  other  microorganisms,  and  it  has  since  become  one 
of  the  standard  media  for  this  purpose.  It  possesses  the  advant- 
age over  many  other  substances  in  favoring  the  development  of  a 
large  variety  of  species,  but  often  suffers  the  disadvantage  of 
becoming  liquefied  by  the  organisms. 

Nutrient  gelatin  may  be  prepared  by  adding  about  3  grams  of 
beef  and  5  grams  of  peptone  to  a  liter  of  distilled  water,  and 
adding  100  grams  of  gelatin.  After  the  gelatin  has  swollen  for 
an  hour  or  so  the  mixture  is  warmed  until  a  solution  is  obtained. 
The  medium  is  made  neutral  or  slightly  alkaline  to  phenol- 
phthalein,  filtered,  distributed  in  tubes,  and  sterilized  by  heating 
in  an  autoclave  at  15  pounds  pressure  (120°C.)  for  15  minutes, 
or  by  intermittent  heating  at  100°  for  30  minutes  on  three  suc- 
cessive days.  The  white  of  an  egg  dissolved  in  30  to  60  c.c.  of 
distilled  water  may  be  beaten  in  to  improve  the  clarity  of  the 
solution.  Mixtures  of  gelatin  with  other  media  are  often  made 
for  special  purposes.  Among  such  mixtures  may  be  mentioned 


APPLICATIONS  OF  GELATIN  575 

agar  gelatin,   wort  gelatin,  whey  gelatin,  soil  extract  gelatin, 
litmus  gelatin,  raisin  gelatin,  and  Hiss  Medium. 

In  determining  the  power  of  microorganisms  to  liquefy  gelatin, 
straight  needle  stab  cultures  are  made  in  gelatin  tubes,  and  the 
latter  incubated  at  20°C.  The  Society  of  American  Bacteri- 
ologists in  1907  recommended  that  gelatin  tubes  be  held  for  6 
weeks  to  determine  liquefaction.  Some  cultures,  however,  will 
liquefy  gelatin  only  after  several  months,1  while  others  will 
require  but  a  day.  Rothberg2  proposes  to  obviate  this  diffi- 
culty by  giving  the  organism  a  preliminary  cultivation  in  a  1 
per  cent  gelatin  solution  at  25  or  37°C.,  then  inoculate  the  surface 
of  gelatin  in  a  test  tube  and  incubate  for  15  days  at  20°. 

3.  Gelatin  Cells  for  Ultrafiltration. — For  the  ultrafiltration  of 
suspensoid  colloids  membranes  made  from  gelatin  or  collodion 
are  often  used.  They  may  very  easily  be  prepared3  by  impreg- 
nating disks  of  a  hard  filter  paper  with  a  solution  of  the  gelatin. 
Care  must  be  taken  not  to  entrap  any  air  bubbles  underneath 
the  paper,  and  to  have  the  paper  uniformly  penetrated  with  the 
sol.  A  2  to  10  per  cent  sol  of  gelatin  may  be  used,  and  the  con- 
taining disk  should  be  kept  on  the  water  bath  at  a  definite  tem- 
perature during  impregnation.  Especial  care  must  be  exercised 
in  this  point,  because  the  porosity  of  the  filter  will  vary  with  the 
temperature  as  well  as  with  the  concentration.  For  uniform 
results  in  any  one  experiment,  therefore,  the  temperature  and 
concentration  must  be  held  rigidly  constant.  After  removing  the 
disks  from  the  liquid  they  are  allowed  to  drain,  turning  constantly 
in  their  own  plane  to  prevent  an  excess  of  gel  forming  on  the 
under  side.  After  the  gel  has  set,  the  papers  are  placed  in  a  2  to 
4  per  cent  solution  of  formaldehyde  for  24  hours  to  render 
insoluble,  the  whole  being  placed  in  the  cooler  during  this  time. 
The  disks  are  then  rinsed  in  cold  water  and  may  be  kept  in  water 
saturated  with  chloroform.  If  they  are  allowed  to  dry,  even 
partially,  they  become  useless.  The  common  forms  of  fat  extrac- 
tion thimble  may  also  be  used  to  advantage  in  many  cases  for 
the  preparation  of  ultrafilters. 

If  experiments  calling  for  varying  gradations  in  the  size  of 
pore,  or  particle  which  will  be  allowed  to  pass  through,  are 

1  F.  W.  TANNER,  "Bacteriology  and  Mycology  of  Foods"  (1919),  111. 

2  W.  ROTHBERG,  Paper  read  before  Soc.  Am.  Bacteriologists  (1917). 

3  E.  HATSCHEK,  "Laboratory  Manual  of  Colloid  Chemistry,"  Philadelphia 
(1920),  69. 


576  GELATIN  AND  GLUE 

designated,  filters  may  be  made  up  as  above  using  several  differ- 
ent concentrations  of  gelatin  between  2  and  10  per  cent  at  the 
same  temperature.  Much  more  concordant  results  are  obtained 
by  this  procedure  than  by  varying  the  temperature  of  a  specified 
concentration. 

4.  Gelatin  in  Analytical  Procedures. — On  account  of  the  diffi- 
culty of  filtering  and  otherwise  handling  analytically  solutions 
that  contain  even  traces  of  gelatin  or  glue,  these  substances  are 
commonly  regarded  as  altogether  impermissible  in  such  solutions, 
and  if  present  must  be  eliminated.     The  very  properties,  how- 
ever, that  make  them  usually  undesirable  may  in  certain  instances 
be  utilized  in  a  determination.     Crete1  has  based  a  volumetric 
estimation  of  phosphorus  as  phosphate  upon  the  particular  type 
of  precipitation  he  is  able  to  obtain  with  molybdate  solutions  in 
the  presence  of  gelatin.     He  finds  that  "an  addition  of  gelatin, 
or  similar  substance  as  peptone,  results  in  a  precipitate  of  phos- 
phomolybdate  that  is  whitish  and  voluminous,  such  that  a  very 
small  amount  of  phosphoric  anhydride,  e.g.,  0.000125  g.,  will 
reveal  itself  as  a  distinct  cloud  in  the  clear  liquid.     By  a  short 
warming  the  gelatin  separates  from  the  phosphomolybdate  and 
the  precipitate  assumes  the  usual  yellow  compact  form  and 
settles  readily  and  quickly.     Upon  a  further  addition  of  a  little 
molybdic  acid  solution,  so  long  as  any  phosphoric  acid  remains, 
a   voluminous   gelatin-containing   precipitate   will   again   come 
down.     It  is  possible  by  this  means,  through  continued  additions 
of  molybdic  acid  and  subsequent  heating  and  settling  of  the 
precipitate  to  titrate  to  a  sharp  end  point."     The  gelatin  was 
added  to  the  slightly  alkaline  solution  of  the  molybdate,  and 
this  run  into  the  solution  of  the  phosphate  containing  definite 
amounts   of    ammonium    nitrate   and    nitric    acid   until,    after 
boiling,  further  additions  produced  no  precipitate.     The   solu- 
tions are  standardized  by  means  of  pure  di-hydrogen  potassium 
phosphate,  and  the  effect  due  to  the  acidity  is  determined  for 
each  set  of  solutions.     Grete  reports  that  he  has  performed  over 
100,000  analyses  by  this  method  with  entire  satisfaction. 

5.  Gelatin  as  a  Medium  for  Demonstrating  Colloidal  Phe- 
nomena.— Gelatin  has  ever  been  held  in  the  highest  regard  as  an 
ideal  medium  for  demonstrating  the  many  phenomena  incident 
to  the  behavior  of  and  the  study  of  colloids.     The  very  name 
"colloid"  was  given  to  that  class  of  substances  by  Graham  in 

1  A.  GRETE,  Ber.t  21  (1888),  2762;  32  (1909),  3106. 


APPLICATIONS  OF  GELATIN  577 

1861  on  account  of  the  similarity  which  they  bore  to  gelatin 
(colloid  from  colla,  glue).  The  two  great  discoveries  of  Graham 
which  definitely  established  colloid  chemistry  as  distinguished 
from  the  chemistry  of  molecularly  dispersed  systems  were 
concerned  with  gelatin.  The  first  of  these  distinguished  between 
the  diffusibility  of  gelatin,  glue,  gum-arabic,  etc.,  and  ordinary 
crystalloids,  while  the  second  had  to  do  with  the  obtaining  in  a 
state  of  an  apparently  homogeneous  solution  substances  that 
were  ordinarily  considered  as  insoluble.  The  protective  action 
of  gelatin  was  here,  in  some  cases,  utilized. 

All  of  the  colloid  chemists  since  the  day  of  Graham  have 
likewise  found  gelatin  of  the  greatest  value  in  the  study  of 
colloids,  and  in  the  demonstration  of  special  colloidal  properties. 
Luppo-Cramer1  has  very  successfully  demonstrated  the  differ- 
ences in  the  color  of  metallic  suspensoids  with  differences  in 
degree  of  dispersion  by  dispersing  the  solutions  in  plates  of 
gelatin.  For  example,  he  obtained  photographic  plates  of  silver 
dispersed  in  gelatin  that  were  yellow,  orange,  red,  violet,  blue, 
and  green.  In  the  solutions  of  highest  dispersion  the  metals 
are  usually  yellow  or  orange.  In  other  words  they  absorb  the 
violet  and  blue  light.  As  the  size  of  the  particle  becomes  greater 
the  color  passes  from  yellow  and  orange  to  red,  violet,  blue,  and 
finally  green.  "The  absorption  maximum  gradually  moves 
towards  the  side  of  the  greater  wave  lengths  as  the  degree  of 
dispersion  decreases."2  Similar  preparations  demonstrating  the 
color  change  in  gold  and  platinum  with  degree  of  dispersion  may 
easily  be  made. 

The  laws  of  the  viscosity  of  emulsoids  have  been  studied,  using 
gelatin  as  the  type,  by  Hatsehek,3  Pauli,4  Wo.  Ostwald,5  and 
many  others.  The  ionization  of  proteins  and  of  colloids  has  been 
concentrated  upon  gelatin  by  Loeb,6  Fischer,7  Bogue8  and  others. 
The  phenomenon  of  the  Liesegang  ring  formation  is  obtained  in 
gelatin  more  easily  than  in  most  other  colloid  gels.  The  enor- 

1  LUPPO-CRAMER,  Kolloid-Z.,  8  (1911),  240. 

2  Wo.  OSTWALD,  Kolloidchem.  Beihefte,  2  (1911),  409. 

3E.  HATSCHEK,  Kolloid-Z.,  7  (1910),  301;  8  (1911),  34;  Trans.  Faraday 
Soc.,  9  (1913),  80. 

4  Wo.  PAULI,  Trans.  Faraday  Soc.,  9  (1913),  54. 

5  Wo.  OSTWALD,  ibid.,  9  (1913),  34. 

6  J.  LOEB,  J.  Gen.  Physiol,  vols.  1,  2,  3. 

7  MARTIN  FISCHER,  J.  Am.  Chem.  Soc.,  40  (1918),  272;  303. 

8  R.  H.  BOGUE,  J.  Am.  Chem.  Soc.,  44  (1922),  1313;  1343. 

37 


578  GELATIN  AND  GLUE 

mous  water  absorption  and  swelling  of  hydrophile  colloids  is 
beautifully  illustrated  by  gelatin.  The  protective  action  of  the 
emulsoid  colloids  is  better  shown  by  gelatin,  as  revealed  by  its 
gold  number,  than  by  any  other  colloid. 

Gelatin  is  indeed  the  type  hydrophile  or  emulsoid  colloid,  and 
whenever,  either  for  new  investigational  research  or  for  demon- 
strational  purposes,  it  is  desired  to  study  the  properties  of  the 
type,  gelatin  is  most  conveniently  and  satisfactorily  employed. 


APPENDIX 

PAGE 

1.  Hydrogen  Ion  Concentration  and  pH 579 

lonization  Equilibria 579 

The  Theory  of  the  Hydrogen  Electrode 583 

The  Electrometric  Determination , 587 

The  Theory  of  Indicators  and  the  Colorimetric  Determination . .  599 

2.  lonization  Constants  of  Acids  and  Bases 606 

3.  The  Conversion  of  MacMichael  Viscosity  Degrees  to  Centipoises. .  608 

4.  Specific  Gravity  in  Degrees  Baume  and  Twaddell 611 

5.  Comparison  of  Centigrade  and  Fahrenheit  Scales. 614 

6.  Conversion  of  Parts  per  Million  to  Grains  per  United  States  and 

Imperial  Gallons  and  to  Per  Cent 614 

7.  Metric  and  American  Equivalents. 615 

8.  Specific   Gravity  and   Percentage   Composition  of   Hydrochloric, 

Nitric,  and  Sulphuric  Acids,  and  Sodium,  Potassium  and  Am- 
monium Hydroxide  Solutions 616 

9.  Table  for  the  Conversion  of  Volume  of  Nitrogen  to  Milligrams. .  .  .    621 

10.  The  Chemical  Elements  and  their  Atomic  Weights 622 

11.  Logarithms  of  Numbers 623 

1.  HYDROGEN  ION  CONCENTRATION  AND  pH 

lonization  Equilibria. — All  acids  and  acidic  substances  are 
characterized  by  the  fact  that  they  dissociate  to  a  greater  or 
lesser  extent  in  aqueous  solution  into  ions,  among  which  are 
included  the  ions  of  hydrogen,  while  bases  and  basic  substances 
dissociate  with  the  formation  of  hydroxyl  ions.  In  the  case  of 
amphoteric  substances  which  may  liberate  both  hydrogen  and 
hydroxyl  ions,  they  are  said  to  be  acid  or  alkaline  according  to  the 
predominance  of  the  hydrogen  or  the  hydroxyl  ions  respectively. 

The  variation  in  the  relative  proportion  of  hydrogen  and 
hydroxyl  ion  concentration  in  any  case  is  due  to  the  fact  that 
these  two  ions  combine  to  form  water,  and  that  the  ionization  of 
water  at  any  given  temperature  is  a  constant  value.  These 
relations  are  expressed  by  the  Mass  Law  of  Guldberg  and  Waage, 
according  to  the  equation : 

[H+]  X  [OH-] 
[H20] 

in  which  the  brackets  signify  ionic  or  molecular  concentrations, 

579 


580  GELATIN  AND  GLUE 

and  k  is  a  constant.  Since  the  ionization  of  water  is  very  slight, 
the  concentration  of  the  undissociated  water  [H20]  will  always  be 
practically  constant,  so  the  equation  may  be  written: 

[H+]  X  [OH-]  =  KUJ 

in  which  Kw  is  known  as  the  ionization  constant  for  pure  water. 
Now  since  the  above  equation  is  an  expression  of  fact,  it  follows 
that  if  there  exists  in  any  solution  an  excess  of  hydrogen  ions, 
then,  in  order  that  Kw  shall  not  change,  the  concentration  of 
hydroxyl  ions  must  become  smaller.  This  is  brought  about  by 
the  combination  of  the  hydroxyl  ions  with  some  of  the  excess 
hydrogen  ions  until  the  product  of  the  concentrations  of  the  two 
is  again  that  represented  by  Kw.  On  the  other  hand,  if  an  excess 
of  hydroxyl  ions  is  present  or  caused  to  be  formed  in  the  solution, 
a  similar  combination  reduces  the  concentration  of  the  hydrogen 
ions  to  that  necessary  in  order  that  the  product  shall  be  equal  to 
Kw.  In  case  neither  of  these  ions  are  present  in  excess  the 
concentrations  of  the  two  will  be  equal,  and  the  solution  is 
spoken  of  as  neutral.  This  is  the  condition  in  pure  water,  and  in 
such  solutions  as  do  not  result  in  any  change  in  this  equilibrium. 
The  value  of  Kw  in  pure  water  has  been  investigated  by  a 
number  of  workers  and  a  number  of  different  methods  have  been 
employed  for  the  determination.  The  very  careful  researches  of 
Michaelis1  gave  it  the  value  of  10~14  at  22°C.  Since  in  pure 
water  [H+]  must  be  equal  to  [OH~],  it  follows  that  the  value  for 
each  of  these  is,  in  water,  10~7  at  22°C.2  As  the  temperature 
changes,  the  ionization  constant  also  changes,  becoming  greater 
as  the  temperature  rises.  A  table  illustrative  of  this  is  given  on 
page  593.  But  when  one  of  these  ions  is  increased,  as  by  the 
addition  of  an  acid,  the  other  ion  must  be  repressed  to  such  an 
extent  that  the  product  of  the  concentrations  of  the  two  shall 
be  always  10~14  (at  22°C.).  For  example,  in  a  normal  solution 
of  a  completely  dissociated  acid  the  ionic  concentration  of  the 
hydrogen  ion  is  1.  The  concentration  of  the  hydroxyl  ion  is 
then : 

[OH-]  =  Kw  I  [H+], 

1  MICHAELIS,  "Die  Wasserstoffionenkonzentration,"(1914:). 

2  The  findings  of  [H+]  in  water  by  other  investigators  has  varied  from 
15.8   X    10~7  to  1.23  X  10-8.     Cf.  Beans  and  Oakes,  J.  Am.  Chem.  Soc., 
42  (1920),  2116. 


APPENDIX  581 

or  10-14/10°  =  10-14.  If  the  solution  is  N/100  acid,  [H+]  = 
1/100  or  10-2,  and  [OH~]  =  10~14/10-2  =  10~12.  Obviously  the 
sum  of  the  exponents  of  10,  representing  the  concentrations  of 
the  two  ions  must  always  be  -14.  If  the  concentrations  were 
expressed  in  whole  numbers,  the  hydroxyl  ion  would  have  the 
value,  in  the  latter  case,  of  N/ 1,000,000,000,000,  which  is  the 
same  as  writing  [OH~]  =  10~12.  In  water  [H+]  and  [OH~]  are 
both  N/10.000,000,  but  this  value  is  more  easily  expressed  as 
10~7. 

The  system  of  expressing  the  hydrogen  ion  concentration  as 
exponents  of  10  does  not  seem  cumbersome  so  long  as  only 
integers  are  necessary,  but  as  soon  as  decimals  are  required  they 
become  less  easy  of  interpretation.  The  electrometric  measure- 
ment of  the  value  is  found  to  involve  the  expression: 

Potential  1 

=  log 


Numerical  factor  [H+] 

and  S0rensen1  in  1909  suggested  that  the  term  "log  fg+i"  be 

denoted  by  the  symbol  pH  and  employed  without  further 
change  for  expressing  the  hydrogen  ion  concentration.  This 
suggestion  has  many  obvious  advantages  and  has  been  almost 
universally  adopted  by  chemists,  physicists  and  biologists. 
The  pH  is  a  linear  function  of  the  hydrogen  electrode  potential 
and  the  errors  in  determination  are  proportional  to  pH  rather  than 
to  [H+].  It  is  at  first  confusing  that  the  pH  varies  inversely  as 
[H+],  but  if  we  remember  that  pH  indicates  the  hydroxyl  as  well 
as  the  hydrogen  ion  concentration  this  apparent  conflict  with  a 
mental  habit  is  at  once  overcome. 

If  [H+]  is  desired  to  be  calculated  from  a  known  pH,  it  may 
be  done  without  trouble.  For  example,  if  pH  =  8.52,  CH  = 
10-8.52  =  io~9  +  °-48  =  10°-48  X  10~9.  10°-48  =  3.02  (antilog  of 
0.48).  Therefore  pH  8.52  is  equivalent  to  [H+]  =  3.02  X  10~9N. 
If  we  have  given  the  latter  value,  to  find  the  pH,  pH  =  log 

=  1  "  lo§  (3'02  X  10"9)'     L°g  X   =  °'  S°  PH  = 


3  Q2 

-  log  3.02  -  log  10~9,  =  -0.48  +  9.00  =  8.52. 

The  general  relationship  between  pH  and  hydrogen  ion  con- 
centration is  shown  in  Table  66.     The  normality  of  the  solution 
may  be  correctly  said  to  be  identical  with  the  hydrogen  ion  con- 
1  S.  SJ^RENSEN,  Comp.  rend.  Lab.  Carlsberg,  8  (1909),  1. 


582 


GELATIN  AND  GLUE 


TABLE  66. — APPROXIMATE  RELATION  OF  [H+]  AND  [OH~]  TO  pH'  AND 

NORMALITY 


Normality1 

pH 

[H+] 

[OH-] 

N  HC1  

0  0 

1  0 

1Q-14 

0.  1  HC1 

1  0 

0  1 

10~13 

0.01  HC1  

2.0 

10-2 

1Q-12 

0.001  HC1  

3  0 

10~3 

10~u 

0  0001  HC1 

4  0 

10~4 

lO-io 

0.00001  HC1  

5.0 

10-5 

10~9 

0.000001  HCL... 

6  0 

10~fl 

10~8 

Neutrality 

7  0 

10~7 

10~7 

0.000001  KOH  

8.0 

10~8 

io-« 

0.00001  KOH  

9  0 

lO'9 

10~5 

0  0001  KOH  . 

10  0 

IQ-io 

10~4 

0  001  KOH 

11  0 

1Q-11 

10~3 

0.01  KOH  
0.1  KOH.. 

12.0 
13  0 

10-i2 
10~13 

lO-2 

10"1 

N  KOH 

14  0 

1Q-14 

1  0 

TABLE  67. — [H+]  AND  pH  OF  SOME  ACIDS  AND  ALKALIES2 


lonogen 

Normality 

[H+] 

pH 

HC1  

1  0 

SOX  10-1 

0  10 

HC1 

0  1 

8  4    X  10~2 

1  071 

HC1  

0.01 

9.5     X  10~3 

2.022 

HC1   

0  001 

9  7    X  10~4 

3  013 

HC1 

0  0001 

9  8    X  10~5 

4  009 

CH3COOH  

1.0 

4.3     X  10~3 

2.366 

CH3COOH  

0.1 

1.36  X  10~3 

2  866 

CH,COOH               .... 

0  01 

4  3     X  10~4 

3  366 

CH3COOH 

0  001 

1  36  X  10~4 

3  866 

NaOH  

1.0 

0.90  X  10~14 

14  05 

NaOH     

0.1 

0  86  X  10~13 

13  07 

NaOH 

0  01 

0  76  X  10~12 

12  12 

NaOH 

0  001 

0  74  X  10~u 

11  13 

NH4OH      

1.0 

1.7    X  10~12 

11.77 

NH4OH                        

0.1 

5.4     X  10~12 

11  27 

NH4OH 

0.01 

1  7    X  1Q-11 

10  77 

NH4OH  

0.001 

5.4     X  10~10 

10.27 

1  Theoretical  value  assuming  complete  dissociation.     See  next  table. 

2  MICHAELIS,  "Die  Wasserstoffionenkonzentration"  (1914),  23. 


APPENDIX 


583 


centration  only  when  100  per  cent  dissociated,  but  for  the  fixing 
of  the  approximate  relationship  in  the  mind  a  few  normality 
expressions  are  included.  In  Table  67  are  shown  the  pH  and 
hydrogen  ion  concentration  of  some  acids  and  alkalies  that  have 
been  experimentally  determined.  As  shown,  the  weaker  the 
electrolyte,  i.e.,  the  less  the  dissociation, — the  further  removed 
will  be  the  pH  from  the  assigned  normality  value.  The  con- 
version of  pH  to  [H+]  is  shown  in  Table  68. 

TABLE  68. — CONVERSION  TABLE  OF  pH  TO  [H+]1 


pH 

[H+] 

pH 

[H+] 

0.00 

1.00  X  10~* 

0.55 

0.28  X  10- 

0.05 

0.89  X  10-* 

0.60 

0.25  X  10- 

0.10 

0.79  X  10-* 

0.65 

0.22  X  10- 

0.15 

0.71  X  10-* 

0.70 

0.20  X  10- 

0.20 

0.63  X  10-* 

0.75 

0.18  X  10- 

0.25 

0.56  X  10-* 

0.80 

0.16  X  10- 

0.30 

0.50  X  10-* 

0.85 

0.14  X  10- 

0.35 

0.45  X  10-* 

0.90 

0.13  X  10- 

0.40 

0.40  X  10-* 

0.95 

0.11  X  10- 

0.45 

0.36  X  10-* 

1.00 

0.10  X  10- 

0.50 

0.32  X  10"* 

1  X  10~7 

0.25  X  10-7or  2.5  X  lO'8 


Example  pH  =  7.00;  [H~ 
pH  =  7.60;  [HH 

The  Theory  of  the  Hydrogen  Electrode.  —  Nernst2  has  pro- 
duced an  equation  by  which  the  exact  relations  obtaining 
between  the  electrode  potential  of  a  solution  and  the  ionic  con- 
centration are  defined.  This  is  based  upon  the  conception  of 
electrolytic  solution  tension,  i.e.,  the  difference  in  potential 
developed  between  a  metal  and  a  solution  of  that  metal  when  the 
two  are  brought  together.  A  tendency  either  for  the  metal  to 
go  into  solution,  or  for  the  metallic  ions  to  come  out  of  solution 
is  manifested  by  the  development  of  a  measurable  potential 
between  the  two  phases.  The  thermodynamic  soundness  of  the 
principle  has  since  been  many  times  demonstrated. 

The  fundamental  equation  may  be  written: 

C 


1  W.  M.  CLARK,  "The  Determination  of  Hydrogen  Ions"  (1920),  307. 

2  W.  NERNST,  Z.  physik.  Chem.,  4  (1889),  129. 


584 


GELATIN  AND  GLUE 


where  #uf  the  electromotive  force  measured  (the  algebratic  sum 
of  the-^-f,  "rode  potentials  of  the  tw^'balf  cells),  R  is  the  gas. 
constai  i. 3 129446  joules  per  degree),  T  is  the  absolute  tem- 
peratur  C.  +  273),  n  is  the  valence  uf  the  ion  (H  =  1),  F 
is  the  Fa  .ay  constant  (96,494  coulombs) ,  In  refers  to  the  natural 
logarithm  and  C  and  C",  are  the  ionic  concentrations  of  the  two 
solutions.  On  substituting  the  constants  in  the  equation  and 
transposing  to  Briggsian  logarithms  (to  the  base  10)  by  dividing 
by  0.43429,  and  referring  to  a  solution  normal  with  respect  to 
the  hydrogen  ion,  we  obtain: 

E  =  0.0001983777  log  .*• 
o 

Since,  however,  it  has  been  found  difficult  to  obtain  a  solution 
absolutely  standard  with  respect  to  hydrogen  ion  it  has  become 
customary  to  employ  for  our  working  standard  a  calomel 
electrode  and  to  calculate  the  difference  of  potential  between 
this  arbitrary  standard  and  the  theoretical  normal  hydrogen 
electrode  by  measurements  made  against  a  solution  of  some 
fractional  normal  hydrogen  ion  concentration.  Such  measure- 
ments have  been  made  with  great  care  using  tenth  normal, 
normal,  and  saturated  potassium  chloride — calomel  electrodes. 
The  most  generally  accepted  values  are  shown  below. 


TABLE  69. — STANDARD  VALUES  FOR  CALOMEL  ELECTRODES1 


Concentration  of  KC1 

Temperature,  °C. 

M/10 

M/l 

Saturated 

18 

0.3380 

0.2864 

0.2506 

20 

0.3379 

0.2860 

0.2492 

25 

0.3376 

0.2848 

0.2464 

30 

0.3373 

0.2837 

0.2437 

40 

0.3360 

The  working  formula,  therefore,  becomes : 
E.  M.  F.  (observed)  —  e  (calomel  electrode) 

0.00019837  T 
1  W.  M.  CLARK,  lib.  tit.,  306. 


APPENDIX  585 

Thus,  by  working  with  an  N/10  calomel  cell  at  a  tern  -ture  of 
25°C.,  the  formula  become. 

E  -  0.3376 
0.05911  -; 
or  at  30°  with  a  normal  cell: 

TT_#-  0.2837  <i 

0.06011" 

E  in  these  cases  representing  the  observed  electromotive  force. 

Influence  of  Barometric  Pressure,  and  Dilution. — Hydrogen  ion 
concentrations  are  usually  based  upon  the  hydrogen  pressure 
of  one  atmosphere  or  760  millimeters  of  mercury.  A  measure- 
ment made  at  any  other  pressure  will  produce  an  error  which 
should  be  corrected  in  exact  physico-chemical  researches,  but 
the  size  of  the  correction  is  so  small  that  it  may  be  safely  dis- 
regarded in  all  but  the  most  exacting  investigations.  Loomis  and 
Acree1  found  that  a  difference  of  40  millimeters  in  the  barometric 
pressure  produced  a  change  of  only  0.0007  volt  in  the  E.  M.  F. 
The  correction  may  be  applied  by  the  formula : 

,   0.0001983771 ,      760 
Eo  =  Eb  ±  -     — 2~-     -  log  -y> 

where  E0  is  the  corrected  E.  M.  F.  at  760  mm.,  and  Eb is  the  E.  M.  F. 
at  a  barometric  pressure  6.  The  factor  is  added  when  b  is  less, 
and  subtracted  when  6  is  greater  than  760  millimeters. 

The  influence  of  dilution  on  the  hydrogen  ion  concentration 
of  a  liquid  or  solution  varies  greatly  with  the  nature  of  the  sub- 
stance in  question.  If  a  pure  strong  inorganic  acid,  as  hydro- 
chloric, is  diluted,  the  pH  follows  very  closely  the  normality  of 
the  solution.  That  is,  a  dilution  of  1  to  10,  as  from  0.1  to  0.01N, 
produces  almost  the  theoretical  change  in  [H+]  and  pH  that 
would  be  expected,  i.e.,  from  1.07  to  2.02.  This  slight  discrep- 
ency  is  adequately  accounted  for  by  the  increasing  dissociation 
upon  dilution.  A  further  dilution  to  0.001N  brings  the  pH  to 
3.01  and  to  0.0001N  to  4.01  which  is  almost  exactly  coincident 
with  the  theoretical  values.  Strong  bases  behave  in  an  entirely 
similar  manner. 

If  a  weak  acid  such  as  acetic  is  used,  the  change  in  pH  upon 
dilution  will  be  smaller.  The  pH  of  a  0.1N  solution  is  in  this 
case  2.87,  which  is  quite  far  removed  from  the  value  1.00  which 
would  be  the  value  in  a  completely  dissociated  acid,  and  on  dilut- 

1  N.  E.  LOOMIS  and  S.  F.  ACREE,  J.  Am.  Chem.  Soc.,  38  (1916),  2391. 


586 


GELATIN  AND  GLUE 


ing  to  0.01  N  the  change  is  only  to  3.37,  or  half  the  change  that 
would  result  in  the  strong  acids.  In  still  weaker  acids  the 
change  in  pH  upon  dilution  becomes  very  small.  The  following 
table  shows  the  effect  of  dilution  upon  the  pH  of  glycine  and 
asparagine.1 

TABLE  70. — CHANGE  IN  pH  UPON  DILUTION 


Glycine 

pH 

Asparagine 

pH 

1.0  M  

6  089 

1  0  M  

2.954 

0  1  M  

6  096 

0  1  M              

2  973 

0  01  M 

6  155 

0  01  M 

3  110 

0  001  M  

6  413 

0  001  M  

3.521 

0  0001  M       .      ... 

6  782 

0  0001  M                .... 

4.166 

The  addition  of  a  salt  with  a  common  ion  to  a  solution  of  a 
weak  acid  is  shown  by  the  Mass  Law  to  result  in  a  decrease  in 
the  hydrogen  ion  concentration.  For  example,  the  ionization 
equilibrium  for  acetic  acid  is: 

[H+]  X  [CH3COO-] 
[CH3COOH] 

If  to  this  system  sodium  acetate  is  added,  which  is  largely  dis- 
sociated into  the  ions  Na+  and  CH3COO~~,  the  concentration  of 
the  CHSCOO~  ions  in  the  system  is  greatly  increased.  In 
order  that  K  shall  remain  constant  under  these  new  conditions 
hydrogen  ions  must  combine  with  some  of  the  excess  acetate 
ions  forming  the  undissociated  acid  until  K  has  again  reached 
its  former  value.  That  is,  [H+]  is  greatly  suppressed  and 
accordingly  the  pH  will  become  greater. 

If  now  such  a  system  be  diluted  with  pure  water  there  will  be 
very  little  further  alteration  in  pH,  because  the  equilibrium  has 
become  one  dependent  to  a  far  greater  extent  upon  the  relative 
concentrations  of  salt  to  acid,  than  upon  acid  to  water.  Dilution 
will  not  alter  the  former  ratio  except  in  so  far  as  it  results  in  an 
increase  in  the  dissociation  of  the  acid.  Walpole2  has  found  that 
the  change  of  pH  resulting  from  a  twenty-fold  dilution  of  an 
acetic  acid — sodium  acetate  mixture,  when  present  in  approxi- 
mately equivalent  proportions,  is  about  0.08  pH. 

1  S.  S0RENSEN,  Compt.  rend.  Irav.  lab.  Carlsberg,  12  (1917),  1. 

2  G.  S.  WALPOLE,  J.  Chem.  Soc.,  105  (1914),  2501;  2521. 


APPENDIX  587 

Buffer  Action. — One  of  the  difficulties  in  obtaining  exact 
measurements  of  the  pH  of  pure  water  lies  in  the  extreme  sensi- 
tivity of  water  to  minute  traces  of  impurities,  such  as  carbon 
dioxide  or  other  gases  that  may  be  dissolved  in  it.  If  1  c.c.  of 
0.01N  hydrochloric  acid  is  added  to  a  liter  of  pure  water  the 
pH  will  be  altered  from  7.0  to  5.0,  i.e.,  a  hundred-fold  increase  in 
[H+],  But  if  the  same  amount  of  acid  is  added  to  a  liter  of  a 
gelatin  solution  at  pH  7.0,  the  change  in  pH  would  be  hardly 
appreciable.  This  power  of  certain  substances  to  resist  the 
change  in  [H+]  upon  the  addition  of  reagents  which  in  pure  water 
would  produce  a  profound  alteration  has  been  spoken  of  by 
S0rensen1  as  due  to  a  buffer  action  of  the  material.  This  term  has 
been  very  generally  taken  up  in  reference  to  any  solution  which 
resists  alteration  in  pH  due  to  the  addition  or  loss  of  acid  or 
alkali.  Colorimetric  methods  for  the  measurement  of  hydrogen 
ion  concentration  all  make  use  of  solutions  of  standard  pH  values 
which  are  made  from  well  buffered  mixtures  that  they  may 
remain  constant  and  that  serious  errors  may  not  be  introduced 
through  slight  dilution. 

The  Electrometric  Determination. — The  equipment  neces- 
sary for  the  carrying  out  of  electrometric  measurements  of 
pH  includes  the  following:  A  hydrogen  electrode  consists  of  a 
strip  of  platinum  (palladium,  iridium  or  gold  are  sometimes  used) 
which  may  be  in  the  form  of  a  flat  foil,  or  as  a  gauze,  or  spiral. 
This  is  coated  with  a  layer  of  platinum  black  (or  of  palladium 
or  iridum  black)  by  electrolysis  in  a  1  per  cent  solution  of  pure 
chlorplatinic  acid,2  or  in  a  1  to  3  per  cent  hydrochloric  acid  solu- 
tion of  platinum  chloride.3  The  current  may  be  supplied  from  a 
4  volt  storage  battery,  and  should  be  reversed  every  2  or  3 
minutes.  The  layer  should  not  be  made  too  thick,  about  ten 
minutes  being  a  sufficient  time  to  allow  for  the  deposition.  This 
coated  electrode  is  then  sealed  into  a  glass  tube,  the  exact  form  of 
which  has  been  largely  determined  by  the  particular  require- 
ments of  the  problem  in  hand.  A  large  number  of  types  have 
been  discussed  in  the  literature. 

The  hydrogen  electrode  vessel  contains  the  solution  under 
investigation  and  into  this  is  dipped  the  hydrogen  electrode. 
The  arrangement  must  provide  for  the  alternate  exposure  of  the 

1  S.  S0RENSEN,  IOC.  tit. 

2  J.  H.  ELLIS,  J.  Am.  Chem.  Soc.,  38  (1916),  737. 
3W.  M.  CLARK,  lib.  tit.,  124. 


588 


GELATIN  AND  GLUE 


electrode  to  a  stream  of  hydrogen  gas  and  to  the  solution  in  the 
vessel.  This  is  often  accomplished  by  requiring  the  gas  to 
bubble  past  the  electrode  in  its  passage  into  the  solution.  Clark 
uses  a  vessel  that  may  be  rocked.  A  few  types 
of  hydrogen  cells  are  shown  in  the  accompany- 
ing illustrations . 

The  standard  half  cell  is  usually  a  calomel 
electrode  but  Acree  has  proposed  a  hydrogen 
electrode  bathed  in  a  standard  buffer  solution 
for  this  purpose.  The  calomel  cells  are  made 
by  placing  a  little  highly  purified  mercury 
in  the  bottom  of  the  cell,  over  this  placing  a 
layer  of  pure  calomel,  and  finally  a  solution 
of  potassium  chloride  of  a  definite  concentra- 
tion. Custom  has  placed  the  concentration 
of  the  latter  solution  at  either  0.1N,  l.ON,  or 
saturated. 

The  calomel  should  be  prepared  as  follows : 
putrified  mercury  is  dissolved  in  redistilled 
nitric  acid,  and  the  solution  poured  into  a 
large  excess  of  distilled  water.  A  redistilled 
20  per  cent  solution  of  hydrochloric  acid  is 
diluted  and  added  slowly  and  with  constant 
stirring  to  the  above.  When  the  precipitate 
has  settled  the  liquid  is  decanted  off,  and  the  residue  washed  by 
decantation  with  distilled  water  for  several  days.  Potassium 
chloride  solution  of  the  strength  desired,  made  from  recrystallized 
highest  purity  salt,  is  then  substituted  for  the  water  and  many 
more  washings  made. 

A  potentiometer  is  the  most  satisfactory  instrument  for  making 
E.  M.F.  measurements,  although  a  milli voltmeter  has  been  used  in 
several  important  researches.  The  type  K  potentiometer  of  the 
Leeds  and  Northrup  Company,  shown  in  Figs.  113  and  114,  is  the 
best  instrument  now  available.  A  less  expensive  form  known 
as  the  Northrup  pH  Pyrovolter  Potentiometer1  is  more  compact 
and  convenient  for  carrying  about  as  the  instrument  requires  no 
storage  battery,  standard  cell,  or  galvanometer.  As  accessories 
to  the  former  instrument,  there  are  required  a  two  volt  storage 
cell,  a  Weston  standard  cadmium  cell,  and  a  galvanometer. 
The  type  R  reflecting  galvanometer  with  a  lamp  and  scale  outfit, 
1  Graham  Chemical  Co.,  Rochester,  N.  Y. 


FIG.  109.— The 
hydrogen  electrode 
of  J.  H.  Hildebrand. 


APPENDIX 


589 


and  provided  with  a  heavy  damping;  resistance,  are  desirable 
for  the  most  accurate  work,  but  the  box-type  enclosed  lamp  and 
scale  galvanometer  is  satisfactory  for  all  work  of  moderate 
precision.  The  Lippman  electrometer  has  in  the  past  served  as 
the  measure  of  current  adjustment,  but  the  much  more  easily 
handled  galvanometers  have  almost  completely  superceded 
that  delicate  but  troublesome  instrument. 


FIG.  110. — Clark's  system  for  the  determination  of  hydrogen  ions.  (From 
W.  M.  Clark,  "The  Determination  of  Hydrogen  Ions,"  Williams  and  Wilkins 
Company,  Baltimore,  1920.) 

The  hydrogen  gas  is  most  conveniently  supplied  from  a  cylinder 
of  the  compressed  gas,  supplied  with  a  reducing  valve,  and 
purified  by  passing  through  solutions  of  alkaline  permanganate, 
concentrated  sulphuric  acid,  and  finally  water  containing  a  little 
barium  chloride.  The  water  leaves  the  gas  moist  so  that  it  will 
not  produce  evaporation  of  the  solution  being  measured  upon 
being  passed  through  it,  while  the  barium  chloride  removes  any 
traces  of  sulphuric  acid  that  may  be  carried  over.  The  use  of 
hydrogen  from  commercial  tanks,  after  proper  purification,  has 
been  employed  successfully  by  a  number  of  investigators.  If  it 
is  desired  to  generate  the  gas  it  may  be  done  by  the  electrolysis 
of  water  or  dilute  sodium  hydroxide.1  The  action  of  acids  on 
metals  is  less  satisfactory. 

1  Cf.  W.  M.  CLARK,  lib.  tit.,  162. 


590  GELATIN  AND  GLUE 

An  air  thermostat  for  maintaining  a  constant  temperature 
should  be  used  as  a  container  for  the  hydrogen  and  calomel  cells 
if  the  highest  accuracy  is  required,  but  for  most  work  it  will  be 
unnecessary  to  take  this  precaution  provided  the  temperature  of 
the  measurements  is  noted  and  the  formula  corrected  accordingly. 

The    Measurement. — The    connections    are    made    upon    the 


FIG.    111.— The    Elliott    single    cell    "lon-O-Meter."     (Kindness    of  Felix    A. 

Elliott.) 

potentiometer  as  indicated,1  using  care  always  to  connect  +  to 
+,  and  attaching  the  +  E.  M.  F.  wire  to  the  calomel  electrode 
and  the  —  wire  to  the  hydrogen  cell.  The  solution  to  be  tested, 
which  should  not  be  too  viscous,  is  placed  in  the  hydrogen  elec- 
trode vessel  and  hydrogen  admitted  as  described,  adopting  what- 

1  For  details  of  wiring  and  measurement  see  Leeds  and  Northrup  cata- 
logue No.  70  (1919),  or  W.  M.  CLARK,  lib.  cit.,  142. 


APPENDIX 


591 


ever  means  that  may  be  necessary  to  insure  the  proper  shaking 
or  stirring  of  the  solution.  After  the  hydrogen  has  been  passing 
for  a  suitable  length  of  time  (about  10  to  15  minutes  in  the  Clark 
cell,  or  30  minutes  in  the  Hildebrand  type),  the  supply  of  gas  is 
shut  off,  the  shaking  stopped,  and  liquid  connection  made 


FIG.  112. — The  Elliott  titration  "lon-O-Meter."     (Kindness  of  Felix  A.  Elliott.) 

between  the  hydrogen  electrode  vessel  and  the  saturated  potas- 
sium chloride,  obtaining  as  large  a  contact  surface  between  the 
two  solutions  as  practicable  in  order  to  reduce  the  contact 
potential  to  a  minimum.  A  liquid  contact  is  also  made  between 
the  calomel  electrode  and  the  saturated  potassium  chloride 
solution.  The  potential  of  the  storage  battery  is  adjusted  against 
that  of  the  standard  cell  by  varying  the  resistance  in  the  rheostat 
of  the  potentiometer  until  the  two  are  identical  as  indicated  by  a 
zero  deflection  of  the  galvanometer  upon  making  the  connection. 


592 


GELATIN  AND  GLUE 


The  connection  with  the  calomel  and  hydrogen  cells  is  then  made, 
and  the  deflection  again  brought  to  zero  by  varying  the  resistance 


FIG.  113. — The  Leeds  and  Northrup  potentiometer  (type  K). 


FIG.  114. — Wiring  of  the  Leeds  and  Northrup  potentiometer  (type  K). 

in  the  potentiometer  wire.  The  readings  may  be  made  to 
tenths  and  estimated  to  hundredth^  of  a  millivolt.  These 
readings  are  then  calculated,  or  read  off  from  a  table,  to  [H+], 
OH-],  or  pH. 


CONVERSION  TABLES  SHOWING  E.M.F.,  pH  [H+]  AND  [OH~] 

The  following  tables  computed  by  Schmidt  and  Hoagland1 
show  the  relation  between  the  E.  M.  F.  of  normal  and  of  tenth 
1  SCHMIDT  and  HOAGLAND,  Univ.  of  Cal.  Pub.  in  Phys.,  6  (1919),  23-69. 


APPENDIX 


593 


normal  potassium  chloride-calomel  cells  and  the  pH,  [H+]and 
[OH~],  of  solutions  measured  at  25°C.,  between  normal  [H+] 
and  normal  [OH~]  in  steps  of  0.002  volt.  If  any  other  tempera- 
ture than  25°  is  used,  the  observed  E.  M.  F.  should  be  multiplied 
by  a  factor  to  obtain  the  E.  M.  F.  it  would  have  at  25°,  as  follows: 


Temperature 

Factor 

Temperature 

Factor 

18 

1.024 

25 

1.000 

19 

1.021 

26 

0.996 

20 

1.017 

27 

0.993 

21 

1.014 

28 

0.990 

22 

1.010 

29 

0.987 

23 

1.007 

30 

0.983 

24 

1.004 

TABLE   71. — TABLES   CONVERTING   VOLTAGES   OBSERVED   WITH   NORMAL 

AND  TENTH  NORMAL  CALOMEL  CELLS  TO  pH,  [H+],  AND  [OH~] 

(Schmidt  and  Hoagland) 


EN 
1 

EN 
10 

pH 

CH+ 
X 
NH+ 

COH~ 

X  10-" 
OH- 

EN 
1 

EN 
10 

pH 

CH+ 
X 
lO-i  H+ 

COH- 

x  10-" 

OH- 

0.283 

0.336 

1.000 

1.01 

0.343 

0.396 

.014 

0.968 

1.05 

'    0.285 

0.338 

6!  034 

0.925 

1.09 

0.345 

0.398 

.048 

0.895 

.13 

0.287 

0.340 

0.068 

0.856 

1.18 

0.347 

0.400 

.082 

0.828 

.22 

0.289 

0.342 

0.101 

0.792 

1.28 

0.349 

0.402 

.116 

0.766 

.32 

0.291 

0.344 

0.135 

0.732 

.38 

0.351 

0.404 

.150 

0.709 

.43 

0.293 

0.346 

0.169 

0.678 

.49 

0.353 

0.406 

.183 

0.656 

.54 

0.295 

0.348 

0.203 

0.627 

.61 

0.355 

0.408 

.217 

0.607 

.67 

0.297 

0.350 

0.237 

0.580 

.75 

0.357 

0.410 

.251 

0.561 

1.80 

0.299 

0.352 

0.270 

0.536 

.89 

0.359 

0.412 

.285 

0.519 

1.95 

0.301 

0.354 

0.304 

0.496 

2.04 

0.361 

0.414 

.319 

0.480 

2.11 

0.303 

0.356 

0.338 

0.459 

2.20 

0.363 

0.416 

.352 

0.444 

2.28 

0.305 

0.358 

0.372 

0.425 

2.38 

0.365 

0.418 

.386 

0.411 

2.46 

0.307 

0.360 

0.406 

0.393 

2.58 

0.367 

0.420 

.420 

0.380 

2.66 

0.309 

0.362 

0.440 

0.364 

2.78 

0.369 

0.422 

.454 

0.352 

2.88 

0.311 

0.364 

0.473 

0.336 

3.01 

0.371 

0.424 

.488 

0.325 

3.11 

0.313 

0.366 

0.507 

0.311 

3.25 

0.373 

0.426 

.521 

0.301 

3.36 

0.315 

0.368 

0.541 

0.288 

3.51 

0.375 

0.428 

.555 

0.278 

3.64 

0.317 

0.370 

0.575 

0.266 

3.80 

0.377 

0.430 

.589 

0.258 

3.92 

0.319 

0.372 

0.609 

0.246 

4.11 

0.379 

0.432 

.623 

0.238 

4.25 

0.321 

0.374 

0.642 

0.228 

4.44 

0.381 

0.434 

.657 

0.221 

4.58 

0.323 

0.376 

0.676 

0.211 

4.80 

0.383 

0.436 

.691 

0.204 

4.96 

0.325 

0.378 

0.710 

0.195 

5.19 

0.385 

0.438 

.724 

0.189 

5.35 

0.327 

0.380 

0.744 

0.180 

5.62 

0.387 

0.440 

.758 

0.175 

5.78 

0.329 

0.382 

0.778 

0.167 

6.06 

0.389 

0.442 

.792 

0.162 

6.25 

0  331 

0.384 

0  811 

0.154 

6.57 

0.391 

0.444 

.826 

0.149 

6.79 

0.333 

0.386 

0.845 

0.143 

7.08 

0.393 

0.446 

.860 

0.138 

7.33 

0.335 

0.388 

0.879 

0.132 

7.67 

0.395 

0.448 

1.893 

0.128 

7.91 

0  337 

0  390 

0  913 

0.122 

8.30 

0.397 

0.450 

1.927 

0.118 

8.58 

0.339 

0.392 

0.947 

0.113 

8.96 

0.399 

0.452 

1.961 

0.109 

9.28 

0.341 

0.394 

0.980 

0.105 

9.64 

0.401 

0.454 

1.995 

0.101 

10.00 

38 


594 


GELATIN  AND  GLUE 
TABLE   71. — Continued 


EN 
1 

EN 
10 

pH 

CH+ 

X  10-2 
H+ 

COH- 
X  10-12 
OH- 

EN 
1 

EN 
10 

pH 

CH+ 

X  10-3 
H+ 

COH~ 

x  10-11 

OH- 

0.403 

0.456 

2.029 

0.036 

1.08 

0.461 

0.514 

3.009 

0.979 

1.03 

0.405 

0.458 

2.062 

0.966 

1.17 

0.463 

0.516 

0.043 

0.906 

1.12 

0.407 

0.460 

2.096 

0.801 

1.26 

0.465 

0.518 

3.077 

0.838 

1.21 

0.409 

0.462 

2.130 

0.741 

1.37 

0.467 

0.520 

3.111 

0.775 

1.31 

0.411 

0.464 

2.164 

0.686 

1.48 

0.469 

0.522 

3.144 

0.717 

1.41 

0.413 

0.466 

2.198 

0.634 

1.60 

0.471 

0.524 

3.178 

0.663 

1.53 

0.415 

0.468 

2.232 

0.587 

1.72 

0.473 

0.526 

3.212 

0.614 

1.65 

0.417 

0.470 

2.265 

0.543 

1.86 

0.475 

0.528 

3.246 

0.568 

1.78 

0.419 

0.472 

2.299 

0.502 

2.02 

0.477 

0.530 

3.280 

0.525 

1.93 

0.421 

0.474 

2.333 

0.465 

2.18 

0.479 

0.532 

3.313 

0.486 

2.08 

0.423 

0.476 

2.367 

0.430 

2.35 

0.481 

0.534 

3.347 

0.450 

2.25 

0.425 

0.478 

2.401 

0.398 

2.54 

0.483 

0.536 

3.381 

0.416 

2.43 

0.427 

0.480 

2.434 

0.368 

2.75 

0.485 

0.538 

3.415 

0.385 

2.63 

0.429 

0.482 

2.468 

0.340 

2.98 

0.487 

0.540 

3.449 

0.356 

2.84 

0.431 

0.484 

2.502 

0.315 

3.21 

0.489 

0.542 

3.483 

0.329 

3.08 

0.433 

0.486 

2.536 

0.291 

3.48 

0.491 

0.544 

3.516 

0.305 

3.32 

0.435 

0.488 

2.570 

0.269 

3.76 

0.493 

0.546 

3.550 

0.282 

3.59 

0.437 

0.490 

2.603 

0.249 

4.06 

0.495 

0.548 

3.548 

0.261 

3.88 

0.439 

0.492 

2.637 

0.231 

4.38 

0.497 

0.550 

3.618 

0.241 

4.20 

0.441 

0.494 

2.671 

0.213 

4.75 

0.499 

0.552 

3.652 

0.223 

4.54 

0.443 

0.496 

2.705 

0.197 

5.14 

0.501 

0.554 

3.685 

0.206 

4.91 

0.445 

0.498 

2.739 

0.183 

5.53 

0.503 

0.556 

3.719 

0.191 

5.30 

0.447 

0.500 

2.772 

0.169 

5.99 

0.505 

0.558 

3.753 

0.177 

5.72 

0.449 

0.502 

2.806 

0.156 

6.49 

0.507 

0.560 

3.787 

0.163 

6.21 

0.451 

0.504 

2.840 

0.145 

6.98 

0.509 

0.562 

3.821 

0.151 

6.70 

0.453 

0.506 

2.874 

0.134 

7.55 

0.511 

0.564 

3.854 

0.140 

7.23 

0.455 

0.508 

2.908 

0.124 

8.16 

0.513 

0.566 

3.888 

0.129 

7.85 

0.457 

0.510 

2.942 

0.114 

8.88 

0.515 

0.568 

3.922 

0.120 

8.43 

0.459 

0.512 

2.975 

0.106 

9.55 

0.517 

0.570 

3.956 

0.111 

9.12 

0.519 

0.572 

3.990 

0.102 

9.92 

APPENDIX 
TABLE   71. — Continued 


595 


EN 
1 

EN 
10 

pH 

CH+ 
X  10-4 
H+ 

COH- 

x  io-»o 

OH- 

EN 
1 

EN 
10 

pH 

CH+ 
X  10-5 
H+ 

COH- 

x  io-» 

OH- 

0.521 

0.574 

4.023 

0.947 

1.07 

0.579 

0.632 

5.004 

0.991 

.02 

0.522 

0.575 

4.040 

0.911 

1.11 

0.580 

0.633 

5.021 

0.953 

.06 

0.523 

0.576 

4.057 

0.876 

1.16 

0.581 

0.634 

5.038 

0.916 

.10 

0.524 

0.577 

4.074 

0.843 

1.20 

0.582 

0.635 

5.055 

0.881 

.15 

0.525 

0.578 

4.091 

0.811 

1.25 

0.583 

0.636 

5.072 

0.848 

.19 

0.526 

0.579 

4.108 

0.780 

1.30 

0.584 

0.637 

5.089 

0.815 

.24 

0.527 

0.580 

4.125 

0.750 

1.35 

0.585 

0.638 

5.106 

0.784 

.29 

0.528 

0.581 

4.142 

0.721 

.40 

0.586 

0.639 

5.122 

0.754 

.34 

0.529 

0.582 

4.159 

0.694 

.46 

0.587 

0.640 

5.139 

0.725 

.40 

0.530 

0.583 

4.176 

0.667 

.52 

0.588 

0.641 

5.156 

0.698 

.45 

0.531 

0.584 

4.193 

0.642 

.58 

0.589 

0.642 

5.173 

0.671 

.51 

0.532 

0.585 

4.210 

0.617 

.64 

0.590 

0.643 

5.190 

0.646 

.57 

0.533 

0.586 

4.226 

0.594 

.70 

0.591 

0.644 

5.207 

0.621 

.63 

0.534 

0.587 

4.243 

0.571 

.77 

0.592 

0.645 

5.224 

0.597 

.70 

0.535 

0.588 

4.260 

0.549 

.84 

0.593 

0.646 

5.241 

0.574 

1.76 

0.536 

0.589 

4.277 

0.528 

1.92 

0.594 

0.647 

5.258 

0.552 

1.83 

0.537 

0.590 

4.294 

0.508 

1.99 

0.595 

0.648 

5.275 

0.531 

1.91 

0.538 

0.591 

4.311 

0.489 

2.07 

0.596 

0.649 

5.292 

0.511 

1.98 

0.539 

0.592 

4.328 

0.470 

2.15 

0.597 

0.650 

5.308 

0.492 

2.06 

0.540 

0.593 

4.345 

0.452 

2.24 

0.598 

0.651 

5.325 

0.473 

2.14 

0.541 

0.594 

4.362 

0.435 

2.33 

0.599 

0.652 

5.342 

0.455 

2.22 

0.542 

0.595 

4.379 

0.418 

2.42 

0.600 

0.653 

5.359 

0.437 

2.32 

0.543 

0.596 

4.395 

0.402 

2.52 

0.601 

0.654 

5.376 

0.421 

2.40 

0.544 

0.597 

4.412 

0.387 

2.61 

0.602 

0.655 

5.393 

0.405 

2.50 

0.545 

0.598 

4.429 

0.372 

2.72 

0.603 

0.656 

5.410 

0.390 

2.59 

0.546 

0.599 

4.446 

0.358 

2.83 

0.604 

0.657 

5.427 

0.374 

2.71 

0.547 

0.600 

4.463 

0.344 

2.94 

0.605 

0.658 

5.444 

0.360 

2.81 

0.548 

0.601 

4.480 

0.331 

3.06 

0.606 

0.659 

5.461 

0.346 

2.92 

0.549 

0.602 

4.497 

0.319 

3.17 

0.607 

0.660 

5.478 

0.333 

3.04 

0.550 

0.603 

4.514 

0.306 

3.31 

0.608 

0.661 

5.495 

0.320 

3.16 

0.551 

0.604 

4.531 

0.295 

3.43 

0.609 

0.662 

5.511 

0.308 

3.29 

0.552 

0.605 

4.548 

0.283 

3.58 

0.610 

0.663 

5.528 

0.296 

3.42 

0.553 

0.606 

4.564 

0.273 

3.71 

0.611 

0.664 

5.545 

0.285 

3.55 

0.554 

0.607 

4.581 

0.262 

3.86 

0.612 

0.665 

5.562 

0.274 

3.69 

0.555 

0.608 

4.598 

0.252 

4.02 

0.613 

0.666 

5.579 

0.264 

3.83 

0.556 

0.609 

4.615 

0.243 

4.16 

0.614 

0.667 

5.596 

0.254 

3.98 

0.557 

0.610 

4.632 

0.233 

4.34 

0.615 

0.668 

5.613 

0.244 

4.15 

0.558 

0.611 

4.649 

0.224 

4.52 

0.616 

0.669 

5.630 

0.235 

4.31 

0.559 

0.612 

4.666 

0.216 

4.69 

0.617 

0.670 

5.647 

0.226 

4.48 

0.560 

0.613 

4.683 

0.208 

4.87 

0.618 

0.671 

5.664 

0.217 

4.66 

0.561 

0.614 

4.700 

0.200 

5.06 

0.619 

0.672 

5.681 

0.209 

4.84 

0.562 

0.615 

4.717 

0.192 

5.27 

0.620 

0.673 

5.697 

0.201 

5.03 

0.563 

0.616 

4.734 

0.185 

5.47 

0.621 

0.674 

5.714 

0.193 

5.24 

0.564 

0.617 

4.750 

0.178 

5.69 

9.622 

0.675 

5.731 

0.186 

5.44 

0.565 

0.618 

4.767 

0.171 

5.92 

0.623 

0.676 

5.748 

0.179 

5.65 

0.566 

0.619 

4.784 

0.164 

6.17 

0.624 

0.677 

5.765 

0.172 

5.88 

0.567 

0.620 

4.801 

0.158 

6.41 

0.625 

0.678 

5.782 

0.165 

6.13 

0.568 

0.621 

4.818 

0.152 

6.66 

0.626 

0.679 

5.799 

0.159 

6.36 

0.569 

0.622 

4.835 

0.146 

6.93 

0.627 

0.680 

5.816 

0.153 

6.61 

0.570 

0.623 

4.852 

0.141 

7.18 

0.628 

0.681 

5.833 

0.147 

6.88 

0.571 

0.624 

4.869 

0.135 

7.50 

0.629 

0.682 

5.850 

0.141 

7.18 

0.572 

0.625 

4.886 

0.130 

7.78 

0.630 

0.683 

5.866 

0.136 

7.44 

0.573 

0.626 

4.903 

0.125 

8.10 

0.631 

0.684 

5.883 

0.131 

7.73 

0.574 

0.627 

4.920 

0.120 

8.43 

0.632 

0.685 

5.900 

0.126 

8.03 

0.575 

0.628 

4.936 

0.116 

•8.72 

0.633 

0.686 

5.917 

0.121 

8.36 

0.576 

0.629 

4.953 

0.111 

9.12 

0.634 

0.687 

5.934 

0.116 

8.72 

0.577 

0.630 

4.970 

0.107 

9.46 

0.635 

0.688 

5.951 

0.112 

9.04 

0.578 

0.631 

4.987 

0.103 

9.83 

0.636 

0.689 

5.968 

0.108 

9.37 

0.637 

0.690 

5.985 

0.104 

9.73 

596 


GELATIN  AND  GLUE 

TABLE   71. — Continued 


EN 
1 

EN 
10 

pH 

CH+ 
X  10-6 
H+ 

COH- 
X  lO-s 
OH- 

EN 
1 

EN 
10 

pH 

CH+ 
X  10-7 
H+ 

COH~ 

x  10-7 

OH- 

0.638 

0.691 

6.002 

0.996 

1.02 

0.698 

0.751 

7.016 

0.964 

.05 

0.639 

0.692 

6.019 

0.958 

1.06 

0.699 

0.752 

7.033 

0.927 

.09 

0.640 

0.693 

6.036 

0.921 

1.10 

0.700 

0.753 

7.050 

0.892 

.13 

0.641 

0.694 

6.052 

0.886 

1.14 

0.701 

0.754 

7.067 

0.858 

.18 

0.642 

0.695 

6.069 

0.852 

1.19 

0.702 

0.755 

7.084 

0.825 

.23 

0.643 

0.696 

6.086 

0.820 

1.23 

0.703 

0.756 

7.100 

0.794 

.27 

0.644 

0.697 

6.103 

0.789 

1.28 

0.704 

0.757 

7.117 

0.763 

.33 

0.645 

0.698 

6.120 

0.758 

1.34 

0.705 

0.758 

7.134 

0.734 

.38 

0.646 

0.699 

6.137 

0.729 

1.39 

0.706 

0.759 

7.151 

0.706 

.43 

0.647 

0.700 

6.154 

0.702 

1.44 

0.707 

0.760 

7.168 

0.679 

.49 

0.648 

0.701 

6.171 

0.675 

1.50 

0.708 

0.761 

7.185 

0.653 

.55 

0.649 

0.702 

6.188 

0.649 

1.56 

0.709 

0.762 

7.202 

0.628 

.61 

0.650 

0.703 

6.204 

0.625 

1.62 

0.710 

0.763 

7.219 

0.604 

.68 

0.651 

0.704 

6.221 

0.601 

1.68 

0.711 

0.764 

7.236 

0.581 

.74 

0.652 

0.705 

6.238 

0.578 

1.75 

0.712 

0.765 

7.253 

0.559 

.81 

0.653 

0.706 

6.255 

0.556 

1.82 

0.713 

0.766 

7.269 

0.538 

.88 

0.654 

0.707 

6.272 

0.535 

1.89 

0.714 

0.767 

7.286 

0.517 

.96 

0.655 

0.708 

6.289 

0.514 

1.97 

0.715 

0.768 

7.303 

0.497 

2.04 

0.656 

0.709 

0.306 

0.495 

2.04 

0.716 

0.769 

7.320 

0.478 

2.12 

0.657 

0.710 

6.323 

0.476 

2.13    ' 

0.717 

0.770 

7.337 

0.460 

2.20 

0.658 

0.711 

6.340 

0.458 

2.21 

0.718 

0.771 

7.354 

0.443 

2.28 

0.659 

0.712 

6.357 

0.440 

2.30 

0.719 

0.772 

7.371 

0.426 

2.38 

0.660 

0.713 

6.373 

0.423 

2.39 

0.720 

0.773 

7.388 

0.409 

2.47 

0.661 

0.714 

6.390 

0.407 

2.49 

0.721 

0.774 

7.405 

0.394 

2.57 

0.662 

0.715 

6.407 

0.392 

2,58 

0.722 

0.775 

7.422 

0.379 

2.68 

0.663 

0.716 

6.424 

0.377 

2.68 

0.723 

0.776 

7.439 

0.364 

2.77 

0.664 

0.717 

6.441 

0.362 

2.80 

0.724 

0.777 

7.455 

0.350 

2.89 

0.665 

0.718 

6.458 

0.348 

2.91 

0.725 

0.778 

7.472 

0.337 

3.00 

0.666 

0.719 

6.475 

0.335 

3.02 

0.726 

0.779 

7.489 

0.324 

3.12 

0.667 

0.720 

6.492 

0.322 

3.14 

0.727 

0.780 

7.506 

0.312 

3.24 

0.668 

0.721 

6.509 

0.310 

3.26 

0.728 

0.781 

7.523 

0.300 

3.37 

0.669 

0.722 

6.526 

0.298 

3.40 

0.729 

0.782 

7.540 

0.288 

3.51 

0.670 

0.723 

6.543 

0.287 

3.53 

0.730 

0.783 

7.557 

0.277 

3.65 

0.671 

0.724 

6.559 

0.276 

3.67 

0.731 

0.784 

7.574 

0.267 

3.79 

0.672 

0.725 

6.576 

0.265 

3.82 

0.732 

0.785 

7.591 

0.257 

3.94 

0.673 

0.726 

6.593 

0.255 

3.97 

0.733 

0.786 

7.608 

0.247 

4.10 

0.674 

0.727 

6.610 

0.245 

4.13 

0.734 

0.787 

7.624 

0.238 

4.25 

0.675 

0.728 

6.627 

0.236 

4.29 

0.735 

0.788 

7.641 

0.228 

4.44 

0.676 

0.729 

6.644 

0.227 

4.46 

0.736 

0.789 

7.658 

0.220 

4.60 

0.677 

0.730 

6.661 

0.218 

4.64 

0.737 

0.790 

7.675 

0.211 

4.80 

0.678 

0.731 

6.678 

0.210 

4.82 

0.738 

0.791 

0.692 

0.203 

4.99 

0.679 

0.732 

6.695 

0.202 

5.01 

0.739 

0.792 

7.709 

0.195 

5.19 

0.680 

0.733 

6.712 

0.194 

5.22 

0.740 

0.793 

7.726 

0.188 

5.38 

0.681 

0.734 

6.728 

0.187 

5.41 

0.741 

0.794 

7.743 

0.181 

5.59 

0.682 

0.735 

6.745 

0.180 

5.62 

0.742 

0.795 

7.760 

0.174 

5.82 

0.683 

0.736 

6.762 

0.173 

5.85 

0.743 

0.796 

7.777 

0.167 

6.06 

0.684 

0.737 

6.779 

0.166 

6.10 

0.744 

0.797 

7.794 

0.161 

6.29 

0.685 

0.738 

6.796 

0.160 

6.32 

0.745 

0.798 

7.810 

0.155 

6.53 

0.686 

0.739 

6.813 

0.154 

6.57 

0.746 

0.799 

7.827 

0.149 

6.79 

0.687 

0.740 

6.830 

0.148 

6.84 

0.747 

0.800 

7.844 

0.143 

7.08 

0.688 

0.741 

6.847 

0.142 

7.13 

0.748 

0.801 

7.861 

0.138 

7.33 

0.689 

0.742 

6.864 

0.137 

7.39 

0.749 

0.802 

7.878 

0.132 

7.67 

0.690 

0.743 

6.881 

0.132 

7.67 

0.750 

0.803 

7.895 

0.127 

7.97 

0.691 

0.744 

6.898 

0.127 

7.97 

0.751 

0.804 

7.912 

0.123 

8.23 

0.692 

0.745 

6.914 

0.122 

8.30 

0.752 

0.805 

7.929 

0.118 

8.58 

0.693 

0.746 

6.931 

0.117 

8.65 

0.753 

0.806 

7.946 

0.113 

8.96 

0.694 

0.747 

0.948 

0.113 

8.96 

8.754 

0.807 

7.963 

0.109 

9.28 

0.695 

0.748 

6.965 

0.108 

9.37 

0.755 

0.808 

7.980 

0.105 

9.64 

0.696 

0.749 

6.982 

0.104 

9.73 

0.756 

0.809 

7.996 

0.101 

10.02 

*  0.697 

0.750 

6.999 

0.100 

10.12 

Neutral  point. 


APPENDIX 


597 


EN 
1 

EN 
10 

pH 

CH+ 
X  10-8 
H+ 

COH- 

x  io-« 

OH- 

EN 

1 

EN 
10 

pH 

CH+ 
X  10-s 
H+ 

COH- 
X  10-5 
OH- 

0.757 

0.810 

8.013 

0.970 

1.04 

0.816 

0.869 

9.011 

0.975 

1.04 

0.758 

0.811 

8.030 

0.933 

1.08 

0.817 

0.870 

9.028 

0.938 

1.08 

0.759 

0.812 

8.047 

0.897 

1.13 

0.818 

0.871 

9.045 

0.902 

1.12 

0.760 

0.813 

8.064 

0.863 

1.17 

0.819 

0.872 

9.062 

0.868 

1.17 

0.761 

0.814 

8.081 

0.830 

1.22 

0.820 

0.873 

9.078 

0.835 

1.21 

0.762 

0.815 

8.098 

0.798 

1.27 

0.821 

0.874 

9.095 

0.803 

1.26 

0.763 

0.816 

8.115 

0.768 

1.32 

0.822 

0.875 

9.112 

0.772 

1.31 

0.764 

0.817 

8.132 

0.739 

1.37 

0.823 

0.876 

9.129 

0.743 

1.36 

0.765 

0.818 

8.149 

0.710 

1.43 

0.824 

0.877 

9.146 

0.714 

.42 

0.766 

0.819 

8.165 

0.683- 

1.48 

0.825 

0.878 

9.163 

0.687 

.47 

0.767 

0.820 

8.182 

0.657 

.54 

0.826 

0.879 

9.180 

0.661 

.53 

0.768 

0.821 

8.199 

0.632 

.60 

0.827 

0.880 

9.197 

0.636 

.59 

0.769 

0.822 

8.216 

0.608 

.66 

0.828 

0.881 

9.214 

0.611 

.66 

0.770 

0.823 

8.233 

0.585 

.73 

0.829 

0.882 

9.231 

0.588 

.72 

0.771 

0.824 

8.250 

0.562 

.80 

0.830 

0.883 

9.248 

0.566 

.79 

0.772 

0.825 

8.267 

0.541 

.87 

0.831 

0.884 

9.264 

0.544 

.86 

0.773 

0.826 

8.284 

0.520 

.95 

0.832 

0.885 

9.281 

0.523 

1.93 

0.774 

0.827 

8.301 

0.500 

2.02 

0.833 

0.886 

9.298 

0.503 

2.01 

0.775 

0.828 

8.318 

0.481 

2.10 

0.834 

0.887 

9.315 

0.484 

2.09 

0.776 

0.829 

8.335 

0.463- 

2.19 

0.835 

0.888 

9.332 

0.466 

2.17 

0.777 

0.830 

8.351 

0.445 

2.27 

0.836 

0.889 

9.349 

0.448 

2.26 

0.778 

0.831 

8.368 

0.428 

2.36 

0.837 

0.890 

9.366 

0.431 

2.35 

0.779 

0.832 

8.385 

0.412 

2.46 

0.838 

0.891 

9.383 

0.414 

2.44 

0.780 

0.833 

8.402 

0.396- 

2.56 

0.839 

0.892 

9.400 

0.398 

2.54 

0.781 

0.834 

8.419 

0.381 

2.66 

0.840 

0.893 

9.417 

0.383 

2.64 

0.782 

0.835 

8.436 

0.367 

2.76 

0.841 

0.894 

9.434 

0.369 

2.74 

0.783 

0.836 

8.453 

0.353 

2.87 

0.842 

0.895 

9.450 

0.354 

2.86 

0.784 

0.837 

8.470 

0.339 

2.99 

0.843 

0.896 

9.467 

0.341 

2.97 

0.785 

0.838 

8.487 

0.326 

3.10 

0.844 

0.897 

9.484 

0.328 

3.09 

0.786 

0.839 

8.504 

0.314- 

3.22 

0.845 

0.898 

9.501 

0.315 

3.21 

0.787 

0.840 

8.521 

0.302 

3.35 

0.846 

0.899 

9.518 

0.304 

3.33 

0.788 

0.841 

8.537 

0.290 

3.49 

0.847 

0.900 

9.535 

0.292 

3.47 

0.789 

0.842 

8.554 

0.279 

3.63 

0.848 

0.901 

9.552 

0.281 

3.60 

0.790 

0.843 

8.571 

0.269 

3.76 

0.849 

0.902 

9.569 

0.270 

3.75 

0.791 

0.844 

8.588 

0.258 

3.92 

0.850 

0.903 

9.585 

0.260 

3.89 

0.792 

0.845 

8.605 

0.248 

4.08 

0.851 

0.904 

9.602 

0.250 

4.05 

0.793 

0.846 

8.622 

0.239 

4.23 

0.852 

0.905 

9.619 

0.240 

4.22 

0.794 

0.847 

8.639 

0.230 

4.40 

0.853 

0.906 

9.636 

0.231 

4.38 

0.795 

0.848 

8.656 

0.221 

4.58 

0.854 

0.907 

9.653 

0.222 

4.56 

0.796 

0.849 

8.673 

0.213- 

4.75 

0  .  855- 

0.908 

9.670 

0.214 

4.73 

0.797 

0.850 

8.690 

0.204 

4.96 

0.856 

0.909 

9.687 

0.206 

4.91 

0.798 

0.851 

8.706 

0.197 

5.14 

0.857 

0.910 

9.704 

0.198 

5.11 

0.799 

0.852 

8.723 

0.189 

5.35 

0.858 

0.911 

9.721 

0.190 

5.33 

0.800 

0.853 

8.740 

0.182 

5.56 

0.859 

0.912 

9.738 

0.183 

5.53 

0.801 

0.854 

8.757 

0.175 

5.78 

0.860 

0.913 

9.755 

0.176 

5.75 

0.802 

0.855 

8.774 

0.168 

6.02 

0.861 

0.914 

9.772 

0.169 

5.99 

0.803 

0.856 

8.719 

0.162 

6.25 

0.862 

0.915 

9.788 

0.163 

6.21 

0.804 

0.857 

8.808 

0.156 

6.49 

0.863 

0.916 

9.805 

0.157 

6.45 

0.805 

0.858 

8.825 

0.150 

6.75 

0.864 

0.917 

9.822 

0.151 

6.70 

0.806 

0.859 

8.842 

0.144- 

7.03 

0.865 

0.918 

9.839 

0.145 

6.98 

0.807 

0.860 

8.859 

0.139 

7.28 

0.866 

0.919 

9.856 

0.139 

7.28 

0.808 

0.861 

8.876 

0.133 

7.61 

0.867 

0.920 

9.873 

0.134 

7.55 

0.809 

0.862 

8.892 

0.128 

7.91 

0.868 

0.921 

9.890 

0.129 

7.84 

0.810 

0.863 

8.909 

0.123 

8.23 

0.869 

0.922 

9.907 

0.124 

8.  16 

0.811 

0.864 

8.926 

0.119 

8.50 

0.870 

0.923 

9.924 

0.119 

8.50 

0.812 

0.865 

8.943 

0.114 

8.88 

0.871 

0.924 

9.941 

0.115 

8.80 

0.813 

0.866 

.8.960 

0.110 

9.20 

0.872 

0.925 

9.958 

0.110 

9.20 

0.814 

0.867 

8.977 

0.106 

9.55 

0.873 

0.926 

9.974 

0.106 

9.55 

0.815 

0.868 

8.994 

0.101 

10.00 

0.874 

0.927 

9.991 

0.102 

9.92 

598 


GELATIN  AND  GLUE 


EN 
1 

EN 
10 

pH 

CH+ 
X  lO-io 
H+ 

COH- 

x  10-" 

OH- 

EN 
1 

EN 
10 

pH 

CH+ 
X  10-u 
H+ 

COH- 
X  10-3 
OH- 

0.875 

0.928 

10.008 

0.981 

.03 

0.935 

0.988 

11.022 

0.950 

1.07 

0.877 

0.930 

10.042 

0.908 

.11 

0.937 

0.990 

11.056 

0.879 

1.15 

0.879 

0.932 

10.076 

0.840 

.20 

0.939 

0.992 

11.090 

0.813 

1.24 

0.881 

0.934 

10.110 

0.777 

.30 

0.941 

0.994 

11.124 

0.752 

1.35 

0.883 

0.936 

10.143 

0.719 

.41 

0.943 

0.996 

11.158 

0.696 

1.45 

0.885 

0.938 

10.177 

0.665 

.52 

0.945 

0.998 

11.191 

0.644 

1.57 

0.887 

0.940 

10.211 

0.615 

.65 

0.947 

.000 

11.225 

0.595 

1.70 

0.889 

0.942 

10.245 

0.569 

.78 

0.949 

.002 

11.259 

0.551 

1.84 

0.891 

0.944 

10.279 

0.526 

.92 

0.951 

.004 

11.293 

0.509 

1.99 

0.893 

0.946 

10.313 

0.487 

2.08 

0.953 

.006 

11.327 

0.471 

2.15 

0.895 

0.948 

10.346 

0.451 

2.24 

0.955 

.008 

11.361 

0.436 

2.32 

0.897 

0.950 

10.380 

0.417 

2.43 

0.957 

.010 

11.394 

0.403 

2.51 

0.899 

0.952 

10.414 

0.386 

2.62 

0.959 

.012 

11.428 

0.373 

2.71 

0.901 

0.954 

10.448 

0.357 

2.83 

0.961 

.014 

11.462 

0.345 

2.93 

0.903 

0.956 

10.481 

0.330 

3.07 

0.963 

.016 

11.496 

0.319 

3.17 

-     0.905 

0.958 

10.515 

0.305 

3.32 

0.965 

.018 

11.530 

0.295 

3.43 

0.907 

0.960 

10.549 

0.282 

3.59 

0.967 

.020 

11.563 

0.273 

3.71 

0.909 

0.962 

10.583 

0.261 

3.88 

0.969 

.022 

11.597 

0.253 

4.00 

0.911 

0.964 

10.617 

0.242 

4.18 

0.971 

.024 

11.631 

0.234 

4.32 

0.913 

0.966 

10.651 

0.224 

4.52 

0.973 

.026 

11.665 

0.216 

4.69 

0.915 

0.968 

10.684 

0.207 

4.89 

0.975 

.028 

11.699 

0.200 

5.06 

0.917 

0.970 

10.718 

0.191 

5.30 

0.977 

.030 

11.732 

0.185 

5.47 

0.919 

0.972 

10.752 

0.177 

5.72 

0.979 

.032 

11.766 

0.171 

5.92 

0.921 

0.974 

10.786 

0.164 

6.17 

0.981 

.034 

11.800 

0.159 

6.36 

0.923 

0.976 

10.820 

0.152 

6.66 

0.983 

.036 

11.834 

0.147 

6.88 

0.925 

0.978 

10.853 

0.  140 

7.23 

0.985 

.038 

11.868 

0.136 

7.44 

0.927 

0.980 

10.887 

0.130 

7.78 

0.987 

.040 

11.901 

0.126 

8.03 

0.929 

0.982 

10.921 

0.120 

8.43 

0.989 

.042 

11.935 

0.116 

8.72 

0.931 

0.984 

10.955 

0.111 

9.12 

0.991 

.044 

11.969 

0.107 

9.46 

0.933 

0.986 

10.989 

0.103 

9.83 

EN 
1 

EN 
10 

pH 

CH+ 

X  10-12 
H+ 

COH- 
'X  10-2 
OH- 

EN 
1 

EN 
10 

pH 

CH+ 
X  10-18 
H+ 

COH- 

x  10-1 

OH- 

0.993 

1.046 

12.003 

0.993 

1.02 

1.053 

1.106 

13.017 

0.961 

1.05 

0.995 

.048 

12.037 

0.919 

1.10 

1.055 

1.108 

13.051 

0.889 

.14 

0.997 

.050 

12.071 

0.850 

1.19 

1.057 

1.110 

13.085 

0.822 

.23 

0.999 

.052 

12.104 

0.786 

1.29 

1.059 

1.112 

13.119 

0.761 

.33 

1.001 

.054 

12.138 

0.728 

.39 

1.061 

1.114 

13.153 

0.704 

.44 

1.003 

.056 

12.172 

0.673 

.50 

1.063 

1.116 

13.186 

0.651 

.55 

1.005 

.058 

12.206 

0.623 

.62 

1.065 

1.118 

13.220 

0.602 

1.68 

1.007 

.060 

12.240 

0.576 

.76 

1.067 

1.120 

13.254 

0.557 

1.82 

1.009 

.062 

12.273 

0.533 

.90 

1.069 

1.122 

13.288 

0.516 

1.96 

1.011 

.064 

12.307 

0.493 

2.05 

1.071 

1  .  124 

13.322 

0.477 

2.12 

1.013 

.066 

12.341 

0.456 

2.22 

1.073 

1.126 

13.355 

0.441 

2.29 

1.015 

.068 

12.375 

0.422 

2.40 

1.075 

.128 

13.389 

0.408 

2.48 

1.017 

.070 

12.409 

0.390 

2.59 

1.077 

.130 

13.423 

0.378 

2.68 

1.019 

.072 

12.443 

0.361 

2.80 

1.079 

.132 

13.457 

0.349 

2.90 

1.021 

.074 

12.476 

0.334 

3.03 

1.081 

.134 

13.491 

0.323 

3.13 

1.023 

.076 

12.510 

0.309 

3.28 

1.083 

.136 

13.524 

0.299 

3.38 

1.025 

.078 

12.544 

0.286 

3.54 

1.085 

.138 

13.558 

0.277 

3.65 

1.027 

.080 

12.578 

0.264 

3.83 

1.087 

.140 

13.592 

0.256 

3.95 

1  .  029 

1.082 

12.612 

0.245 

4.13 

1.089 

.142 

13.626 

0.237 

.4.27 

1.031 

1.084 

12.645 

0.226 

4.48 

1.091 

.144 

13.660 

0.219 

4.62 

1.033 

1.086 

12.679 

0.209 

4.84 

1.093 

.146 

13.693 

0.203 

4.99 

1.035 

1.088 

12.713 

0.194 

5.22 

1.095 

.148 

13.727 

0.187 

5.41 

1.037 

1.090 

12.747 

0.179 

5.65 

1.097 

.150 

13.761 

0.173 

5.85 

1.039 

1.092 

12.781 

0.166 

6.10 

1.099 

.152 

13.795 

0.160 

6.32 

1.041 

1.094 

12.814 

0.153 

6.61 

1.101 

.154 

13.829 

0.148 

6.84 

1.043 

1.096 

12.848 

0.142 

7.13 

1.103 

.156 

13.863 

0.137 

7.39 

1.045 

1.098 

12.882 

0.131 

7.73 

1.105 

.158 

13.896 

0.127 

7.97 

1.047 

1.100 

12.916 

0.121 

8.36 

1.107 

1.160 

13.930 

0.117 

8.65 

1.049 

1.102 

12.950 

0.112 

9.04 

1.109 

1.162 

13.964 

0.109 

9.28 

1.051 

1.104 

12.983 

0.104 

9.73 

1.111 

1.164 

13.998 

0.101 

10.02 

xio-n 

X  N 

H+ 

OH- 

1.113 

1.166 

14.032 

0.930 

1.09 

APPENDIX  599 

The  Theory  of  Indicators  and  the  Colorimetric  Determina- 
tion.— The  hydrogen  ion  concentration  of  glue  and  gelatin  solu- 
tions may  be  determined  with  somewhat  less  accuracy  but  with 
greater  rapidity  by  the  colorimetric  than  by  the  electrometric 
procedure.  For  purposes  of  plant  control  where  the  niceties 
obtainable  by  the  latter  method  are  not  necessary,  a  reasonably 
close  approximation  may  be  obtained  in  a  minimum  of  time  by 
the  colorimetric  procedure. 

This  method  depends  upon  the  fact  that  many  organic  sub- 
stances are  capable  of  internal  tautomeric  rearrangements  of 
their  molecules  under  the  influence  of  hydrogen  or  hydroxyl  ions 
with  a  corresponding  change  in  color.  These  compounds, 
commonly  designated  as  indicators,  are  for  the  most  part  very 
weak  acids.  They  are  consequently  undissociated  in  the  pres- 
ence of  a  certain  concentration  of  hydrogen  ion.  but  as  that 
value  becomes  less,  as  by  the  addition  of  alkali,  they  become 
dissociated.  Their  value  as  indicators  lies  in  the  fact  that  the 
undissociated  molecule  has  a  color  which  is  different  from  that 
of  the  tautomeric  rearrangement  which  takes  place  coincident 
with  the  formation  of  its  ions.  The  useful  range  in  color  change 
of  the  indicators  is  usually  about  1.5  pH.  Thus  phenol  red  is 
yellow  in  solutions  more  acid  than  pH  6.6,  and  is  red  in  solutions 
more  alkaline  than  pH  8.2.  Between  these  values,  however,  the 
change  in  shade  of  color  is  gradual  and  the  development  of  any 
given  shade  or  virage  signifies  a  definite  and  fixed  pH  of  the 
solution. 

A  great  variety  of  indicators  have  been  reported  by  Salm,1 
S0rensen,2  Thiel3  and  others.  Clark  and  Lubs1  have  developed 
the  sulphonphthalein  series  which  have  proved  of  exceptional 
value  on  account  of  their  great  brilliance  of  color  and  sharply 
defined  color  changes.  These  give  a  useful  pH  range  of  from 
1.2  to  10.0.  The  following  table  shows  the  chemical  name,  the 
common  name,  the  most  favorable  concentration  for  use,  the 
color  change,  and  the  pH  range  of  this  series. 

In  Fig.  115  is  shown  the  dissociation  of  these  indicators  as 
expressed  by  Clark,  and  the  effective  range  for  each. 

The  indicators  listed  below  are  obtainable  from  any  chemical 

*E.  SALM,  Z  physik.  Chem.,  57  (1906),  471. 

2S.  S0RENSEN,  Biochem  Z,  21  (1909),  131,  201. 

3  A.  THIEL,  Sammlung,  chem.  chemtech.  Vortrdge,  16  (1911),  307. 

« W.  CLARK  and  H.  LUBS,  /.  Bact.,  2  (1917),  1;  109;  191. 


600 


GELATIN  AND  GLUE 


w 

PH 


OOCOOOOCO^OOCOOO 


? 


OtX)(N 


0)030505 


000000000 


c 


APPENDIX 


601 


suppty  house  in  the  form  of  a  dry  powder,  and  in  stock  solutions, 
and  tablets.  Alcoholic  solutions  may  be  used  where  the  alcohol 
is  not  objectionable  and  where  the  free  acid  dye  in  the  indicator 
solution  does  not  vitiate  accuracy, 
but  Clark  and  Lubs  prefer  the 
use  of  aqueous  solutions  of  alkali 
salts.  Rather  concentrated  solu- 
tions are  conveniently  made  up, 
and  diluted  to  the  appropriate 
strength  for  use  from  time  to  time 
as  needed. 

The  stock  solutions  are  prepared 
as  follows:1  One  decigram  (0.1 
gram)  of  the  dry  powder  is  ground 
in  an  agate  mortar  with  the  quanti- 
ties of  N/20  NaOH  listed  in  Table 
73.  When  solutions  are  complete 
they  are  diluted  to  25  c.c. 

After  making  up  to  25  c.c.  the 
dye  is  present  as  a  0.4  per  cent 
solution.  For  use  in  testing  10  c.c. 
portions  of  a  solution  with  five 
drops  of  indicator  solution,  5  c.c. 
portions  of  brom  phenol  blue,  brom 
cresol  purple,  thymol  blue,  and 
brom  thymol  blue,  and  2.5  c.c. 
portions  of  phenol  red,  cresol  red, 
and  methyl  red  are  made  up  to  a 
volume  of  50  c.c.  each.  This 
results  in  a  concentration  of  0.04 
per  cent  for  the  former  and  0.02  per 
cent  for  the  latter  set  listed.  Ortho 
cresol  phthalein  (and  phenol- 
phthalein  if  used)  are  made  up 
into  0.02  per  cent  solutions  in  95 
per  cent  alcohol. 

In  using  indicator  solutions  pre- 
pared as  above,  five  drops  are 

added  to  10  c.c.  of  the  solution  to  be  tested,  and  the  color  com- 
pared with  that  developed  by  the  addition  of  a  similar  amount 

1  From  directions  of  W.  M.  CLARK,  lib.  cit. 


0  2b  50          75          100 

.      D(s5ociation,percen+ 

FIG.  115. — Indicator  curves 
and  significant  pH  values  (Clark 
and  Lubs).  (From  W.  M.  Clark, 
"  The  Determination  of  Hydrogen 
Ions,"  Williams  and  Wilkins 
Company,  Baltimore,  1920.) 


602  GELATIN  AND  GLUE 

TABLE  73. — PREPARATION  OF  INDICATOR  SOLUTIONS 


Molecular  weight 

Indicator 

N/20  NaOH  per  deci- 
gram, cubic  centimeters 

354.17 

Phenol  red 

5.7 

669  .  82 

Brom  phenol  blue 

3.0 

382.17 

Cresol  red 

5.3 

540.01 

Brom  cresol  purple 

3.7 

466.30 

Thymol  blue 

4.3 

624.12 

Brom  thymol  blue 

3.2 

269.12 

Methyl  red 

7.4 

of  indicator  solution  to  10  c.c.  portions  of  solutions  of  known 
standard  pH.  For  this  purpose  solutions  possessing  a  decided 
buffer  action  (vide  page  587)  are  selected,  as  they  may  be  accu- 
rately reproduced  and  represent  pH  values  that  do  not  alter 
through  trivial  variations  in  operation.  Several  such  series  of 
buffer  solutions  have  been  suggested  which  vary  in  their  pH, 
as  determined  by  electrometric  calibration,  by  0.2.  This  varia- 
tion-is arbitrarily  fixed  as  most  convenient,  but  may  be  altered 
at  will. 

The  set  described  by  Clark  and  Lubs1  seem  more  convenient 
of  preparation  and  somewhat  more  satisfactory  in  service  than 
the  others,  and  alone  will  be  described.  It  is  composed  of  the 
following  mixtures : 

Potassium  chloride  +  hydrochloric  acid. 
Acid  potassium  phthalate  +  hydrochloric  acid. 
Acid  potassium  phthalate  +  sodium  hydroxide. 
Acid  potassium  phosphate  +  sodium  hydroxide. 
Boric  acid  +  potassium  chloride  +  sodium  hydroxide. 

The  following  stock  solutions  are  required : 

M/5  potassium  chloride  solution,  prepared  from  recrystallized  and  thor- 
oughly dried  (at  120°C.  for  2  days)  material,  using  14.912  grams  per  liter 
of  solution. 

M/5  acid  potassium  phthalate  solution,  prepared  from  recrystallized  and 
well  dried  material,  using  40.828  grams  per  liter  of  solution. 

M/5  acid  potassium  phosphate  solution,  prepared  from  recrystallized  and 
well  dried  material,  using  27.232  grams  per  liter  of  solution. 

M/5  boric  acid  M/5  potassium  chloride  solution.  The  boric  acid  should 
be  recrystallized  and  dried  in  thin  layers  between  filter  paper.  A  liter  of  the 
solution  should  contain  12.4048  grams  of  boric  acid  and  14.912  grams  of 
potassium  chloride. 

1  W.  M.  CLARK  and  H.  LUBS,  J.  Biol.  Chem.,  25  (1916),  479. 


APPENDIX 


603 


M/5  sodium  hydroxide  must  be  prepared  as  free  as  possible  from  carbon- 
ates, and  preserved  in  paraffined  bottles,  protected  from  the  air  with  soda- 
lime  tubes. 

M/5  hydrochloric  acid  solution  is  made  in  the  usual  way  and  carefully 
checked  against  the  sodium  hydroxide. 

The  standard  .solutions  of  pH  varying  from  1.2  to  10.0  by 
intervals  of  0.2  pH  are  made  up  from  the  above  stock  solutions 
according  to  the  following  table.  By  employing  buffer  solutions 
varying  by  0.2  pH  it  is  easily  possible  to  estimate  values  in 
unknown  solutions  to  the  nearest  tenth.  The  solutions  are 
best  kept  in  200  c.c.  bottles  each  provided  with  a  cork  stopper 
in  which  is  inserted  a  10  c.c.  pipette,  sealed  with  a  closed  rubber 
tube  at  the  upper  end. 

The  approximate  pH  of  the  solution  to  be  tested  is  first  noted 
by  adding  a  few  drops  of  several  indicators  to  small  portions  of 
the  solution  in  a  test  tube.  That  indicator  which  more  nearly 


TABLE  74. — CLARK  AND  LUBS'  BUFFER  SOLUTIONS1 


KC1-HC1  Mixtures 


pH 
1.2 
1.4 
1.6 
1.8 
2.0 
2.2 

50  c.c.  M/5  KC1 
50  c.c.  M/5  KC1 
50  c.c.  M/5  KC1 
50  c.c.  M/5  KC1 
50  c..c.  M/5  KC1 
50  c.c.  M/5  KC1 

64.5  c.c.  M/5  HC1 
41.5  c.c.  M/5  HC1 
26.3  c.c.  M/5  HC1 
16.6  c.c.  M/5  HC1 
10.6  c.c.  M/5  HC1 
6.7  c.c.  M/5  HC1 

Dilute  to  200  c.c. 
Dilute  to  200  c.c. 
Dilute  to  200  c.c. 
Dilute  to  200  c.c. 
Dilute  to  200  c.c. 
Dilute  to  200  c.c. 

Phthalate-HCl  Mixtures 


2.2 

50  c  c    M/5  KH  Dhthalate 

46.  70  c.c.  M/5HC1 

Dilute  to  200  c.c. 

2.4 

50  c 

a 

M/5  KH  phthalate 

39.60  c.c.  M/5  HC1 

Dilute  to  200  c.c 

2.6 

50  c 

o 

M/5  KH  phthalate 

32.95  c.c.  M/5  HC1 

Dilute  to  200  c.c 

2.8 

50  c 

c 

M/5  KH  phthalate 

26.42  c.c.  M/5  HC1 

Dilute  to  200  c.c 

3.0 

50  c 

0 

M/5  KH  phthalate 

20.32  c.c.  M/5HC1 

Dilute  to  200  c.c 

3.2 

50  c 

tt 

M/5  KH  phthalate 

14.70  c.c.  M/5  HC1 

Dilute  to  200  c.c 

3.4 

50  c 

f 

M/5  KH  phthalate 

9.  90  c.c.  M/5  HC1 

Dilute  to  200  c.c 

3.6 

50  c 

a 

M/5  KH  nhthalate 

5.97  c.c.  M/5HC1 

Dilute  to  200  c.c 

3.8 

50  c.c.  M/5  KH  phthalate 

2.63  c.c.  M/5  HC1 

Dilute  to  200  c.c. 

Phthalate-NaOH  Mixtures 


4.0 

50  c.c.  M/5  KH  phthalate 

0.40  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

4.2 

50  c.c.  M/5  KH  phthalate 

3.70  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

4.4 

50  c.c.  M/5  KH  phthalate 

7.50  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

4.6 

50  c.c.  M/5  KH  phthalate 

12.  15  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

4.8 

50  c.c.  M/5  KH  phthalate 

17.70  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

5.0 

50  c.c.  M/5  KH  phthalate 

23.85  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

5.2 

50  c.c.  M/5  KH  phthalate 

29.95  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

5.4 

50  c.c.  M/5  KH  phthalate 

35.45  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

5.6 

50  c.c.  M/5  KH  phthalate 

39.85  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

5.8 

50  c.c.  M/5  KH  phthalate 

43.00  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

6.0 

50  c.c.  M/5  KH  phthalate 

45.45  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

6.2 

50  c.c.  M/5  KH  phthalate 

47.00  c.c.  M/5  NaOH 

Dilute  to  200  c  c. 

1  W.  M.  CLARK,  lib.  cit.,  75. 


604 


GELATIN  AND  GLUE 


KH2PO4-NaOH  Mixtures 


5.8 

50  c.c.  M/5  KH2PO4 

3.72  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

6.0 

50  c.c.  M/5  KH2PO4 

5.70  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

6.2 

50  c.c.  M/5  KH2P04 

8.60  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

6.4 

50  c.c.  M/5  KH2PO4 

12.60  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

6.6 

50  c.c.  M/5  KH2PO4 

17.80  c 

0 

M/5  NaOH 

Dilute  to  200  c.c 

6.8 

50  c.c.  M/5  KH2PO4 

23.65c 

0 

M/5  NaOH 

Dilute  to  200  c.c 

7.0 

50  c.c.  M/5  KH2PO4 

29.63  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

7.2 

50  c.c.  M/5  KH2P04 

35.00  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

7.4 

50  c.c.  M/5  KH2PO4 

39.50c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

7.6 

50  c.c.  M/5  KH2P04 

42  .  80  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

7.8 

50  c.c.  M/5  KH2PO4 

45.20  c 

c 

M/5  NaOH 

Dilute  to  200  c.c 

8.0 

50  c.c.  M/5  KH2PO4 

46.80  c 

0 

M/5  NaOH 

Dilute  to  200  c.c 

Boric  acid-KCl-NaOH  Mixtures 


7.8 

50  c.c.  M/5  HsBOs,  M/5  KC1 

2.61  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

8.0 

50  c.c.  M/5  HsBOs,  M/5  KC1 

3.97  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

8.2 

50  c.c.  M/5  HsBOs,  M/5  KC1 

5.  90  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

8.4 

50  c.c.  M/5  HsBOs,  M/5  KC1 

8.50  c.c    M/5  NaOH 

Dilute  to  200  c.c. 

8.6 

50  c.c.  M/5  H3BO3,  M/5  KC1 

12.00  c.c.  M/5  NaDH 

Dilute  to  200  c.c. 

8.8 

50  c.c.  M/5  HsBOs,  M/5  KC1 

16.30  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

9.0 

50  c.c.  M/5  HsBOs,  M/5  KC1 

21.30  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

9.2 

50  c.c.  M/5  HjBOs,  M/5  KC1 

26.  70  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

9.4 

50  c.c.  M/5  HsBOs,  M/5  KC1 

32.00  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

9.6 

50  c.c.  M/5  HsBOs,  M/5  KC1 

36.  85  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

9.8 

50  c.c.  M/5  HsBOs,  M/5  KC1 

40.80  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

10.0 

50  c.c.  M/5  HsBOs,  M/5  KC1 

43.90  c.c.  M/5  NaOH 

Dilute  to  200  c.c. 

measures  the  pH  noted  is  then  added  to  10  c.c.  portions  of  the 
buffer  solutions  of  pH  values  on  either  side  of  the  approximate 
value  observed,  and  the  color  developed  in  10  c.c.  of  the  unknown 
solution  by  the  addition  of  five  drops  of  the  indicator  compared 
to  that  of  the  buffer  mixtures.  The  assumption  is  made  that  if 
the  same  virage  is  produced  in  any  two  solutions,  the  pH  of  the 
two  is  the  same.  This  is  not  always  strictly  true,  as  will  be 
described  later. 

Where  only  approximate  results  are  required,  the  buffer 
solutions  may  be  dispensed  with  and  a  color  chart,  such  as  shown 
in  Clark's  book  (lib.  tit.}  referred  to.  After  considerable  practice 
the  " color  memory"  alone  may  be  safely  relied  upon  for  rough 
work. 

Sources  of  Error  in  Colorimetric  Determinations. — If  there  are 
present  in  the  solution  being  tested  substances  that  adsorb  the 
indicator,  or  otherwise  remove  it  from  the  field  of  action,  the 
equilibria  upon  which  the  color  depends  may  be  altered  to  such  an 
extent  that  not  only  the  intensity  but  even  the  quality  of  the 
color  will  be  quite  different  from  that  observed  in  a  solution  of  the 
same  pH  in  which  such  disturbing  influences  are  not  present. 
In  all  such  cases  of  discrepancy  the  electrometric  estimation  is 
taken,  whether  correctly  or  not,  as  the  basic  value.  These  non- 


APPENDIX  605 

conformities,  known  as  protein  errors,  on  account  of  their  fre- 
quent occurrence  in  proteins,  are  said  to  be  larger  the  more 
complex  and  concentrated  the  proteins,  and  less  noticeable  in  the 
products  of  hydrolysis.  Benedict  and  Elliott1  have  shown  in  a 
study  upon  gelatin  that  some  indicators  show  a  higher  pH 
than  the  hydrogen  electrode,  some  show  a  lower  pH,  and  that  no 
stated  correction  may  be  properly  applied  because  the  electro- 
metric  and  colorimetric  curves  at  different  pH  values  do  not  run 
parallel.  In  some  instances  the  curves  may  even  cross,  e.g., 
at  one  pH  the  colorimetric  method  may  show  a  higher,  and  at 
another  pH  a  lower  value  than  the  corresponding  determination 
by  electrometric  means. 

Clark2  has  reported  the  exact  relation  between  pH  obtained 
colorimetrically  with  methyl  red  and  by  the  hydrogen  electrode 
of  casein  solutions  between  pH  2  and  6. 

Another  discrepancy  between  the  two  methods  for  measuring 
pH  is  found  to  be  due  to  the  presence  of  inorganic  salts  in  different 
amounts.  Perfectly  neutral  salts  such  as  sodium  chloride  are 
capable,  in  some  way,  of  affecting  the  equilibria  so  that  different 
intensities  and  shades  of  color  are  produced  with  the  different 
indicators.  No  adequate  explanation  has  been  offered  to  account 
for  this  so-called  salt  effect.  In  measurements  where  the  salt 
effect  or  protein  effect  are  constant,  they  may  be  eliminated  by  a 
calibration  of  such  solutions  with  the  indicators  against  the 
hydrogen  electrode,  but  if  the  salt  or  protein  content  is  variable 
such  a  procedure  becomes  impracticable.  The  best  way  to  deal 
with  such  cases  seems  to  be  to  select  only  such  indicators  as 
give  a  relatively  small  error,  and  keep  in  mind  the  general  order 
of  magnitude  expected.  But  for  the  most  reliable  work  the 
electrometric  procedure  must  be  used. 

A  natural  strong  coloration  or  turbidity  in  the  solution 
makes  colorimetric  determinations  very  difficult.  A  considerable 
dilution  may  be  employed  (at  least  to  1.0  per  cent)  but  if  such  a 
solution  is  still  highly  colored  or  turbid  the  virage  on  adding  the 
indicator  will  be  markedly  different  from  that  produced  in  a  clear 
and  colorless  solution  of  the  same  pH.  There  have  been  a 
number  of  schemes  suggested  to  make  comparisons  possible  in 
such  cases,  and  with  a  fair  degree  of  success.  The  simplest,  and 

1  A.  BENEDICT  and  F.  ELLIOTT,  paper  read  at  60th  Gen.  Meeting,  Am. 
Ghem.  Soc.,  Chicago,  1920. 

2  W.  M.  CLARK,  et.  al,  J.  Ind.  Eng.  Chem.,  12  (1920),  1163. 


606  GELATIN  AND  GLUE 

perhaps  most  effective,  method  of  overcoming  the  color  and 
turbidity  errors  is  by  the  use  of  a  color  comparator  as  proposed  by 
Hurwitz,  Meyer,  and  Ostenberg.1  This  consists  merely  of  a 
block  of  wood  (Fig.  116),  painted  dull  black  in  which  are  bored 
six  holes  in  pairs,  parallel  to  each  other,  of  such  size  as  will  hold 
an  ordinary  test  tube.  Adjacent  pairs  are  bored  very  close 


FIG.  116. — A  color  comparator. 

together.  Smaller  holes  are  bored  perpendicular  to  these  and 
passing  through  each  pair.  In  the  front  center  hole  is  placed  the 
tube  containing  the  solution  being  tested  plus  the  indicator. 
Behind  this  is  placed  a  tube  of  clear  water.  On  either  side  in 
front  are  placed  the  standard  buffer  solutions  colored  with  the 
indicator,  and  behind  each  of  these  a  tube  of  the  solution  being 
tested,  but  without  any  indicator.  Both  color  and  turbidity 
are  in  that  way  properly  compensated  in  all  solutions.  Turbid 
solutions  should  always  be  viewed  through  a  thin  layer,  that  is, 
through  the  sides  of  test  tubes  rather  than  from  the  top. 

2.  IONIZATION  CONSTANTS  OF  ACIDS  AND  BASES 

Figure  117  shows  graphically  the  relation  between  hydrogen 
ion  concentration  and  pH;  together  with  the  ranges  of  some  com- 
mon indicators  and  the  approximate  points  on  the  curve  of 
some  common  substances. 

1  S.  HURWITZ,  K.  MEYER,  and  Z.  OSTENBERG,  Proc.  Soc.  Exptl  Boil. 
Med.,  13  (1915),  24. 


APPENDIX 


607 


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608  GELATIN  AND  GLUE 

TABLE  75. — IONIZATION  CONSTANTS  OF  ACIDS  AND  BASES1 


lonogen 


Acetic  acid 

Amino  acetic  acid 

Amino  acetic  acid 

Ammonium  hydroxide 

Aspartic  acid 

Aspartic  acid 

Barium  hydroxide 

Boric  acid 

Calcium  hydroxide 

Carbonic  acid 

Carbonic  acid 

Citric  acid 

Citric  acid 

Citric  acid 

Formic  acid 

Hydrochloric  acid 

Lactic  acid . 

Lysine 

Lysine 

Nitric  acid 

Nitrous  acid 

Oxalic  acid 

Oxalic  acid 

Phosphoric  acid 

Phosphoric  acid 

Phosphoric  acid 

Potassium  hydroxide . . 
Sodium  hydroxide .... 

Sulphuric  acid 

Sulphuric  acid 


K0 
Ka 


Ka 

K6 


KB1 
K02 

K01 
Ka2 

Ka3 

Ka 

Ka 

K0 

Ka 

Kb 

Ka 

Ka 

Kol 


Ka3 


Kaj 


8  X 
8  X 
8  X 
8  X 
4  X 
1.2  X 
3.0  X 
7.0  X 
3.0  X 
3.0  X 
7.0  X 
8.2  X 
3.2  X 

7.0  X 

2.1  X 
1.0 
1.4  X 
1.0  X 
1.0  X 
1.0 
6.0  X 

1.0  X 

4.1  X 
1.0  X 
2.0  X 


10~5 

10-io 

10-12 

io-5 

10~4 

1Q-12 


0  X 
0 
0 
0 


10-io 

10~2 

10~7 

10~1J 

10~4 

IO-5 

10~7 

10~4 

io-4 

10~u 


10~4 

io-1 
io-5 

10-2 

10~7 
10~13 


3.0  X  1C-2 


K  0  =  acid  ionization;   K&  =  basic   ionization;    1,    2,    3   refer   to   primary, 
secondary,  and  tertiary  ionization. 


3.  THE  CONVERSION  OF  MACMICHAEL  VISCOSITY  DEGREES  TO 

CENTIPOISES 

The  MacMichael  viscosimeter  is  most  conveniently  calibrated 
to  absolute  viscosity  units,  or  centipoises,  by  taking  the  viscosity 
in  MacMichael  degrees,  at  a  number  of  different  temperatures, 
of  a  liquid  the  absolute  viscosity  of  which,  at  the  temperatures 

1  J.  STIEGLITZ,  "Qualitative  Chemical  Analysis,"  (1916),  104;  106;  CLARK, 
lib.  cit.,  308. 


APPENDIX 


609 


employed,  has  been  definitely  established.  At  the  advice  of  the 
United  States  Bureau  of  Standards,1  castor  oil  of  the  highest 
purity  is  recommended  as  the  calibrating  liquid. 

The  speed  of  rotation  of  the  cup  should  be  regulated  to  a 
stated  velocity  between  20  and  60  revolutions  per  minute,  and 
at  all  times  ascertained  to  revolve  at  the  stated  speed.  Each 
instrument  must  be  calibrated  separately  for  each  wire  that  is  to 
be  employed. 

The  author  has  found  the  conversion  curve  of  MacMichael 
degrees  to  centipoises  to  be  a  straight  line,  and  it  is,  therefore, 
necessary  to  take  readings  at  only  three  or  four  different  tem- 
peratures to  establish  the  curve  for  all  temperatures.  This 
done,  the  absolute  viscosity  in  centipoises  may  be  read  off  directly 
from  the  graph  for  any  corresponding  MacMichael  degree. 

The  viscosity  in  centipoises  of  castor  oil  at  numerous  tem- 
peratures from  5  to  100°C.  is  reported  in  the  United  States 
Bureau  of  Standards  Technologic  Paper  No.  112  (1919),  pages 
24  and  25.  A  part  of  the  data  there  given  is  reproduced  below: 


TABLE  76. — ABSOLUTE  VISCOSITY  OF  CASTOR  OIL 


Temperature 

Viscosity  in 
centipoises 

Temperature 

Viscosity  in 
centipoises 

18.0 

1162.5 

32.0 

394.0 

20.0 

986.0 

34.0 

340.0 

22.0 

834.0 

36.0 

294.0 

24.0 

706.0 

38.0 

258.0 

26.0 

604.0 

40.0 

231.0 

28.0 

521.0 

65.6 

60.5 

30.0 

451.0 

100.0 

16.9 

The  conversion  curve  for  the  author's  instrument  employing 
wire  E  and  revolving  at  a  speed  of  69  revolutions  per  minute  is 
shown  in  the  accompanying  figure.  By  the  use  of  wire  No.  27 
at  58  r.p.m.  the  viscosity  in  centipoises  could  be  read  off  directly. 

1  U.  S.  Bureau  of  Standards,  Personal  Communication. 


610 


GELATIN  AND  GLUE 


IIUU 

1000 
900 
800 
100 

S  600 
to 

•|"500 

S 

u 

400 
300 
?00 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

100 
0 

/ 

7 

/ 

100    200     500     400     500     GOO     TOO      800 
Mac  Michael  Degrees 

FIG.  118. — Calibration  of  MacMichael  viscosimeter  with  castor  oil  (Bureau  of 
Standards  Bull.  112).     Wire  E.  revolutions  69  per  minute.     Cp. 


APPENDIX 


611 


TABLE    77. — SPECIFIC    GRAVITY    IN    DEGREES   BAUME   AND    TWADDELL 
LIQUIDS  HEAVIER  THAN  WATER 


Degrees 
Twaddell 

Degrees 
Baume' 

Specific 
gravity 

Degrees 
Twaddell 

Degrees 

Bauni6 

Specific 
gravity 

0 

0.0 

.000 

50 

28.8 

1.250 

1 

0.7 

.005 

51 

29.3 

1.255 

2 

1.4 

.010 

52 

29.7 

.260 

3 

2.1 

.015 

53 

30.2 

.265 

4 

2.7 

.020 

54 

30.6 

.270 

5 

3.4 

.025 

55 

31.1 

.275 

6 

4.1 

.030 

56 

31.5 

.280 

7 

4.7 

1.035 

57 

32.0 

.285 

8 

5.4 

1.040 

58 

32.4 

.290 

9 

6.0 

1.045 

59 

32.81 

.295 

10 

6.7 

.050 

60 

33.3 

.300 

11 

7.4 

.055 

61 

33.7 

.305 

12 

8.0 

.060 

62 

34.2 

.310 

13 

8.7 

.065 

63 

34.6 

.315 

14 

9.4 

.070 

64 

35.0 

.320 

15 

10.0 

.075 

65 

35.4 

.325 

16 

10.6 

.080 

66 

35.8 

.330 

17 

11.2 

.085 

67 

36.2 

.335 

18 

11.9 

.090 

68 

36.6 

.340 

19 

12.4 

.095 

69 

37.0 

.345 

20 

13.0 

.100 

70 

37.4 

.350 

21 

13.6 

.105 

71 

37.8 

.355 

22 

14.2 

.110 

72 

38.2 

.360 

23 

14.9 

.115 

73 

38.6 

.365 

24 

15.4 

.120 

74 

39.0 

.370 

25 

16.0 

.125 

75 

39.5 

.375 

26 

16.5 

.130 

76 

39.8 

.380 

27 

17.1 

.135 

77 

40.1 

.385 

28 

17.7 

.140 

78 

40.5 

.390 

29 

18,3 

.145 

79 

40.8 

.395 

30 

18.8 

.150 

80 

41.2 

.400 

31 

19.3 

.155 

81 

41.6 

.405 

32 

19.8 

.160 

82 

42.0 

.410 

33 

20.3 

.165 

83 

42.3 

.415 

34 

20.9 

.170 

84 

42.7 

.420 

35 

21.4 

.175 

85 

43.1 

.425 

36 

22.0 

.180 

86 

43.4 

.430 

37 

22.5 

.185 

87 

43.8 

.435 

38 

23.0 

.190 

88 

44.1 

1.440 

39 

23.5 

.195 

89 

44.4 

1.445 

40 

24.0 

.200 

90 

44.8 

1.450 

41 

•24.5 

.205 

91 

45.1 

1.455 

42 

25.0 

.210 

92 

45.4 

1.460 

43 

25.5 

.215 

93 

45.8 

1.464 

44 

26.0 

.220 

94 

46.1 

.470 

45 

26.4 

.225 

95 

46.4 

.475 

46 

26.9 

.230 

96 

46.7 

.480 

47 

27.4 

.235 

97 

47.1 

.485 

48 

27.9 

.240 

98 

47.4 

.490 

49 

28.4 

.245 

99 

47.8 

.495 

612 


GELATIN  AND  GLUE 
TABLE  77. — (Continued} 


Degrees 
Twaddell 

Degrees 
Baume 

Specific 
gravity 

Degrees 
Twaddell 

Degrees 
Baume' 

Specific 
gravity 

100 

48.1 

.500 

140 

59.5 

1.700 

101 

48.4 

.505 

141 

59.7 

1.705 

102 

48.7 

.510 

142 

60.0 

1.710 

103 

49.0 

.515 

143 

60.2 

1.715 

104 

49.4 

.520 

144 

60.4 

1.720 

105 

49.7 

.525 

145 

60.6 

1.725 

106 

50.0 

.530 

146 

60.9 

1.730 

107 

50.3 

.535 

147 

61.1 

1.735 

108 

50.6 

.540 

148 

61.4 

1.740 

109 

50.9 

.545 

149 

61.6 

1.745 

110 

51.2 

.550 

150 

61.8 

1.750 

111 

51.5 

.555 

151 

62.1 

1.755 

112 

51.8 

.560 

152 

62.3 

1.760 

113 

52.1 

.565 

153 

62.5 

1.765 

114 

52.4 

.570 

154 

62.8 

1.770 

115 

52.7 

.575 

155 

63.0 

1.775 

116 

53.0 

.580 

156 

63.2 

1.780 

117 

53.3 

.585 

157 

63.5 

1.785 

118 

53.6 

.590 

158 

63.7 

1.790 

119 

53.9 

.595 

159 

64.0 

1.795 

120 

54.1 

.600 

160 

64.2 

1.800 

121 

54.4 

.605 

161 

64.4 

1.805 

122 

54.7 

.610 

162 

64.6 

1.810 

123 

55.0 

.615 

163 

64.8 

1.815 

124 

55.2 

.620 

164 

65.0 

1.820 

125 

55.5 

.625 

165 

65.2 

1.825 

126 

55.8 

.630 

166 

65.5 

1.830 

127 

56.0 

.635 

167 

65.7 

1.835 

128 

56.3 

.640 

168 

65.9 

1.840 

129 

56.6 

.645 

169 

66.1 

1.845 

130 

56.9 

.650 

170 

66.3 

1.850 

131 

57.1 

.655 

171 

66.5 

1.855 

132 

57.4 

.660 

.  .  . 

70.0 

1.933 

133 

57.7 

.665 

.  .  . 

71.0 

1.959 

134 

57.9 

.    .670 

72.0 

1.986 

135 

58.2 

.675 

73.0 

2.014 

136 

58.4 

.680 

74.0 

2.042 

137 

58.7. 

.685 

. 

75.0 

2.071 

138 

58.9 

.690 

77.0 

2.132 

139 

59.2 

.695 

... 

79.0 

2.197 

140 


Specific  gravity  =  T> A  o    i    i  o/-t  Ior  liquids  lighter  than  water. 

Specific  gravity  =  -..  _  p ,  0  for  liquids  heavier  than  water. 
140 


Baum6  = 


Sp.  Gr. 


—  130  for  liquids  lighter  than  water. 


Baum6  =  145  — 


145 


g 


for  liquids  heavier  than  water. 


APPENDIX 


613 


TABLE  78. — SPECIFIC  GRAVITIES  IN  DEGREES  BAUME,  LIQUIDS  LIGHTER 

THAN  WATER 


Degrees  Baume 

Specific 
gravity 

Degrees 
Baume" 

Specific 
gravity 

Degrees 
Baume" 

Specific 
gravity 

10 

1.000 

15 

0.966 

20 

0.933 

11 

0.993 

16 

0.959 

21 

0.927 

12 

0.986 

17 

0.952 

22 

0.921 

13 

0.979 

18 

0.946 

23 

0.915 

14 

0.972 

19 

0.940 

24 

0.909 

25 

0.903 

30 

0.875 

35 

0.849 

26 

0.897 

31 

0.870 

36 

0.843 

27 

0.892 

32 

0.864 

37 

0.838 

28 

0.886 

33 

0.859 

38. 

0.833 

29 

0.881 

34 

0.854 

39 

0.828 

40 

0.824 

48 

0.787 

58 

0.745 

41 

0.819 

50 

0.778 

60 

0.737 

42 

0.814 

52 

0.769 

65 

0.718 

43 

0.805 

54 

0.761 

70 

0.700 

46 

0.796 

56 

0.753 

75 

0.683 

Specific  gravity  = 


Tw.°  +  200 
200 


Twaddell  =  (200  X  sp.  gr.)  -  200. 


,  0 


614  GELATIN  AND  GLUE 

TABLE  79. — COMPARISON  OF  CENTIGRADE  AND  FAHRENHEIT  SCALES 


Centi- 
grade 

Fahrenheit 

Centi- 
grade 

Fahrenheit 

Centi- 
grade 

Fahrenheit 

Centi- 
grade 

Fahrenheit 

-30 

-22.0 

10 

50.0 

50 

122.0 

90 

194.0 

-28 

-18.4 

12 

53.6 

52 

125.6 

92 

197.6 

-26 

-14.8 

14 

57.2 

54 

129.2 

94 

201.2 

-24 

-11.2 

16 

60.8 

56 

132.8 

96 

204.8 

-22 

-   7.6 

18 

64.4 

58 

136.4 

98 

208.4 

-20 

-   4.0 

20 

68.0 

60 

140.0 

100 

212.0 

-18 

-   0.4 

22 

71.6 

62 

143.6 

105 

221.0 

-16 

3.2 

24 

75.2 

64 

147.2 

110 

230.0 

-14 

6.8 

26 

78.8 

66 

150.8 

115 

239.0 

-12 

10.4 

28 

82.4 

68 

154.4 

120 

248.0 

-10 

14.0 

30 

86.0 

70 

158.0 

125 

257.0 

-    8 

17.6 

32 

89.6 

72 

161.6 

130 

266.0 

-    6 

21.2 

34 

93.2 

74 

165.2 

135 

275.0 

-    4 

24.8 

36 

96.8 

76 

168.8 

140 

284.0 

-    2 

28.4 

38 

100.4 

78 

172.4 

145 

293.0 

0 

32.0 

40 

104.0 

80 

176.0 

150 

302.0 

2 

35.6 

42 

107.6 

82 

179.6 

155 

311.0 

4 

39.2 

44 

111.2 

84 

183.2 

160 

320.0 

6 

42.8 

46 

114.8 

86 

186.8 

165 

329.0 

8 

46.4 

48 

118.4 

88 

190.4 

170 

338.0 

Fahrenheit  =  -^  +  32. 
Centigrade  =  ~  (F  -  32). 


TABLE  80. — CONVERSION  OF  PARTS  PER  MILLION  TO  GRAINS  PER  UNITED 
STATES  AND  IMPERIAL  GALLONS  AND  TO  PER  CENT 


Parts  per 
million 

Grains  per 
United 
States 
gallon 

Grains  per 
imperial 
gallon 

Per  cent 

Parts 
per 
million 

Grains 
per 
United 
States 
gallon 

Grains 
per 
imperial 
gallon 

Per  cent 

1 

0.  0583 

0.  0700 

0.0001 

40 

2.3327 

2.8000 

0.004 

2 

0.1166 

0.1400 

0.  0002 

50 

2.9159 

3.  5000 

0.005 

3 

0.  1749 

0.2100 

0.  0003 

60 

3.4990 

4.2000 

0.006 

4 

0.  2332 

0.  2800 

0.  0004 

70 

4.0882 

4.9000 

0.007 

5 

0.2916 

0.  3500 

0.  0005 

80 

4.6654 

5.6000 

0.008 

6 

0.  3499 

0.  4200 

0.  0006 

90 

5.2486 

6.  3000 

0.009 

7 

0.4082 

0.  4900 

0.  0007 

100 

5.8318 

7.000 

0.01 

8 

0.4665 

0.5600 

0.  0008 

200 

11.6630 

14.000 

0.02 

9 

0.  5248 

0.  6300 

0.  0009 

300 

17.4950 

21.000 

0.03 

10 

0.  5831 

0.  7000 

0.001 

400 

23.3270 

28.000 

0.04 

20 

1.1663 

1.4000    ' 

0.002 

500 

29.  1590 

35.  000 

0.05 

30 

1.7495 

2.  1000 

0.003 

1000 

58.3180 

70.  000 

0.1 

APPENDIX 


615 


TABLE  81. — METRIC  AND  AMERICAN  EQUIVALENTS 


1  centimeter  =  0.3937  inch 

1  meter  =  39.37  inches  =  1.0936  yards 

1  meter  =  3.20883  feet 

1  kilometer  =  0.62137  mile 

1  square  centimeter  =  0.1550  square  inch 

1  square  meter  =  1.96  square  yards 

1  cubic  centimeter  =  0.0610  cubic  inch 

1  cubic  meter  =  1.308  cubic  yards 

1  liter  (1000  c.c.)  =  0.908  quart,  dry 

1  liter  =  1.0567  quarts,  liquid 

1  liter  =  0.26418  gallon 

1  miililiter  =  0.033815  U.  S.  fluid  ounce 

1  gram  =  0.035274  ounce  av. 

1  gram  =  15.4324  grains 

1  kilogram    (1000  grams)  =  2.20462   pounds 

(av.) 
1  metric  ton  =  1.1023  English  tons 


1  inch  =  2.5400  centimeters 

yard  =  0.9144  meter 

foot  =  0.3048  meter 

mile  =  1.6093  kilometers 

square  inch  =  6.452  square  centimeters. 

square  yard  =  0.8631  square  meter 

cubic  inch  =  16.3872  cubic  centimeters 

cubic  yard  =  0.7646  cubic  meter 

quart  dry  =  1.101  liters 

quart  liquid  =  0.9463  liter 

gallon  =  3.78533  liters 

U.  S.  fluid  ounce  =  29.573  milliliters. 

ounce  av.  =  28.350  gram 

grain  =  0.064799  grams 

pound   (av.)  =  0.45359  kilogram   (453.6 

grams) 
1  English  ton  =  0.9072  metric  ton. 


616 


GELATIN  AND  GLUE 


TABLE  82. — SPECIFIC  GRAVITY  AND  PERCENTAGE  COMPOSITION  OF  HYDRO- 
CHLORIC, NITRIC  AND  SULPHURIC  ACIDS 


Specific  gravity 

HC1 

HNOa 

H2S04 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

.000 

0.16 

0.16 

0.10 

0.10 

0.09 

0.10 

.005 

1.15 

1.2 

1.00 

1.0 

0.83 

0.80 

.010 

2.14 

2.2 

1.90 

1.9 

1.57 

1.6 

.015 

3.12 

3.2 

2.80 

2.8 

2.30 

2.3 

.020 

4.13 

4.2 

3.70 

3.8 

3.03 

3.1 

.025 

5.15 

5.3 

4.60 

4.7 

3.76 

3.9 

.030 

6.15 

6.4 

5.50 

5.7 

4.49 

4.6 

.035 

7.15 

7.4 

6.38 

6.6 

5.23 

5.4 

.040 

8.16 

8.5 

7.26 

7.5 

5.96 

6.2 

.045 

9.16 

9.6 

8.13 

8.5 

6.67 

7.1 

.050 

10.17 

10.7 

8.99 

9.4 

7.37 

7.7 

.055 

11.18 

11.8 

9.84 

10.4 

8.07 

8.5 

.060 

12.19 

12.9 

10.68 

11.3 

8.77 

9.3 

.065 

13.19 

14.1 

11.51 

12.3 

9.47 

10.2 

.070 

14.17 

15.2 

12.33 

13.2 

10.19 

10.9 

.075 

15.16 

16.3 

13.15 

14.1 

10.90 

11.7 

.080 

16.15 

17.4 

13.95 

15.1 

11.60 

12.5 

.085 

17.13 

18.6 

14.74 

16.0 

12.30 

13.3 

.090 

18.11 

19.7 

15.53 

16.9 

12.99 

14.2 

.095 

19.06 

20.9 

16.32 

17.9 

13.67 

15.0 

.100 

20.01 

22.0 

17.11 

18.8 

14.35 

15.8 

.105 

20.97 

23.2 

17.89 

19.8 

15.03 

16.6 

.110 

21.92 

24.3 

18.67 

20.7 

15.71 

17.5 

.115 

22.86 

25.5 

19.45 

21.7 

16.36 

18.3 

.120 

23.82 

26.7 

20.23 

22.7 

17.01 

19.1 

.125 

24.78 

27.8 

21.00 

23.6 

17.66 

19.9 

.130 

25.75 

29.1 

21.77 

24.6 

18.31 

20.7 

.135 

26.70 

30.3 

22.54 

25.6 

18.96 

21.5 

.140 

27.66 

31.5 

23.31 

26.6 

19.61 

22.3 

.145 

28.61 

32.8 

24.08 

27.6 

20.26 

23.1 

.150 

29.57 

34.0 

24.84 

28.6 

20.91 

23.9 

.155 

30.55 

35.3 

25.60 

29.6 

21.55 

24.8 

.160 

31.52 

36.6 

26.36 

30.6 

22.19 

25.7 

.165 

32.49 

37.9 

27.12 

31.6 

22.83 

26.6 

.170 

33.46 

39.2 

27.88 

32.6 

23.47 

27.5 

.175 

34.42 

40.4 

28.63 

33.6 

24.12 

28.3 

.180 

35.39 

41.8 

29.38 

34.7 

24.76 

29.2 

.185 

36.31 

43.0 

30.13 

35.7 

25.40 

30.1 

.190 

37.23 

44.3 

30.88 

36.7 

26.04 

31.0 

.195 

38.16 

45.6 

31.62 

37.8 

26.68 

31.9 

.200 

39.11 

46.9 

32.36 

38.8 

27.32 

32.8 

.205 

33.09 

39.9 

27.95 

33.7 

.210 

• 

33.82 

40.9 

28.58 

34.6 

.215 

34.55 

42.0 

29.21 

35.5 

.220 

.  .  .  .  . 

35.28 

43.0 

29.84 

36.4 

.225 

36.03 

44.1 

30.48 

37.3 

.230 

'....'. 

.  .'.  . 

36.78 

45.2 

31.11 

38.2 

.235 

37.53 

46.3 

31.70 

39.1 

.240 

38.29 

47.5 

32.28 

40.0 

245 

39.05 

48.6 

32.86 

40.9 

.250 

39^82 

49  8 

33  43 

41  8 

.255 

40.58 

50.9 

34.00 

42.6 

1.260 

41.34 

52.1 

34.57 

43.5 

1.265 

....'. 

42.10 

53.3 

35.14 

44.4 

1.270 

42.87 

54.4 

35.71 

45.4 

1.275 

.  .  .  .  . 

.  .  .  . 

43.64 

55.6 

32.29 

46.2 

1.280 

44.41 

56.8 

36.87 

47.2 

1.285 

45.18 

58.1 

37.45 

48.1 

1.290 

45.95 

59.3 

38.03 

49.0 

1.295 

46.72 

60.5 

38.61 

50.0 

1.300 

r 

47.49 

61.7 

39.19 

51.0 

1.305 

48.26 

63.0 

39.77 

51.9 

1.310 

49.07 

64.3 

40.35 

52.9 

1.315 

49.89 

65.6 

40.93 

53.8 

1.320 



50.71 

66.9 

41.50 

54.8 

APPENDIX 
TABLE  82.— (Continued) 


617 


Specific  gravity 

HC1 

HN03 

H2SO4 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

1.325 
1.330 
1.335 
.340 
.345 
.350 
.355 
.360 
.365 
.370 
.375 
.380 
.385 
.390 
.395 
.400 
.405 
.410 
.415 
.420 
.425 
.430 
.435 
.440 
.445 
.450 
.455 
.460 
.465 
.470 
.475 
.480 
.485 
.490 
.495 
.500 
.505 
.510 
.515 
.520 
.525 
.530 
.535 
.540 
.545 
.550 
.555 
.560 
.565 
.570 
.575 
.580 
.585 
.590 
.595 
.600 
.605 
.610 
.615 
.620 
.625 
.630 
.635 
.640 
.645 
1.650 

:::: 

51.53 
52.37 
53.22 
54.07 
54.93 
55.79 
56.66 
57.57 
58.48 
59.39 
60.30 
61.27 
62.24 
63.23 
64.25 
65.30 
66.40 
67.50 
68.63 
69.80 
70.98 
72.17 
73.39 
74.68 
75.98 
77.28 
78.60 
79.98 
81.42 
82.90 
84.45 
86.05 
87.70 
89.60 
91.60 
94.09 
96.39 
98.10 
99.07 
99.67 

68.3 
69.7 
71.0 
72.5 
73.9 
75.3 
76.8 
78.3 
79.8 
81.4 
82.9 
84.6 
86.2 
87.9 
89.6 
91.4 
93.3 
95.2 
97.1 
99.1 
101.1 
103.2 
105.3 
107.5 
109.8 
112.1 
114.4 
116.8 
119.3 
121.9 
124.6 
127.4 
130.2 
133.5 
136.9 
141.1 
145.1 
148.1 
150.1 
151.5 

42.08 
42.66 
43.20 
43.74 
44.28 
44.82 
45.35 
45.88 
46.41 
46.94 
47.47 
48.00 
48.53 
49.06 
49.59 
50.11 
50.63 
51.15 
51.66 
52.15 
52.63 
53.11 
53.59 
54.07 
54.55 
55.03 
55.50 
55.97 
56.43 
56.90 
57.37 
57.83 
58.28 
58.74 
59.22 
59.70 
60.18 
60.65 
61.12 
61.59 
62.06 
62.53 
63.00 
63.43 
63.85 
64.26 
64.67 
65.08 
65.49 
65.90 
66.30 
66.71 
67.13 
67.59 
68.05 
68.51 
68.97 
69.43 
69.89 
70.32 
70.74 
71.16 
71.57 
71.99 
72.40 
72.82 

55.7 
56.7 
57.7 
58.6 
59.6 
60.5 
61.4 
62.4 
63.3 
64.3 
65.3 
66.2 
67.2 
68.2 
69.2 
70.2 
71.1 
72.1 
73.0 
74.0 
75.0 
75.9 
76.9 
77.9 
78.9 
79.8 
80.8 
81.7 
82.7 
83.7 
84.6 
85.6 
86.5 
87.6 
88.5 
89.6 
90.6 
91.6 
92.6 
93.6 
94.6 
95.7 
96.7 
97.7 
98.7 
99.6 
100.6 
101.5 
102.5 
103.5 
104.4 
105.4 
106.4 
107.5 
108.5 
109.6 
110.7 
111.8 
112.8 
113.9 
115.0 
116.0 
117.0 
118.1 
119.2 
120.2 



::::: 

::::.' 

'.'.'.'.'. 

•  •••• 



'.'/.'.'. 

• 

:::: 

!  '.  '.  '.  '. 

::::: 

618 


GELATIN  AND  GLUE 

TABLE  82— (Concluded) 


Specific  gravity 

HC1 

HNOs 

H2S04 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

Per  cent 
by  weight 

Grams  of 
acid  per 
100  c.c. 

1.655 
1.660 
1.665 
1.670 
1.675 
1.680 
.685 
.690 
.695 
.700 
.705 
.710 
.715 
.720 
.725 
.730 
.735 
.740 
1.745 
1.750 
1.755 
1.760 
1.765 
1.770 
1.775 
1.780 
1.785 
1.790 
1.795 
1.800 
1.805 
t  810 
t.815 
1.820 
1.825 
1.830 
1.835 
1.840 
1  .  8405 
1.8410 
1.8415 
1.8410 
1  .  8405 
1  .  8400 
1.8395 
1  .  8390 
1  .  8385 

73.23 
73.64 
74.07 
74.51 
74.97 
75.42 
75.86 
76.30 
76.73 
77.17 
77.60 
78.04 
78.48 
78.92 
79.36 
78.90 
80.24 
80.68 
81.12 
81.56 
82.00 
82.44 
82.88 
83.32 
83.90 
84.50 
85.10 
85.70 
86.30 
86.90 
87.60 
88.30 
89.05 
90.05 
91.00 
92.10 
93.43 
95.60 
95.95 
97.00 
97.70 
98.20 
98.79 
99.20 
99.45 
99.70 
99.95 

121.2 
122.2 
123.3 
124.4 
125.6 
126.7 
127.8 
128.9 
130.1 
131.2 
132.3 
133.4 
134.6 
135.7 
136.9 
138.1 
139.2 
140.4 
141.6 
142.7 
143.9 
145.1 
146.3 
147.5 
148.9 
150.4 
151.9 
153.4 
154.9 
156.4 
158.1 
159.8 
162.1 
163.9 
166.1 
168.5 
171.3 
175.9 
176.5 
178.6 
179.9 
180.8 
181.6 
182.5 
183.0 
183.4 
183.8 

•  •  •  •  • 

APPENDIX 


619 


TABLE  83. — SPECIFIC  GRAVITY  AND  PERCENTAGE  COMPOSITION  OF  SODIUM 
AND  POTASSIUM  HYDROXIDE  SOLUTIONS  (BY  LUNGE) 


Specific    gravity 

Sodium  hydroxide 

Potassium  hydroxide 

Per  cent  NaOH 

Grams  NaOH 
per    liter 

Per  cent  KOH 

Grams  KOH 
per  liter 

1.007 

0.61 

6 

0.9 

9 

1.014 

1.20 

12 

1.7 

17 

1.022 

2.00 

21 

2.6 

26 

1.029 

2.70 

28 

3.5 

36 

1.036 

3.35 

35 

4.5 

46 

1.045 

4.00 

42 

5.6 

58 

.052 

4.64 

49 

6.4 

67 

.060 

5.29 

56 

7.4 

78' 

.067 

5.87 

63 

8.2 

83 

.075 

6.55 

70 

9.2 

99 

.083 

7.31 

79 

10.1 

109 

.091 

8.00 

87 

10.9 

119 

.100 

8.68 

95 

12.0 

132 

.108 

9.42 

104 

12.9 

143 

.116 

10.06 

112 

13.8 

153 

.125 

10.97 

123 

14.8 

167 

.134 

11.84 

134 

15.7 

178 

.142 

12.64 

144 

16.5 

183 

.152 

13.55 

156 

17.6 

203 

.162 

14.37 

167 

18.6 

216 

.171 

15.13 

177 

19.5 

228 

.180 

15.91 

188 

20.5 

242 

.190 

16.77 

200 

21.4 

255 

.200 

17.67 

212 

22.4 

269 

.210 

18.58 

225 

23.3 

282 

.220 

19.58 

239 

24.2 

295 

.231 

20.59 

253 

25.1 

309 

.241 

21.42 

266 

26.1 

324 

.252 

22.64 

283 

27.0 

338 

.263 

23.67 

299 

28.0 

353 

.274 

24.81 

316 

28.9 

368 

.285 

25.80 

332 

29.8 

385 

.297 

26.83 

348 

30.7 

398 

.308 

27.80 

364 

31.8 

416 

.320 

28.83 

381 

32.7 

432 

.332 

29.93 

399 

33.7 

449 

.345 

31.22 

420 

34.9 

469 

.357 

32.47 

441 

35.9 

487 

.370 

33.69 

462 

36.9 

506 

.383 

34.96 

483 

37.8 

522 

.397 

36.25 

506 

38.9 

543 

.410 

37.47 

528 

39.9 

563 

.424 

38.80 

553 

40.9 

582 

.438 

39.99 

575 

42.1 

605 

.453 

41.41 

602 

43.4 

631 

.468 

42.83 

629 

44.6 

655 

.483 

44.38 

658 

45.8 

679 

.498 

46.15 

691 

47.1 

706 

.514 

47.60 

721 

48.3 

731 

.530 

49.02 

750 

49.4 

756 

.546 

50.6 

779 

.563 

51.9 

811 

.580 

53.2 

840 

597 

54.5 

870 

.615 

., 

55.9 

905 

.634 

57.5 

940 

620 


GELATIN  AND  GLUE 


TABLE  84. — SPECIFIC  GRAVITY  AND  PERCENTAGE  COMPOSITION  OF  AQUA 
AMMONIA  (By  W.  C.  FERGUSON) 


Specific 
gravity 
60°/60°F. 

Per  cent 
NH3 

Specific 
gravity 
60°/60°F. 

Per  cent 
NH3 

Specific 
gravity 
60°/60°F. 

Per  cent 
NH3 

1.0000 

0.00 

0.9492 

13.02 

0.9032 

27.44 

0.9982 

0.40 

0.9475 

13.49 

0.9018 

27.93 

0.9964 

0.80 

0.9459 

13.96 

0.9003 

28.42 

0.9947 

1.21 

0.9444 

14.43 

0.8989 

28.91 

0.9929 

1.62 

0.9428 

14.90 

0  .  8974 

29.40 

0.9912 

2.04 

0.9412 

15.37 

0.8960 

29.89 

0.9894 

2.46 

0.9396 

15.84 

0.8946 

30.38 

0.9876 

2.88 

0.9380 

16.32 

0.8931 

30.87 

0.9859 

3.30 

0.9365 

16.80 

0.8917 

31.36 

0.9842 

3.73 

0.9349 

17.28 

0.8903 

31.85 

0.9825 

4.16 

0.9333 

17.76 

0.8889 

32.34 

0.9807 

4.59 

0.9318 

18.24 

0.8875 

32.83 

0.9790 

5.02 

0.9302 

18.72 

0.8861 

33.32 

0.9773 

5.45 

0.9287 

19.20 

0.8847 

33.81 

0.9756 

5.88 

0  .  9272 

19.68 

0.8833 

34.30 

0.9739 

6.31 

0.9256 

20.16 

0.8819 

34.79 

0.9722 

6.74 

0.9241 

20.64 

0.8805 

35.28 

0  .  9705 

7.17 

0.9226 

21.12 

0  .  9689 

7.61 

0.9211 

21.60 

0.9672 

8.05 

0.9195 

22.08 

0.9655 

8.49 

0.9180 

22.56 

0  .  9639 

8.93 

0.9165 

23.04 

0  .  9622 

9.39 

0.9150 

23.52 

0  .  9605 

9.83 

0.9135 

24.01 

0  .  9589 

10.28 

0.9121 

24.50 

0.9573 

10.73 

0.9106 

24.99 

0.9556 

11.18 

0.9091 

25.48 

...... 

0  .  9540 

11.64 

0  .  9076 

25.97 

0  .  9524 

12.10 

0.9061 

26.46 

0  .  9508 

12.56 

0  .  9047 

26.95 

APPENDIX 


621 


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in  <~4 
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o  m  «n  m  o 

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m  in  in 

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m  m"  m  m  m 


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m  m  m  m  in 

o'  d  d  d  d 


m  in  in  in  in 

d  d  d  d  d 


in  in  in  in  in 

d  d  d  d  d 


tA    tA    tA    tA    tA 


in  in  in  in  in 
o'  d  o'  d  d 


in  in  m  m  m 
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o  o'  o  d  o 

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m  m  m  m  m 


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622 


GELATIN  AND  GLUE 


TABLE    86. — THE    CHEMICAL   ELEMENTS    AND    THEIR   ATOMIC    WEIGHTS 

(Compiled  by  the  International  Committee  on  Atomic   Weights) 

Corrected  to  1921 


Name 

Symbol 

Atomic 
weight 

Name 

Symbol 

Atomic 
weight 

Aluminum  

Al 

27.1 

Molybdenum  

Mo 

96.0 

Antimony  
Argon  

Sb 
A 

120.2 
39  9 

Neodymium  
Neon  

Nd 

Ne 

144.3 
20.2 

Arsenic  

As 

74  96 

Nickel  

Ni 

58.68 

Barium 

Ba 

137  37 

Niton 

Nt 

222.4 

Bismuth 

Bi 

208  0 

N 

14.008 

Boron 

B 

10  9 

Os 

190.9 

Bromine 

Br 

79  92 

Oxygen                 .    .    . 

o 

16.00 

Cadmium  

Cd 

112.40 

Palladium  

Pd 

106.7 

Calcium  

Ca 

40.07 

Phosphorus  

P 

31.04 

Carbon  
Cerium  .... 

C 

Ce 

12.005 
140  25 

Platinum  
Potassium  

Pt 
K 

195.2 
39.10 

Cesium  
Chlorine 

Cs 

Cl 

132.81 
35  46 

Praesodymium  

Pr 
Ra 

140.9 
226.0 

Chromium 

Cr 

52  0 

Rh 

102.9 

Cobalt 

Co 

58  97 

Rubidium 

Rb 

85.45 

Columbium 

Cb 

93  1 

Ruthenium              .... 

Ru 

101.7 

Copper  .  .  . 

Cu 

63  57 

Samarium         

Sa 

150.4 

Dysprosium 

Dy 

162  5 

Scandium   

Sc 

45.1 

Erbium  
Europium  

Er 
Eu 

167.7 
152.0 

Selenium  
Silicon  

Se 
Si 

79.2 
28.3 

Fluorine  

F 

19.0 

Silver  

Ag 

107.  88 

Gadolinium 

Gd 

157  3 

Na 

23.00 

Gallium  

Ga 

70.1 

Strontium  

Sr 

87.63 

Germanium 

Ge 

72  5 

Sulphur                     .  .  . 

s 

32.06 

Glucinum 

Gl 

9  1 

Tantalum         

Ta 

181.5 

Gold  

Au 

197  2 

Tellurium     

Te 

127.5 

Helium  
Holmium  
Hydrogen  

He 
Ho 
H 

4.00 
163.5 
1.008 

Terbium  
Thallium  
Thorium  

Tb 
Tl 
Th 

159.2 
204.0 
232.  15 

Indium  

In 

114.8 

Thulium  

Tm 

168.5 

Iodine 

I 

126  92 

Tin 

Sn 

118.7 

Iridium 

Ir 

193  1 

Ti 

48.1 

Iron  ...    . 

Fe 

55  84 

Tungsten 

W 

184.0 

Krypton  

Kr 

82  92 

Uranium               .    ... 

u 

238.2 

Lanthanum  
Lead  
Lithium  
Lutecium 

La 
Pb 
Li 
Lu 

139.0 
207.  20 
6.94 
175  0 

Vanadium  
Xenon  
Ytterbium  .  
Yttrium 

V 
Xe 
Yb 

Yt 

51.0 
130.2 
173.5 

89.33 

Magnesium 

Mg 

24  32 

Zn 

65.37 

Manganese 

Mn 

54  93 

Zr 

90.6 

Mercury  

Hg 

200.6 

APPENDIX 
TABLE  87. — LOGARITHMS  OF  NUMBERS 


623 


Natural 
numbers 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  parts 

• 

2 

3 

4 

5 

6 

7 

8 

9 

10 

0000 

0043- 

0086 

0128 

0170 

0212 

0253 

0294 

0334 

0374 

4 

8 

12 

17 

21 

25 

29 

3337 

11 

9414 

0453 

0492 

0531 

0569 

0607 

0645 

0682 

0719 

0755 

4 

8 

11 

15 

19 

23J26 

3034 

12 

0792 

0828 

0864 

0899 

0934 

0969 

1004 

1038 

1072 

1106 

3 

7 

10 

14 

17 

212412831 

13 

1139 

1173 

1206 

1239 

1271 

1303 

1335 

1367 

1399 

1430 

3 

(5 

10 

13 

10 

19 

23 

26 

29 

14 

1461 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732 

3 

6 

g 

12 

15 

18 

21 

24 

27 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

3 

6 

8 

11 

14 

17 

20 

22 

25 

16 

2041 

2068 

2095 

2122 

2148 

2175 

2201 

2227 

2253 

2279 

3 

5 

8 

11 

13 

16 

18 

21 

24 

17 

2304 

2330 

2355 

2380 

2405 

2430 

2455  2480 

2504 

2529 

2 

5 

7 

10 

12 

15 

17 

20 

22 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695  2718 

2742 

2765 

2 

5 

7 

g 

12 

14 

16 

19 

21 

19 

2788 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

2 

4 

7 

9 

11 

13 

16 

18 

20 

20 

3010 

3032 

3054 

3075 

3096 

3118 

3139 

3160 

3181 

3201 

2 

4 

6 

8 

11 

13 

15 

17 

19 

21 

3222 

3243 

3263 

3284 

3304 

3324 

3345^3365 

3385 

3404 

2 

4 

6 

8 

10 

12 

14 

16 

IS 

22 
23 

3424 
3617 

3444 
3636 

3464 
3655 

3483 
3674 

3502 
5692 

3522  3541 
3711J3729 

3560 
3747 

3579 
3766 

3598 
3784 

2 
2 

4 

4 

(1 
6 

8 

7 

10 

e 

12 
11 

14 
13 

15 
15 

17 

17 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

3945 

3962 

2 

4 

5 

7 

g 

11 

12 

14 

16 

25 

3979 

3997 

4014 

4031 

4048 

4065  4082 

4099 

4116 

4133 

2 

3 

5 

7 

9 

10 

12 

14 

16 

26 

4150 

4166 

4183 

4200 

4216 

4232  4249 

4265 

4281 

4298 

2 

3 

5 

7 

8 

10 

11 

13 

15 

27 

4314 

4330 

4346 

4362 

4378 

4393  4409 

4425 

4440 

4456 

2 

3 

5 

6 

8 

9 

11 

13 

14 

28 

4472 

4487 

4502 

4518 

4533 

4548  4564 

4579 

4594 

4609 

2 

8 

5 

6 

-s 

9 

11 

12 

14 

29 

4624 

4639 

4654 

4669 

4683 

4698 

4713 

4728 

4742 

4757 

1 

3 

4 

6 

7 

9 

10 

12 

13 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

] 

3 

4 

6 

7 

9 

10 

11 

13 

31 

4914 

4928 

4942 

4955 

4969 

4983  4997 

5011 

5024 

5038 

3 

4 

6 

7 

8 

10 

11 

12 

32 

5051 

5065 

5079 

5092 

5105 

5119|5132 

5145 

5159 

5172 

3 

4 

5 

7 

8 

9 

11 

12 

33 

5185 

5198 

5211 

5224 

5237 

525015263 

5276 

5289 

5302 

3 

4 

5 

(i 

8 

9 

10 

12 

34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

3 

4 

5 

6 

8 

9 

10 

11 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551 

2 

4 

5 

6 

7 

9 

10 

11 

36 

5563 

5575 

5587 

5599 

5611 

5623  5635 

5647 

5658 

5670 

2 

4 

5 

6 

7 

8 

10 

11 

37 

5682 

5694 

5705 

5717 

5729 

574015752 

5763 

5775 

5786 

2 

3 

5 

(i 

7 

s 

9 

10 

38 

5798 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5899 

2 

3 

6 

8 

7 

8 

g 

10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

2 

3 

4 

5 

7 

8 

9 

10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

2 

3 

4 

6 

6 

8 

9 

10 

41 
42 

6128 
6232 

6138 
6243 

6149 
6253 

6160  6170 
626316274 

6180 
6284 

6191 
6294 

6201 
6304 

6212  6222 
6314|6325 

2 
2 

3 
3 

4 
4 

B 
6 

6 
6 

7 
7 

8 
8 

9 
9 

43 

6335 

6345  6355 

6365  6375 

6385 

6395 

6405 

641516425 

2 

3 

4 

B 

6 

7 

8 

9 

44 

6435 

6444  6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

2 

3 

4 

6 

G 

7 

8 

9 

45 

6532 

6542  6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

1 

2 

3 

4 

B 

6 

7 

8 

9 

46 

6628 

6637  6646 

6656  6665 

6675 

6684 

6693 

6702 

6712 

1 

2 

3 

4 

5 

6 

7 

7 

8 

47 

6721 

6730  6739 

6749,6758 

6767 

6776 

6785  '6794 

6803 

1 

2 

3 

4 

6 

.-> 

6 

7 

8 

48 

6812 

6821 

6830 

6839:6848 

6875 

6866 

6875 

6884 

6893 

1 

2 

3 

4 

4 

5 

(i 

7 

8 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

1 

2 

3 

4 

-1 

6 

« 

7 

8 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

1 

2 

3 

3 

4 

5 

6 

7 

8 

51 

7076 

7084 

7093 

710117110 

7118 

7126 

7135 

7143 

7152 

1 

2 

3 

8 

4 

B 

0 

7 

8 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

1 

2 

2 

3 

4 

5 

(i 

7 

7 

53 

7243 

7251 

7259 

7267 

7275 

7284 

7292 

730 

7308 

7316 

1 

2 

2 

3 

4 

6 

8 

6 

7 

54 

7324 

7332 

7340 

7348 

7356 

7364 

7372 

7380 

7388 

7396 

1 

2 

2 

a 

4 

5 

8 

6 

7 

624 


GELATIN  AND  GLUE 
TABLE  87.— (Concluded) 


Natural 
numbers 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  parts 

1 

2 

3 

4 

5 

6 

7 

8 

9 

55 

7404 

7412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

1 

.2 

2 

3 

4 

5 

5 

0 

7 

56 

7482 

7490 

7497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

1 

2 

2 

3 

4 

5 

5 

G 

7 

57 

7559 

7566 

7574 

7582 

7589 

7597 

7604 

7612 

7619 

7627 

1 

2 

2 

3 

4 

5 

5 

6 

7 

58 

7634 

7642 

7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

1 

1 

2 

3 

4 

4 

5 

G 

7 

59 

7709 

7716 

7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

1 

1 

2 

3 

4 

4 

5 

G 

7 

60 

7782 

7789 

7796 

7803 

7810 

7818 

7825 

7832 

7839 

7846 

1 

1 

2 

3 

4 

4 

6 

6 

6 

61 

7853 

7860 

7868 

7875 

7882 

7889 

78"96 

7903 

7910 

7917 

1 

1 

2 

3 

4 

4 

6 

6 

6 

62 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980 

7987 

1 

1 

2 

3 

3 

4 

6 

6 

6 

63 

7993 

8000 

8007 

8014 

8021 

8028 

8035 

8041 

8048 

8055 

1 

1 

2 

3 

3 

4 

5 

5 

G 

64 

8062 

8069 

8075 

8082 

8089 

8096 

8102 

8109 

8116 

8122 

1 

1 

2 

3 

3 

4 

5 

5 

G 

65 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8176 

8182 

8189 

2 

3 

3 

4 

5 

5 

G 

66 

8195 

8202 

8209 

8215 

8222 

8228 

8235 

8241 

8248 

8254 

2 

3 

3 

4 

5 

5 

G 

67 

8261 

8267 

8274 

8280 

8287 

8293 

8299 

8306 

8312 

8319 

2 

3 

3 

4 

5 

5 

G 

68 

8325 

8331 

8338 

8344 

8351 

8357 

8363 

8370 

8376 

8382 

2 

3 

3 

4 

4 

5 

8 

69 

8388 

8395 

8401 

8407 

8414 

8420 

8426 

8432 

8439 

8445 

2 

2 

3 

4 

4 

5 

6 

70 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

8494 

8500 

8506 

2 

2 

3 

4 

4 

5 

6 

71 

8513 

8519 

8525 

8531 

8537 

X543 

8549 

8555 

8561 

8567 

2 

2 

3 

4 

4 

5 

5 

72 

8573 

8579 

8585 

8591 

8597 

8603 

8609 

8615 

8621 

8627 

2 

2 

3 

4 

4 

5 

5 

73 

8633 

8639 

8645 

8651 

8657 

8663 

8669 

8675 

8681 

8686 

2 

2 

3 

4 

4 

5 

5 

74 

8692 

8698 

8704 

8710 

8716 

8722 

8727 

8733 

8739 

8745 

2 

2 

3 

4 

4 

5 

6 

75 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

8797 

8802 

2 

2 

3 

3 

4 

5 

5 

76 

8808 

8814 

8820 

8825 

8831 

8837 

8842 

8848 

8854 

8859 

2 

2 

3 

3 

4 

5 

5 

77 

8865 

8871 

8876 

8882 

8887 

8893 

8899 

8904 

8910 

8915 

2 

2 

8 

3 

4 

4 

5 

78 

8921 

8927 

8932 

8938 

8943 

8949 

8954 

S9C.O 

8965 

8971 

2 

2 

3 

3 

4 

4 

5 

79 

8976 

8982 

8987 

8993 

8998 

9004 

9009 

9015 

9020 

9026 

2 

2 

3 

3 

4 

4 

5 

80 

9031 

9036 

9042 

9047 

9053 

9058 

9063 

9069 

9074 

9079 

2 

2 

3 

3 

4 

4 

5 

81 

9085 

9090 

9096 

9101 

9106 

9112 

9117 

9122 

9128 

9133 

2 

2 

3 

3 

4 

4 

5 

82 

9138 

9143 

9149 

9154 

9159 

9165 

9170 

9175 

9180 

9186 

1 

1 

2 

2 

3 

3 

4 

4 

5 

83 

9191 

9196 

9201 

9206 

9212 

9217 

'.1222 

9227 

9232 

9238 

1 

1 

2 

2 

3 

3 

4 

4 

5 

84 

9243 

9248 

9253 

9258 

9263 

9269 

9274 

9279 

9284 

9289 

1 

1 

2 

2 

3 

3 

4 

4 

5 

85 

9294 

9299 

9304 

9309 

9315 

9320 

9325 

9330 

9335 

9340 

1 

1 

2 

2 

3 

3 

4 

4 

5 

86 

9345 

9350 

9355 

9360 

9365 

9370 

9375 

9380 

9385 

9390 

1 

1 

2 

2 

3 

3 

4 

4 

5 

87 

9395 

9400 

9405 

9410 

9415 

9420 

9425 

9430 

9435 

9440 

0 

11  1 

2 

2 

3 

3 

4 

4 

88 

9445 

9450 

9455 

9460 

9465 

9469 

9474,9479 

9484 

9489 

0 

1 

1 

2 

2 

3 

3 

4 

4 

89 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

9533 

9538 

0 

1 

1 

2 

2 

3 

3 

4 

4 

90 

9542 

9547 

9552 

9557 

9562 

9566 

9571 

9576 

9581 

9586 

0 

1 

2 

2 

3 

3 

4 

4 

91 

9590 

9595 

9600 

9605 

9609 

9614 

9619 

9624 

9628  9633 

0 

1 

2 

2 

3 

3 

4 

4 

92 

9638 

9643 

9647 

9652 

9657 

9661 

9666 

9671 

967519680 

0 

2 

2 

3 

3 

4 

4 

93 

9685 

9689 

9694 

9699 

9703 

9708 

9713 

9717 

9722  9727 

0 

2 

2 

3 

3 

4 

4 

94 

9731 

9736 

9741 

9745 

9750 

9754 

9759 

9763 

9768 

9773 

0 

2 

2 

3 

3 

4 

4 

95 

9777 

9782 

9786 

9791 

9795 

9800 

9805 

9809 

9814 

9818 

0 

2 

2 

3 

3 

4 

4 

96 

9823 

9827 

9832 

9836 

9841 

9845 

9850 

9854 

9859  9863 

n 

2 

2 

3 

3 

4 

4 

97 

9868 

9872 

9877 

9881 

9886 

9890 

9894 

9899 

990319908 

o 

2 

2 

3 

3 

4 

4 

98 

9912 

9917 

9921 

9926 

9930 

9934 

9939 

9943 

9948 

9952 

0 

2 

2 

3 

3 

4 

4 

99 

9956 

9961 

9965 

9969 

9974 

9978 

9983 

9987 

9991 

9996 

0 

2 

2 

3 

3 

3 

4 

SUBJECT  INDEX 


Abrasive  wheels,  545 
Adamkiewicz  reaction,  47 
Adhesive  strength,  411,  517-538 

conditions  affecting,  517-526 
methods  of  testing,  526-534 
significance     in     evaluation, 

493,  495 
Adsorption,  314 
Agar-agar,  127 
Alanine,  21,  35,  36,  65 
Albumin,  280 

Albuminoids,  19,  20,  73,  83,  85,  86 
Albumoses,  469 
Alcohol  number,  249 

precipitation,  463-464,  471 
Aluminum  oxide,  440-441 
Alum-treated  glues,  190,  191,  198 
Alutes,  571 
Amicrons,  161 
Amide  nitrogen,  39,  40 
Amine  nitrogen,  38 
Amino-acids,  21-23 

apparatus,  31-33 

calculation  of  molecular  weight 
from,  112-113 

condensation  of,  222 

conversion  table  for,  621 

description,  21 

determination,  25-28,  448-449, 
451-452 

distribution  in  keratins,  65 

enumeration      and      formulas, 
21-23 

estimation,  28-29 

formaldehyde  titration,  30 

in  proteins,  36 

nitrogen  in  free  groups,  221 

nitrous  acid  method,  30-34 


Amino-acids,  separation,  34-37 
synthesis,  18 

Van  Slyke  distribution,  38-45 
Amino  nitrogen,  38,  39,  40,  42 
Ammonia  nitrogen,  36,  39,  40 
Ammonium  hydroxide,  620 
Amphoteresis,  219-223 
Amyloid,  76-77,  78-80 
Analysis  of  ash,  425-463 
gelatin,  433-448 
organic,  448-450 
proximate,  429-433 
sampling,  427-429 
special  tests,  450-456 
traces  of  metals,  456-463 
Animal  gum,  71 
Antiseptics,  291 
Arginine,  22,  38,  40,  41,  65 
Arsine,  457-459 
Artificial  ivory,  546 
leather,  549 
silk,  549 
Ash  analysis,  433-448 

aluminum  oxide,  440-441 
barium  oxide,  444 
calcium  oxide,  441-442 
chloride,  447-448 
ferric  oxide,  449-441 
lead  oxide,  444 
magnesium  oxide,  442-443 
phosphoric  acid,  434-440 
potassium  oxide,  444-446 
silica,  434 

sodium  oxide,  444-446 
sulphate,  447 
zinc  oxide,  444 
Ash-free  gelatin,  102 
significance,  289 
Aspartic  acid,  22,  35,  36,  39,  65 
Atmidkeratin,  64 


625 


40 


626 


SUBJECT  INDEX 


Atmidkeratose,  64 
Atomic  weights,  622 
Autoclaves,  303,  314 
Avogadro's  constant,  200 

law,  101 
Azotobacter,  15 

B 

Bacteria,  action  on  gelatin,  60-61 
decomposition  of  glue  by,  515- 

517 

in  gelatin  and  ice  cream,  566 
in  hide  stock,  280-281,  313 
Barium  oxide,  444 
Barrel  mill,  278 
Basic  nitrogen,  38 
Baume"  degrees,  611-613 
Beating  engine,  278,  313 
Bergman  process,  306,  317 
Bibliography,  fish  glues,  365-366 
glue  and  gelatin  manufacture, 

313-317 
TL  S.  patents  on  casein  adhe- 

sives,  340-344 
Billiard  balls,  546 
Biuret  reaction,  45 
Blood  albumin,  288,  344 
glue,  344-348 

application,  345-347 
comparison      with      other 

glues,  362-363 
dry  glue  process,  347-348 
formula  for  making,  345 
preparation,  345 
press  for  joining,  346 
Boiling  point,  106-107,  110 
Bone  glue,  300-303 
Bones,  87-88 

collagen  from,  51 
composition,  88 

of  inorganic  portion,  87 
degreasing,  300-303 
glue  from,  2,  4,  6,  316-317 
leaching,  303-306 
marrow  of,  87 
Bone  tankage,  307 
Bookbinding,  537 
Breman  blue,  540 


Bristles,  64 

Bromine  precipitation,  472-473 
Brownian  movement,  109,  124 
Buffer  action,  587 


Calcium  oxide,  441-442 

Calomel  electrode,  588 

Calsomine,  544-545 

Cambon's  fusiometer,  405-406 

Caprine,  21,  36 

Capsules,  570 

Carbohydrates,  15 

Cartilage,  70,  86 

Cartons,  536 

Casein,    acid    coagulation    method 

for,  321 

acidity,  331-332 
ash,  330-331 

borax  solubility  test,  332-333 
color,  328 
electrolysis,  228 
fat,  329-330 
fineness,  328-329 
government    specifications   for, 

325 

grain  curd  method  for,  322-324 
influence  of  method  of  manu- 
facture, 325-327 
lactic  acid  method  for,  320-321 
manufacture,  320-325 
methods  of  analysis  and  testing, 

327-333 
moisture,  329 
nitrogen,  331 

of  free  amino  groups,  221 
odor,  328 
rennet  coagulation  method  for, 

321-322 

sampling  for  analysis,  328 
test  in  size,  483-486 
Casein  glue,  318-344 

application,  337-339 
ash  content,  326 
bibliography  of  U.  S.  patents 

on,  340-344 

comparison  with  other  glues, 
362-363 


SUBJECT  INDEX 


627 


Casein  glue,  dry-mix,  333-337 

formulas  for  making,  334-336 

life,  326 

mixer  for,  337-338 

properties,  333-337 

sodium  fluoride  in,  336-337, 

343-344 

strength      and      water-resis- 
tance, 339-340 
wet-mix,  333-337 
Caseinogen,  36,  38,  123 
Castor  oil,  609 
Cellulose,  49 

filters,  288 

Centigrade-Fahrenheit  table,  614 
Charcoal  filters,  288 
Chitin,  49 
Chitinoids,  20 
Chloride,  447-448 
Chlorine  precipitation,  472 
Chocolate,  480 
Chondrigin,  73-74 
Chondrin,  73-74,  68-80 
Chondroalbuminoid,  86 
Chondroitic  acid,  69-70,  73,  86 
Chondroitin,  70 
Chondromucoid,  70,  73,  86 
Chondroprotein,  70-71 
Chondrosin,  70 
Chum,  359 
Coagulation,  239 
Coefficient  of  diffusion,  92 

of  penetration,  170 
Coffee,  481 
Collagen,  49-52 

classification,  20 

derivation,  1 

elementary  composition,  50 

formula  for,  49 

from  cartilage,  86 

from  connective  tissue,  85 

from  fish  scales,  77,  78,  88 

from  ossein,  87 

from  skin,  82 

hydrolysis,  51 

preparation,  51 

properties,  51 

reaction  with  enzymes,  51-52 

relation  to  chondrigin,  73 


Collodion  membranes,  97 
Colloid,  classification,  161-163 

definition,  160 

derivation,  1,  17 
Colloidal  electrolytes,  266 
Color  comparator,  606 
Color  reactions,  42-47 
Colorimetric  determination,  599-606 
Commercial  gelatin,  571-576 
Composition  cork,  546 
Conductivity,  249,  257,  289,  313 
Cone  mill,  278 
Coney  stock,  272 
Conjugated  proteins,  69 
Connective  tissue,  82-86 
Copper,  459-460 
Coriin,  84 
Corium,  79,  82,  84 
Country  bone,  273,  275 
Court  plaster,  571    . 
Crazed  glue,  422-424 
Cream,  477-478 
Crystaline  liquide,  138 
Cystine,  22,  35-38,  40,  41,  65-67 


Degree  of  dispersion,  162 
Cental  caries,  72 
Depilation,  280,  313,  314 
Desiccation  of  hide  stock,  277 
Detection  of  gelatin  or  glue,  470-486 
in  meat  and  meat  products, 

470-477 
in    milk,    cream    and    ice 

cream,  477-478 
in    other    food    products, 

478-483 
in  size,  483-486 
Deuteroalbuminose,  123 
Deuteroelastose,  68 
Dextrin,  123 
Dialysis,  95-97 
Diffusion,  91-95 
thimbles,  96 
Dineric  interface,  214 
Disinfection  of  hides,  313,  314 
Dispersed  phase,  162 
Dispersion  medium,  162 


628 


SUBJECT  INDEX 


Distinction    between    gelatin-  and 

glue,  62 
Donnan  equilibrium,  128-132,  158- 

159,  168,  184-185 
Doublet,  135 
Drying  alleys,  294-300 


E 


Edestine,  38,  43,  221 

Edible  gelatin,  556-571 

Egg  albumin,  38,  92,  106,  115,  123, 

133,  227,  288 
globulin,  123,  134 
membrane,  64,  65 

Elasticity,  411-413 

Elastin,  36,  67-69,  78-80,  85 

Electro-metallurgic  process,  550 

Electrometric    determination,    587- 
599 

Emery  paper,  545 

Emulsification  test,  482 

Emulsifying    agent,    212-218,    567- 

569 

\Emulsin,  92 
-"Emulsions,  212-218 

Emulsoid,  163,  203 

Equivalent  weight,  108 

Erepsin,  24  X 

Estimation  of  gelatin,  463-470 

alcoholic  precipitation,  463-464 
indirect  methods,  467-468 
picric  acid  precipitation,   466- 

467 

salt  precipitation,  468-470 
tannin  precipitation,  464-466 

Evaluation,  487-505 

Evaporation,  4,  290-292,  315 

Eye,  76 


Fahrenheit-Centigrade  table,  614 
Fat,  15,  455-456 

extractor,  301 
Feathers,  64-65 
Feeding  stuffs,  481 
Ferric  oxide,  440-441 
Fibrin,  182-183 


Fibroids,  20,  36,  43 

Fining,  355,  569 

Fischer's  esterification  method,  35, 

449-450 

Fish  gelatin,  349 
glue,  356-366 

bibliography,  365-366 
comparison  with  other  glues, 

362-363 

composition,  361-364,  435 
constitution,  28 
hydroscopicity,  360 
manufacture,  358-359 
preparation,  4 
production,  6-7 
properties,  364-365 
raw  materials,  357-358 
tests  for  quality,  359-361 
uses,  365 
Van  Slyke  determination,  43, 

46 

Fixed  acids,  451 
Flake  glue,  298 
Fleshings,  272,  274 
Fluorides,  336-337,  343-344 
Foam,  416-418,  501 
Foamite,  554 
Foils,  571 
Formaldehyde,    action    on    gelatin, 

57-58,  570 

on  glue,  190,  192,  198,  199 
determination,  454 
in  water-resistant  glues,  318 
precipitation  by,  473 
titration,  30 
Formo-gelatin,  570 
Free  mineral  acids,  451 
organic  acids,  451 
Freezing  point,  106-107 
Fibrinogen,  229 
Frosted  glass,  538 
Fruit  products,  478 
Fur,  64 


G 


Gamboge,  201 

Gay  ley  dry-blast  process,  298,  316 

Gelatin,  action  of  enzymes  on,  59 


SUBJECT  INDEX 


629 


Gelatin,  action  of  halogens  on,  56-57 

of  oxidizing  agents  on,  59 
alcohol  number  of,  154 
amino-acid  composition  of,  36 
analysis,  425-463 
as  a  culture  medium,  474-475 
as  a  food,  556-558 
as  a  glaze  on  coffee,  481 
as  an  emulsifying  agent,  567- 

569 

ash-free,  102 
bacteria  in,  566 
bibliography    on    manufacture, 

313-317 

boiling  point,  106-107 
bromine  precipitation,  473 
brownian  movement,  109 
by-products,  307-313 
cells  for  ultrafiltration,  575-576 
change  in  viscosity  with  time, 

149-151 

colloidal  phenomena,  576-578 
color  reactions,  60 
combination  with  acids,  256 
combining  capacity,  110-112 
commercial,  571-576 
constitution,  28-29,  58-60 
crystals,  143 
decomposition  by  bacteria,  60- 

61 

derivation,  1 
detection,  470-486 
dialysis,  95-97 
diffusion,  91 

distinction  from  glue,  62,  502 
edible,  4,  556-571 
elementary  composition,  50,  111 
emulsification  test,  482 
estimation,  463-470 

in  papers,  56 
exports,  5-8,  11-12 
foam,  153 
foils,  571 

formulas,  49,  111,  112 
freezing  point,  106-107 
gelation,  164-186 
gold  number,  121-123 
Hausmann  numbers,  38 
hydration,  49 


Gelatin,  hydrolysis,  25 
imports,  5-8,  10-11 
in  analytical  procedures,  576 
in  chocolate,  480 
in  feeding  stuffs,  481 
in  fining,  569 
in  food  products,  567 
in  fruits  and  fruit  products,  478 
in  glues,  465 
in  gums,  480 

in  ice  cream,  561-567  -^ 

in    meat    and    meat    products, 

470-477 
in  milk,  cream  and  ice  cream, 

477-378 
in  paper,  486 
in  pharmaceutical  preparations, 

569-571 

in     photography     and     photo- 
lithography, 571-574 
index  of  refraction,  119-121 
inorganic  composition,  435 
jelly  strength,  154 
manufacture,  306-307 
melting  point,  54 
molecular  weight,  107-114 
mutarotation,  156-157 
nitrogen  in  free  amino  groups, 

221 

optical  rotation,  116-119 
osmosis,  97-104 
osmotic  pressure,  98-99 
physico-chemical        properties, 

91-132 

preparation,  61 
production,  5-9 
properties,  52-55,  78-80 
protective  action,  123,  558-567 
purification,  61 
raw  materials,  7 
reactions,  56 

Rontgen  photograph,  144 
size  of  ultramicrons,  125 
sol-gel   equilibrium,    148,    151- 

152,  207,  211-212 
solubility,  55 

of  salts  in,  54 
solution,  164-186 
specific  rotation,  116-117 


630 


SUBJECT  INDEX 


Gelatin,  structure,  132-159 

sulphur  dioxide  in,  453 

surface  tension,  114-116 

swelling,  153,  164-186 

testing,  369-424 

turbidity,  154 

Tyndall  effect,  123-128 

ultramicroscopy,  123-128 

Van  Slyke  determination,  43-46 

vapor  pressure,  104-105 

veneers,  550 

/       viscosity,  153,  186-212 
^Gelatinizing  agents,  478-479 
/(  Gibbs'  theorem,  115 
V-      Gliadin,  36,  38,  43,  221 
Glucosamine,  70 
Glue,  adhesive,  517-538 

analysis,  425-463 

application,  536-538 

bacterial    decomposition,    515- 
517 

bibliography    on    manufacture, 
313-317 

binding  agent,  545-547 

by-products,  307-313 

colloidal  gel,  547-550 

comparison   of   different  glues, 
362-363 

constitution,  28 

course  through  plant,  312 

derivation,  1 

detection,  470-486 

distinction  from  gelatin,  62,  502 

effect  of  pressure,  523 

estimation,  463-470 

exports,  5-8,  11-12 

extraction  apparatus,  286,  314 

gelatin  content,  465 

gold  number,  123 

handling,  506-517 

imports,  5-8,  10-11 

in  oils,  486 

inorganic  composition,  435 

losses  from   overheating,    510- 
515 

manufacture,  271-303 

prices,  7,  12 

production,  5,  7-9 

protective  action,  550 


Glue,  raw  materials,  7,  271-277 
selection  for  service,  534-536 
sizing  agent,  538-545 
testing,  369-424 
uses  and  applications,  506-554 
Van  Slyke  composition,  43 
determination,  43-46 

Glue  room  economy  and  technology, 
506-510 

Glutamic  acid,   22,   35,   36,   39,   65 

Glutem,  1 

Glutenin,  38,  43 

Glycine,  21,  35,  36,  65 

Glycoproteins,  69,  123 

Glycuronic  acid,  70 

Gold  number,  121-123 

Grade  designation,  502-504 

Grain  curd  casein,  322-324 

Grease,  308-309,  418-419,  455-456, 
501,  541 

Green  bone,  273,  303 

Grillo-Schroeder  process,  303,  317 

Gum  arabic,  165 

Gum  tragacanth,  164 

Gums,  480 


H 


Hair,  amino-acid  content,  65 
cystine  content,  64 
elementary  composition,  64 
melanin  content,  76 
relation  to  skin,  82-83,  279 
test  for,  484 

Van  Slyke  determination,  43 
Hair  tankage,  307-308 
Half-round  mill,  279 
Handling  of  glue,  506-517 
Hausmann  numbers,  37-38,  449 
Hectograph  plates,  548 
Hemocyanin,  43,  421 
Hemoglobin,  43,  221 
Hide  glue,  bleaching,  290,  314,  315 
boiling,  282-287,  314 
chilling  and  spreading,  292- 

294 
clarification     and     filtration, 

287-290,  314 
colored,  291 


SUBJECT  INDEX 


631 


Hide  glue,  drying,  294-300 

evaporation,  290-292,  315 
liming,  279-281,  313 
preparation  of  stock,  277-278 
raw  materials,  271-277 
soaking    and   washing,    278- 

279,  313 
special,  291 

washing  and  deliming,  281- 
282,  314 

Histidine,  22,  36,  39,  40,  42,  65 

Historical  considerations,  1-5 

Hollander,  278 

Hoof,  64,  82,  83,  279,  301 

Hooke's  law,  178 

Horn,  64,  65,  82,  83,  301 

Hornpiths,  275 

Humagsolan,  67 

Humin,  38,  39,  41,  74-76 

Hydration,  127,  152,  157,  159,  183, 
238 

Hydrochloric  acid,  616 

Hydrogen  electrode,  583-591 

ion    concentration,     450,     500, 
576-606 

I 

Ice  cream,  477-478,  561-567 
Ichthylepidin,  77-80,  88 
Imitation  rubber,  547 
India  ink,  554 
Indicators,  599-606 
Infant  dietary,  558-561 
Invalid  dietary,  558-561 
Invertin,  92 

lonization  constants,  606-608 
lonization  equilibria,  579-583 
Ion  series,  243-245 
Isinglass,  349-356 

composition,    properties,    uses, 
354-356 

constitution,  28 

elementary  composition,  50 

forms,  349-353 

gold  number,  123 

manufacture,  350-354 

preparation,  4,  90 

surface  tension,  115 

Van  Slyke  determination,  43,  46 


Isinglass  gelatin,  349 
Isoelectric  gelatin,  254,  255 

point,  175,  246-247 
Isoleucine,  22,  36 


Jelly  strength,  367-380 

effect  of  added  substances  on,    yf 
402  Xs 

significance     in     evaluation, 

500-501 

tests  for,  285,  370-380 
variation  with  pH,  265 
Joint  work,  534-536 
Junk  bone,  273,  275 

K 

Keratin,  20,  36,  63-67,  78-80,  83,  88 
Kind-Landesmann     machine,     293, 

315 

Kinematic  viscosity,  384 
Kip  stock,  272,  274 
Koilin,  65 


Lace  leather,  273,  274 
Laminaria,  165 
Law  of  mass  action,  220 
Leach  liquors,  473-440 
Lead,  462-463 
oxide,  444 
Leather,  2,  51 

Leather-metal  adhesive,  537 
Legumin,  36 
Leucine,  21,  35,  36,  65 
Liesegang  rings,  93-95 
Ligamentum  nuchse,  67,  68,  84,  85 
Lime,  277 
Liquid  crystals,  138 
Logarithms,  623-624 
Log  mill,  278 
Loom  pickers,  273 
Losses  due  to  overheating,  519-515 
Lysine,  22,  36,  39,  40,  42,  65 

M 

Magnesium  oxide,  442-443 
Mastic,  201 
Matches,  545 


632 


SUBJECT  INDEX 


Meat,  470-477 

extracts,  470-477 
Melanin,  39,  41,  74-76,  78-80 
Melting  point,  399-411 

definition,  118 

determination,  207-208 

effect  of  hydrolysis  on,  401- 
404 

evaluation  by,  496 

methods        of        measuring, 
404-411 

of  gelatin,  54 

scientific      basis      of      test, 

400-404 

Menhaden,  78,  88 
Metals  in  gelatin,  456-463 
Metric-American  equivalents,  615 
Micelle  theory,  264-268 
Milk,  477-478,  559 
Millon's  reaction,  46 
Molecular  weight,  107-114 
Molisch's  reaction,  47 
Monoamino  nitrogen,  38,  40 
Moulding  material,  546 
Mucins,  69-73,  78-80,  280,  288 
Mucoids,  69-73,  85 
Mutarotation,    156-157,    209,    400- 
401,  413-415 


X 


Nails,  64 
Neurokeratin,  64 
Night-blue,  192 
Nitric  acid,  616-617 
Nitrogen,  determination,  431-433 
distribution  as  ammo-acids,  36 
as    Hausmann    numbers,    38 
as  protein,  proteose,  peptone, 

and  amino-acids,  25-29 
by    method    of    Van    Slyke, 

38-45 

in  gelatin,  433 
in  meat  extracts,  476 
total  in  proteins,  17 
Nitrous    acid    method    for    amino- 
acids,  30 

Nonamino  nitrogen,  38,  40,  42 
Noodle  glue,  298 


0 


Occlusion  theory,  131,  157-159 
Optical  rotation,   116-119,  413-415 
Organic    analysis    of    gelatin,    Fis- 
cher's esterification 
method,  35,  449-450 
Hausmann    numbers,    37- 

38,  449 

protein,  proteose,  peptone, 
and  amino  acid,   25-28, 
448-449 
Van  Slyke  analysis,  38-46, 

449 

Ornithine,  39 
Osmosis,  97-104 
Osmotic     pressure,     determination, 

103-104 
Donnan       equilibrium       on, 

130-131 

membranes  for,  97 
molecular  weight  by,  108-110 
of    gelatin    salts,     99,     249, 

256-257 

swelling  phenomena,  171-173 
Ossein,  composition,  87 
content  in  bone,  88 
manufacture,  303-306,  317 
mucoid  from,  70 
relation  to  cartilage,  86 
Osseoalbuminoid,  86,  87 
Osseomucoid,  70,  87 
Ovomucoid,  92,  123 
Oxyproline,  23,  35,  36,  40 


Packer  bone,  273 
Paints,  544-545 
Panel  work,  534-536 
Paper,  56,  485-486,  538-542 

boxes,  536 

Parmelee  catchall,  291,  292 
Pepsin,  16,  24,  51-52 
Peptids,  23,  24 

Peptone,  21,  23,  25-28,  127,  448-449 
Peter  Cooper  grades,  369 
Pharmaceutical   preparations,    569- 

571 
Phenylalanine,  22,  35,  36,  65 


SUBJECT  INDEX 


633 


Phosphate  by-product,  309-313,  317 

Phosphoric  acid,  434-440 

Photography,  571-574 

Photolithography,  571-574 

Pickle  baths,  554 
bone,  273 

Picric   acid  precipitation,   466-467, 
477 

Pills,  570 

Plasticity,  207-212 

Poise,  384 

Potassium  hydroxide,  619 
oxide,  444-446 

Potential  difference,  129-131 

Potentiometer,  588,  592 

Preservatives,  277,  281,  291 

Printing  rollers,  547 

Proline,  22,  36,  40,  65 

Protective    action,     123,     127-128, 
550-554,  558-567 

Protein  error,  605 

Proteins,  amino-acid  composition,  36 
amphoteric  character,  219,  236 
classification,  18-19 
cleavage  products,  20-21 
color  reactions,  42-47 
combining  capacity,  229,  232 
description,  15 

determination,  25-28,  448-449 
effect  of  inorganic  ionogens,  236, 

268 

electrolysis,  227-229 
electrophoresis,  227-229 
elementary  composition,  17 
Hausmann  numbers,  38 
hydrolysis,  16,  24 
ionization,  225-227 
ion  series,  243-245 
isoelectric  point,  246-247 
precipitation    and    coagulation, 

236-239 
types,  16 
Van  Slyke  determination,  38-45 

Protein  salts,  acid  and  metal  salts, 

248-253 

depressing     action,     260-264 
equilibrium,  245-253 
formulas,  223-226,  232,  232- 
235,  240-243 


Protein   salts,    hydrolytic   dissocia- 
tion, 232-234 

influence  of  valency,  253-260 
mechanism      of      formation, 

223-225 
theories   of  ionization,    225- 

227,  234-235,  239-243 
Proteoelastose,  68  t^v\f -m$( 

Proteose,  20,  23,  25-28,  240  24£T 
Proximate  analysis,  429-433 
Psychrometric  tables,  294,  295,  296 
Pulp  color,  539 


R 


Refined  isinglass,  349 
Refractive  index,  119-121 
Refrigeration,  315 
Reynold's  criterion,  383 
Roese-Gotlieb      method     for     fat, 

329-330 
Roller  mill,  278 
Rosin,  484 
Rubber,  130,  538,  552-554 


8 


Saliva,  70,  72 
Salivary  calculi,  72 
Salmine,  38 
Salt  effect,  605 

precipitation,    25-28,    468-470, 

475-477 
Salts,  182-186 
Sand  paper,  545 
Sclero-proteins,  19-20 
Scyllium  stellare,  65 
Semplar,  135 
Sepia,  76 
Sericin,  36 

Serine,  22,  35,  36,  65 
Serum  albumin,  36 

globulin,  36 
Setting  point,  118,  403 

rate,  416 
Sheet  glue,  298 
Silica,  434 
Silicic  acid,  133 
Size,  538-545 


634 


SUBJECT  INDEX 


in,  1-2,  79-84,  88-90,  279 
Skivings,  272 
Snail,  70,  71 

Soap,  115,  138-141,  215-216 
Sodium  hydroxide,  619 

oxide,  444-446 
Solubility,  54-55 
of  bases,  42 
product,  220 

Solution,  164,  169-171,  186 
Solvation,  173,  193-194 

potential,  205 

Solvents  for  degreasing,  301-302 
S^rensen  value,  pH,  581 

of  isoelectric  point,  175 
of  pure  gelatin,  62 
significance,  579-606 
Sounds,  90 
Specific    gravity    tables,     611-613, 

616-620 

rotation,  116,  117 
Splits,  272,  274 
Spray  drying,  299-300,  316 
Starch,  123,  129,  165 
Statistical  considerations,  5-12 
Steamed  bone,  273 
Sterilization  of  hide  stock,  277 
Stokes'  law,  201 
Strip  glue,  298 
Structure,  124,  132-159 
Submaxillary,  70,  71 
Sulphate,  447 
Sulphur,  201 

dioxide,  absorption  of  bones,  303 
as  bleaching  agent,  4,  290 
determination,  452-454 
for  leaching  bones,  306;  317 
for  neutralizing  lime,  282 
in  acid  treatment,  276 
reaction,  47 

Sulphuric  acid,  616-618 
Surface  tension,  114-116,  205 
Suspensoid,  163,  201 
Swelling,  164-186 

capacity,  415-416 

chemical   phenomena    in,    173- 

182 

effect   of   electrolytes   on,    153, 
182-186' 


Swelling,  effect  of  gelatin  salts,  249- 
250,  257-258,  265 

of  hide  stock,  282 

osmotic  phenomena  in,  171-173 

physical  equilibria,  164-171 

theories,  176-186 
Syneresis,  172 


Tannic  acid,  51 

Tannin  precipitation,  464-466,  474- 
475,  476 

Tendomucoid,  70,  86 

Tendon,  70,  85 

Tensile  strength,  403,  411-413 

Testing  of  glue  and  gelatin,  367-424 
adhesive  strength,  411,  526-534 
appearance,  etc.,  420-421 
foam,  416-418 
grease,  418-419 
inspection  test,  421-422 
jelly  strength,  369-380 
melting  point,  399-411,  496 
optical  rotation,  413-415 
rate  of  setting,  416 
reaction,  419-420,  500 
swelling  capacity,  415-416 
tensile  strength,  411-413 
viscosity,  380-399 

Textiles,  542-544 

Theory  of  indicators,  599-606 

Thermostat,  590 

Thickeners,  478-479 

Tin,  461-462 

Tooth  preservation,  72-73 

Tortoise  shell,  64 

Total  acidity,  450 
alkalinity,  450 

Trypsin,  16,  24,  51-52 

Tryptophane,  23,  36,  39,  40,  65 

Tumbler,  278 

Twaddell  degrees,  611-613 

Tyndall  effect,  54,  123-128 

Tyrosine,  22,  35,  36,  65 

U 
Ultrafiltration,  575-576 


SUBJECT  INDEX 


635 


Ultramicroscope,  126,  127 
Ultramicroscopy,  123-128 
Urea,  39 


Vacuum  dryers,  299 

evaporators,  290 
Valine,  21,  36,  65 
Van  Slyke  analysis,  38-46,  449 
van't  Hoff's  law,  101,  105 
Vapor  pressure,  104-105 
Vegetable  glue,  362-363 
Viscosimeter,  B  auntie"  and  Vigneron, 
390-391 

Bingham,  389-391 

capillary  tube  type,  384-392 

Cope,  391-392 

Couette,  395-396 

Engler,  384-387 

Kahrs,  387-388 

MacMichael,  148,  189,  397-399, 
496-499,  608-610 

oscillating    cylinder    and    disk 
type,  394-395 

Ostwald,  193,  389-391 

Redwood,  384-387 

Rideal-Slotte,  390-391 

rising  bubble  and  falling  sphere 
types,  392-394 

Saybolt,  384-387 

Stormer,  396-397 

torsional  type,  394-399 
Viscosity,  186-212,  380-399 

change  on  standing,  149-151 

conditions  affecting,  187-200 

effect  of  electrolytes  on,  153 

evaluation  by,  496-497 


Viscosity,  instruments  for  measuring, 

384-399 
MacMichael     conversion,    608, 

610 

of  castor  oil,  609 
of  gelatin  solutions,   138,  259, 

262-265 

plasticity  relationship,  207-212 
relation  to  melting  point,  402 
tests  for,  285 

theories  of,  200-207,  381-384 
Volatile  acids,  451 
Volume  comparison  table,  614 

W 

Water,  275-276,  314 

Water-resistant  glues,  318-348 
animal  glue,  537-538 
blood  albumin  glue,  344-348 
casein  glue,  319-344 

Whalebone,  64 

White  glue,  291 

Wool,  64-65,  484 

X 

Xanthoproteic  reaction,  47 

Y 
Yield  value,  210 

Z 

Zein,  36,  221 
Zinc,  460-461 
oxide,  443 


AUTHOR  INDEX 


Abderhalden,  23,  24,  36,  64,  65,  78 

Abel,  75,  76 

Abrahms,  341 

Abt,  313 

Acree,  585 

Alexander,  20,  49,  193,  314,  373-374, 

388,  488-489,  491,  558,  559 
Alexandrow,  106 
Allen,  57,  472 
Almy,  365 
Anders,  36 
Anderson,  315 
Arkwright,  145 
Arney,  4 
Arnheim,  59 
Arnold,  392 
Arrhenius,  178 
Arrowood,  315 


B 


Bachmann,  127,  141,  142 

Bacon,  375 

Badische,  A.,  314 

Badische,  S.,  314 

Bahntje,  551 

Baker,  322 

Baldwin,  178 

Bancelin,  201 

Bancroft,  115,  116,  121,  127,  141- 

142,    163,    214-215,    218,    314, 

483,  552 
Barcroft,  98 
Barraclough,  484 
Bauer,  64 
Baume,  391 
Baxter,  315 
Bayliss,  102,  109 
Beans,  580 
Beatty,  36 
Bechhold,  54,  95,  134,  206 


Bechmann,  473 

Behrend,  343 

Bellamy,  341 

Benedict,  41,  605 

Benzinger,  182 

Bergel,  341 

Bernstein,  342 

Berrar,  113,  466 

Bert,  315 

Berthelot,  58 

Betts,  551,  552 

Bevan,  56,  485 

von  Biehler,  36 

Biltz,  102,  114,  192,  197 

Bingham,  209,  383,  385,  391 

Birchard,  221 

Blasel,  62,  223,  226 

Blish,  39,  43,  75,  449 

Bogue,  25,  27,  43-46,  57,  120,  127, 
145-159,  178,  184,  187-189, 
192,  194,  197,  200,  204-206,  210, 
217,  239,  264,  401,  410,  423, 
428,  429,  432,  433,  435,  449, 
469,  493,  495,  511,  523,  577 

Booth,  314 

Bordet,  145 

Born,  315 

Botazzi,  204 

Bourcart,  569 

Bourgeois,  74,  113,  433 

Brabrook,  343,  344 

Bracket,  313 

Braconnot,  59 

Bradford,  94,  143 

Bredig,  220 

Bremer,  341 

Briggs,  56,  214,  485 

Brillouin,  381 

Brito,  228 

Brooke,  57 

Brooks,  342 

Broquette,  340 

Browne,  328,  338,  346 


637 


638 


AUTHOR  INDEX 


Buchner,  143,  167 

Buchtala,  64 

Buerger,  85,  86 

Bugarsky,  226 

Bugge,  114 

Buglia,  192 

Bullowa,  558 

Bunting,  72 

Bunzel,  316 

Burmeister,  328 

Burnett,  107 

Burr,  328 

Burton,  126,  176 

Butschli,  132 

Butterman,  320,  325,  326,  335,  343 


Cambon,  317 

Cammen,  315 

Campbell,  316 

Carles,  464 

Carpenter,  J.,  396 

Carpenter,  W.,  341 

Carrier,  316 

Carthans,  315 

Cavalier,  315 

Cerri,  491 

Chercheffski,  207,  404 

Chick,  204,  332 

Child,  217 

Childs,  340 

Ching,  313 

Chittenden  ,  50,  64,  68,  433 

Clapp,  36 

Clark,  A.  W.,  407 

Clark,  E.  D.,  365 

Clark,  F.  C.,  485 

Clark,  W.  M.,  248,  320,  322,  323, 

327,   328,   500,   583,   584,   587, 

589,  590,  599-605 
Clayton,  491 
Clowes,  215 
Coffman,  152,  182 
Cohn,  77 

Congdon,  478-479 
Cope,  392 
Cormack,  286,  314 
Couette,  382,  395 


Coulomb,  381 
Cross,  56,  485 
Cutter,  70 


Dahlberg,  321,  322,  327 

Dakin,  34-37,  59,  112,  114 

Daubine,  316 

David,  569 

Davidson,  314 

Davis,  75,  76,  149 

Dawidowski,   48,   74,   83,   435,   548 

DeBary,  116 

DeBruyn,  123 

Deeley,  383 

DelQuercio,  569 

Dennis,  41 

DesCondres,  215 

Devos,  315 

Dietrich,  365 

Donnan,  128,  168,  184 

Doolittle,  394-395 

Dorner,  317 

Dorpinghaus,  36 

Dreaper,  66 

DuBois,  407 

Duclaux,  101,  110 

Duerling,  316 

Dumanski,  206 

Dunham,  A.,  342,  343 

Dunham,  H.,  317,  342.  344 

Dutton,  314 


E 


Ebstein,  65 

Eckweiler,  452 

Eder,  572 

Ehrenberg,  177 

Einstein,  93,  200 

Ekman,  366 

Ellenberger,  313 

Elliott,  55,  122,  127,  590-591,  605 

Ellis,  587 

Emery,  475 

Emmett,  50 

Erlwein,  316 


AUTHOR  INDEX 


639 


d'Errico,  204 


Fahrion,  455,  467 

Fales,  178 

Falk,  452 

Fargas-Oliva,  342 

Faust,  50,  392 

Fels,  372,  387,  491 

Ferguson,  620 

Fernbach,  318,  370,  388,  419,  487, 

488 

Ferney,  343 
Fick,  92 
Field,  62,  102 
Finsterwalder,  316 
Fischel,  314 
Fischer,  E.,  317 
Fischer,  Emil.,  18,  35,  36 
Fischer,  M.,  92,  98,  120,  144,  152, 

155,  156,  159,  161,  162,  163,  172, 

182-184,    191,    194,    208,    214, 

216-217,  250,  568,  577 
Fischer,  R.,  393 
Flade,  141 
Fletcher,  343,  344 
Flowers,  381,  383,  393-394 
Fohn,  72 
Forrester,  316 
Framm,  116 
Fransden,  562 
Frankenheim,  132 
Fremy,  50 

Freundlich,  95,  138-139,  163 
Friedenthal,  106 
Frost,  512 
Fuchs,  67 
Funk,  36 


G 


Gamble,  366 

Gantlier,  317 

Gantter,  464 

Garner,  128,  184 

Garrett,  133-134,  135,  146,  196,  394 

Garry,  418 

Gayley,  316 


Gibbs,  115 

Gibson,  393 

Gies,  50,  67,  70,  85,  86 

Gill,  411,  523,  526,  527-529,  534 

Girsenwald,  317 

Gokun,  197 

Goldsmith,  341 

Gomberg,  229 

Gortner,  39,  75 

Govers,  342 

Graham,  17,  92,  95,  160,  172,  206 

Grant,  569 

Grasser,  313 

Green,  E.,  77,  78,  88,  366 

Green,  H.,  209 

Greifenhagen,  468 

Grete,  576 

Grillo,  317 

Grindley,  315 

Grosvenor,  316 

Groth,  366 

Gruendler,  341 

Grune,  340 

Gudeman,  453 

Guldberg,  220 

Guthrie,  105 


Hackford,  417,  468 

Hall,  341,  342 

Halla,  433,  465 

Hammerstein,  70,  72 

Handovsky,  241 

Hardy,  133,  142,  227,  236 

Harmed,  178 

Harris,  37,  128,  184 

Harrison,  201 

Hart,  68,  316,  456 

Hatschek,    94,    95,    122,    164,    165, 

200-206,  395,  575,  577 
Hauck,  314 
Hauseman,  534 
Haussner,  313 
Hawk,  70,  86 
Hayes,  383,  398,  435 
Haynes,  341 
Hedenberg,  568 
Heicke,  273-275 


640 


AUTHOR  INDEX 


Heim,  343 

Heinemann,  532 

Henley,  475 

Henri,  163 

Henzold,  478 

Herold,  297,  404,  406 

Herschel,  383,  398 

Herschfeld,  223,  229,  230 

Herter,  560 

Herz,  484 

Herzberg,  484 

Herzog,  92 

Hewitt,  341 

Heyl,  36 

Higgins,  397 

Hildebrand,  314 

Hill,  98 

Hirsch,  340 

Hoagland,  592-598 

Hoffman,  15 

Hofman,  215 

Hofmann,  65 

Hofmeister,  49,  50,  60,  100,  105,  166, 

199,  243-245 
Hohenstein,  341 
Holborn;  227 
Holmes,  95,  194,  217 
Hooker,  182,  183,  216,  568 
Hopfner,  328 
Hopke,  456 
Hopkins,  57 
Hopp,  412 
Hoppe-Seyler,  76 
Horbaczewski,  68 
Home,  453 
Howell,  229 
Hiifner,  206 
Hulbert,  376 
Hurwitz,  606 
Husbrand,  315 


Isaacs,  341,  342,  343,  344 
Ives,  422 


Jacobi,  559 


Jacobs,  393 

Jean,  464 

Jenkins,  317 

Jones,  H.,  147,  155,  173,  237 

Jones,  P.,  568 

Jovignot,  343,  344 

Just,  341,  342 

K 

Kahlenberg,  96 

Kahrs,  387-388,  489-490,  519,  527- 

528 

Kauffman,  557 
Keane,  533 
Kelvin,  487 
von  Kerkhoff,  64 
Kern,  551 
Kerr,  315 
Kimberlin,  342 
Kind,  315 
King,  125,  127 
Kirchman,  556 
Kissling,  207,  372,  405 
Kister,  316 
Klug,  60 
Knudsen,  366 
Kochetkov,  317 
Kohlrausch,  159,  227 
Konig,  468 
Konovalov,  124 
Koplik,  560 
Korner,  64,  169 
Kossel,  17,  234 
Krafft,  106,  107 
Krawkow,  76 
Krombholz,  560 
Kriiger,  116 
Krukenberg,  74 
Krummacher,  556 
Kuhne,  64 
Kutscher,  472 
Kiittner,  207,  406 


Ladenberg,  392 
Laing,  137,  148 
Lambert,  317 


AUTHOR  INDEX 


641 


Landwehr,  71,  74 

La  Wall,  454 

Leavenworth,  223 

LeChattelier,  5? 

LeConnt,  65 

Leffman,  454 

Lehmann,  286,  314 

Levene,  36,  59,  69 

Levi,  483 

Levites,  197 

Lewis,  171,  200,  201,  383,  398 

Lewis,  W.,  316 

Liebermann,  226 

Liesegang,  93 

Lillie,  97,  98, 100,  101,  102,  260 

Lindauer,  343 

Lindemuth,  366 

Linder,  512 

Lipowitz,  371 

Lloyd,  129,  143,  144-145,  167-168 
231-232 

Lodeman,  569 

Loeb,  55,  "62,  91,  97,  98,  100,  102, 
110,  114,  128.  129-132,  137,  147, 
149,  157-159,  174,  175,  177,  182 
184-186,  187,  195,  199,  219,  241, 
245,  247-253,  254,  255,  256,  257, 
258,  259,  260,  268,  500,  573,  577 

Loomis,  585 

Low,  A.,  317 

Low,  W.,  377 

Lowenstein,  315 

Lubs,  323,  599-605 

Lucke,  315 

Liideking,  53,  104,  107,  164 

Lumiere,  57,  572 

Lunge,  533,  619 

Luppo-Cramer,  577 

M 

McBain,  137-141,  147,  148,  264-268 

Macintire,  315 

McLaurin,  341 

McMeekan,  341 

MacMichael,  397 

Majert,  341 

Markham,  562 

Marlow,  316 

Marr,  316 


Martici,  191 

Martin,  125,  204,  332 

Mathews,  17,  81,  87 

Matula,  62,  223,  226 

Mauerhofer,  286,  314 

Mavrojannis,  468 

Maxwell,  403 

Mehler,  114 

Meinford,  315 

Menz,  122,  127 

Merkle,  431 

Merrell,  315 

Merrill,  I.,  316 

Merrill,  O.,  316 

Meyer,  606 

Michaelis,  175,  195,  247,  314,  580 

582 

Millikan,  105 
Miyake,  170 
Mohler,  313 
Moore,  H.,  315 
Moore,  R.,  102,  313,  560 
Morawska,  316 
Morner,  50,  57,  64,  70,  73,  74,  77 

86,  88,  366 
Morochowetz,  73 
Mulder,  64,  433 
Muller,  A.,  70,  463,  465 
Miiller,  E.,  551 
Miirlin,  556-557 
Mutscheller,  314 

N 

von  Nageli,  132 

Neil,  314 

Nelson,  178 

Nencki,  76 

Nernst,  97,  129,  166,  171,  583 

Neuberg,  76 

Neumann,  163 

Newman,  213 

Newton,  425 

Northrup,  25 

Nowak,  342 

Noyes,  A.,  163,  169 

Noyes,  H.,  452 

O 
Oakes,  149,  379,  580 


642 


AUTHOR  INDEX 


Obernethy,  302 

Oden,  201 

Oesterle,  316 

Okuda,  322,  366 

Onfrey,  480 

Oryng,  226 

Osborne,  35,  36,  37,  223 

Ostenberg,  606 

Ostwald,  Walther,  213 

Ostwald,  Wilhelm,  92,  94,  390 

Ostwald,  Wolfgang,  92,  93,  98,  108, 
109,  110,  114,  135,  159,  160,  161, 
162,  163,  172,  174,  187,  191,  194, 
577 

Ota,  173,  237 

Oyamo,  366 


Paal,  110,  111,  113,  433 

Fade,  453 

Paessler,  84 

Parker,  366 

Parr,  383 

Patrick,  477 

Pauli,  54,   100,  157,  159,  166,  174, 

194,  195,  199,  223,  226,  228,  229, 

230,  241,  244,  260,  577 
Peck,  340 
Perrin,  105,  163 
Phelps,  314 

Pickering,  214,  215,  315 
Pickles,  313 
Pitman,  397 
Plateau,  214 
Plimmer,  21,  51 
Poetschke,  454 
Poiseuille,  381 
Poma,  178 
von  Portheim,  340 
Posnjak,  165 
Post,  533 
Posternak,  244 
Potter,  354 
Pregl,  64,  65 
Procter,  52,  110,  111,  114,  128,  129, 

136,  147,  172,  176-182,  185,  258, 

314 
Prollius,  354,  366 


Q 


Quincke,  53,  114,  133,  134,  141,  164, 
173,  214 

R 

Ramsden,  115 

Rankin,  391 

Rauppach,  341 

Rayleigh,  125,  126 

Reid,  98,  101 

Reimer,  84 

Reinke,  165 

Reiss,  119 

Renken,  342 

Reuter,  327,  343 

Reynolds,  382 

Ricevuto,  55 

Richards,  50,  67 

Richardson,  471 

Richmond,  331 

Rideal,  56,  60-61,  391,  472 

Rigg,  396 

Roaf,  98,  102 

Robertson,  101,  107,  108,  116,  119, 
121,  123,  136-137,  146,  170,  173, 
177,  206-207,  213,  223,  226,  229, 
230,  233,  239 

Roehm,  313 

Rogers,  396 

Rollet,  84 

Roma,  244 

Ross,  C.,  340 

Ross,  J.,  340 

Rothberg,  575 

Roy,  316 

Royal,  342 

Rozens,  316 

Rudeloff,  529 

Ruggles,  316 


Sabenejew,  106 
Sabin,  396 
Sadtler,  315 
Sakidoff,  50,  433 
Salm,  599 

Salmon,  137,  147,  265 
Samec,  54,  228 


AUTHOR  INDEX 


643 


Sammett,  407 

Samuley,  75 

Sanford,  341,  343 

Scarpa,  314 

Schereschewsky,  560 

Scherrer,  144 

Schmidt,  214,  484,  592-598- 

Scholer,  481 

Scholl,  468 

Schorer,  64 

von  Schroeder,  55,  84,  135,  143,  167, 

189,  196,  197,  317 
Schryver,  25,  27,  122.  225 
Schiitzenberger,  74,  113,  433 
Schwarz,  67,  68 
Schwickerath,  366 
Scott,  373,  378-379 
Scott,  W.,  443,  444,  462 
Searle,  57,  472 
Seeman,  59 
Seidell,  309,  310 
Seidenberg,  477 
Setterberg,  411,  533 
Seyewetz,  57,  572 
Shattermarm,  416 
Shaw,  328 
Sheppard,  122,  149,  152,  208,  378- 

379,  392,  403,  407-410,  422 
Sherman,  557 
Sherrick,  526 

Shuey,  271-317,  435,  437-440,  448 
Siebel,  315 
Sieber,  76 
Siedentopf,  124 
Sindall,  375 
Skraup,  36 
Slough,  342 
Smith,  66,  87 
Smith,  C.  R.,  50,  102,  110,  114,  117, 

156,  173,  207,  212,  377,  392,  400, 

407,  413-415,  433,  496 
Smith,  E.,  375-376 
Smith,  G.,  237 
Smits,  104 

von  Smoluchowski,  93,  109 
Snell,  314 
Solley,  413 
Stfrensen,  27,  30,  245,  581,  586,  587, 

599 


Stacy,  316 

Starkweather,  315 

Statham,  343 

Steinitzer,  548 

Stefan,  92 

Steffans,  340 

Stelling,  372,  463 

Stewart,  56,  60-61,  472 

Stieglitz,  97,  220,  608 

Stirling,  228 

Stohmann,  58 

Stokes,  382,  392 

Stokes,  A.,  477 

Strauss,  228 

Street,  317 

Sturtz,  106 

Stutzer,  471 

Supp, 343 

Sutherland,  134,  139 

Svedberg,  93 

Sweet,  208,  378-379,  403,  407-410 


Taggert,  88 

Tague,  452 

Talman,  342 

Tammann,  105 

Tanner,  575 

Taylor,  265,  391 

Tellotson,  35 

Thiel,  599 

Thiele,  7,  12,  286,  314,  491 

Thomas,  112,  125,  178,  213,  218 

Thompson,  143 

Thube,  342 

Tidley,  316 

Tiebackx,  314 

Tiemann,  316 

Tilanus,  64 

Tower,  77,  78,  88,  366 

Tressler,  349,  356-366,  435 

Trillat,  480 

Trotman,  417,  468 

Trunkel,  117,  466 

Tyndall,  298,  316 


II 


Uhler,  237 


C44 


AUTHOR  INDEX 


Ulrich,  207,  406 
Upton,  286,  314 


Valenta,  372-373 

Vamvakas,  480 

Van  Bemmelen,  132,  191 

Vandergrift,  85 

Van  Laar,  64,  97 

Van  Name,  50,  433 

Van  Slyke,  24,  27,  30-34,  38-45,  59, 

221,  322,  621 
von  Vegesack,  102,  192 
Venuleth,  313 
Vernon,  222 
Vigneron,  391 
Viktorin,  366 
Vining,  340 
Vogt,  73 

Voightlander,  92,  206 
Voitinovici,  65,  78 
Vorlander,  138 

W 

Waage,  220 
Wagner,  241,  481 
Walker,  171,  390 
Walpole,  119-120,  586 
Washburn,  562,  564 
Weelands,  343 
Weidenbusch,  533 
Weimar,  322 
von  Weimarn,  161 
Wells,  65 


Welmers,  532 

White,  350,  355,  366 

Whitney,  169 

Wiedemann,  53,  164 

Wiegand,  553-554 

Wiglow,  107 

Williams,  K.,  366 

Williams,  O.,  562,  563,  564 

Wilson,  J.  A.,  81,  88,  111,  114,  128, 

143, 176,  178,  179-182,  185,  258, 

324 

Wilson,  W.  H.,  179-182,  314 
Winkelblech,  116,  207,  217-218,  406, 

482-483 
Winton,  453 
Wittkowsky,  341 
Wolff,  76,  143,  167 
Wright,  315 


Youle,  391 
Young,  76 


Zaensdorf,  537 

Zdenek,  76 

Zeynek,  76 

Ziegler,  54,  134,  206 

Zlobicki,  115 

Zoller,  322,  324,  328,  332,  563 

Zomer,  4 

Zsigmondy,  54,  92,   121,   124,    127, 

141,  161 
Zuntz,  66,  81 


14  DAY  USE 

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UOV2019659 


REC'D 


NOV19'65-11AM 


LOAN  DEP 

I  • 

JAN  2  1  2006 


LD  21A-60m-3,'65 
(F2336slO)476B 


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University  of  California 

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